Extracellular matrix

The extracellular matrix is composed of collagen and, in some cases, elastic fibres, embedded in a highly hydrated but stiff ground substance (Fig. 5.1). The components are unique to cartilage, and endow it with unusual mechanical properties. The ground substance is a firm gel, rich in carbohydrates and predominantly acidic. The chemistry of the ground substance is complex. It consists mainly of water and dissolved salts, held in a meshwork of long interwoven proteoglycan molecules together with various other minor constituents, mainly proteins or glycoproteins.

Fig. 5.1 Fine structural organization of hyaline cartilage matrix. Depicted are large proteoglycan complexes and type II collagen fibres (cross-banded and of different diameters). Proteoglycan complexes bind to the surface of these fibres via their monomeric sidechains and link them together. The arrangement of glycosaminoglycans and core protein of the proteoglycan monomer is illustrated in the expansion.

Hyaline cartilage

Hyaline cartilage has a homogeneous glassy, bluish opalescent appearance. It has a firm consistency and some elasticity. Costal, nasal, some laryngeal, tracheobronchial, all temporary (developmental) and most articular, cartilages are hyaline. The arytenoid cartilage changes from hyaline at its base, to elastic cartilage at its apex. Size, shape and arrangement of cells, fibres and proteoglycan composition vary at different sites and with age. The chondrocytes are flat near the perichondrium and rounded or angular, deeper in the tissue. They are often grouped in pairs, sometimes more, forming cell nests (isogenous cell groups) which are daughter cells of a common parent chondroblast: apposing cells have a straight outline. The matrix is typically basophilic (Fig. 5.2) and metachromatic, particularly in the lacunar capsule, where recently formed, territorial matrix borders the lacuna of a chondrocyte. The paler-staining interterritorial matrix between cell nests is older synthetically. Fine collagen fibres are arranged in a basket-like network (Fig. 5.3), but are often absent from a narrow zone immediately surrounding the lacuna. An isogenous cell group, together with the enclosing pericellular matrix, is sometimes referred to as a chondron.

Fig. 5.2 Sections through hyaline cartilage. A, Low-power view of human rib, showing perichondrium (P, left), young chondroblasts (Cb) embedded in pale-staining interterritorial matrix and mature chondrocytes (Cc) embedded in the basophilic interterritorial matrix (centre and right). B, Higher magnification of hyaline cartilage in human bronchial wall, showing isogenous groups of chondrocytes (C). Note the more deeply-stained basophilic zones (B) (rich in acidic proteoglycans) around the cell clusters, with older, paler-staining matrix (M) between clusters.

Fig. 5.3 Electron micrograph of chondroblasts in rabbit femoral condylar cartilage. The central cell has an active euchromatic nucleus with a prominent nucleolus, and its cytoplasm contains concentric cisternae of rough endoplasmic reticulum, scattered mitochondria, lysosomes and glycogen aggregates. The plasma membrane bears numerous short filopodia which project into the surrounding matrix. The latter shows a delicate feltwork of collagen fibrils within finely granular interfibrillary material. No pericellular lacuna is present; the matrix separates the central chondroblast from the cytoplasm of two adjacent chondroblasts (left, and crescentic profile).

(Preparation by courtesy of Susan Smith, Department of Anatomy, GKT School of Medicine, London.)

After adolescence, hyaline cartilages are prone to calcification, especially in costal and laryngeal sites. In costal cartilage, the matrix tends to fibrous striation, especially in old age when cellularity diminishes. The xiphoid process and the cartilages of the nose, larynx and trachea (except the elastic cartilaginous epiglottis and corniculate cartilages) resemble costal cartilage in microstructure. The regenerative capacity of hyaline cartilage is poor.

Articular hyaline cartilage

Articular hyaline cartilage covers articular surfaces in synovial joints (Fig. 5.4). It provides an extremely smooth, resistant surface bathed by synovial fluid, which allows almost frictionless movement. Its elasticity, together with that of other articular structures, dissipates stresses, and gives the whole articulation some flexibility, particularly near extremes of movement. Articular cartilage is particularly effective as a shock-absorber, and resists the large compressive forces generated by weight transmission, especially during movement.

Fig. 5.4 Articular cartilage from the anterior region of the lateral femoral condyle of a young adult human female. Shown are the articular surface (top), articular cartilage and subchondral bone (below). Note the changes in size and spatial distribution of articular chondrocytes through the thickness of the cartilage. 3-D digital volumetric fluorescence imaging of serially sectioned, eosin-Y and acridine orange-stained tissue.

(Provided by courtesy of Professor Robert L Sah, Drs Won C Bae, Kyle D Jadin, Benjamin L Wong, Kelvin W. Li and Mrs Barbara L. Schumacher, Department of Bioengineering and Whitaker Institute of Biomedical Engineering, University of California, San Diego.)

Articular cartilage does not ossify. It varies from 1 to 7 mm in thickness and is moulded to the shape of the underlying bone, indeed it often accentuates and modifies the surface geometry of the bone. It is thickest centrally on convex osseous surfaces, and the reverse is true of concave surfaces. Its thickness decreases from maturity to old age. The surface of articular cartilage lacks a perichondrium; synovial membrane overlaps and then merges into its structure circumferentially (see Fig. 5.32).

Fig. 5.32 A section of a synovial joint and its associated highly vascular (red) synovial membrane in a human fetal hand. The two articular cartilage surfaces (A, arrowed) are separated on the right by a layer of synovial fluid (S) secreted by the synovial membrane (SM) which extends a short distance into the joint space from the capsule (C).

Adult articular cartilage shows a structural zonation with increasing depth from the surface. The arrangement of collagen fibres has been variously described as plexiform, helical, or in the form of serial arcades which radiate from the deepest zone to the surface, where they follow a short tangential course before returning radially. If the surface of an articular cartilage is pierced by a needle, a longitudinal split-line remains after withdrawal. For any given joint, the patterns of split-lines are constant and distinctive and follow the predominant directions of collagen bundles in tangential zones of cartilage. These patterns may reveal tension trajectories set up in surrounding cartilage during joint movement.

Zone 1 is the superficial or tangential layer. The free articular surface is a thin, cell-free layer, 3 μm thick, which contains fine collagen type II fibrils covered superficially by a protein coating. The cells are small, oval or elongated and parallel to the surface, relatively inactive, and surrounded by fine tangential fibres. The collagen fibres deeper within this zone are regularly tangential, their diameters and density increase with depth. Zone 2 is the transitional or intermediate layer. The cells are larger, rounder, and are either single or in isogenous groups. Most are typical active chondrocytes, surrounded by oblique collagen fibres. Deeper still, in the radiate layer (zone 3), cells are large, round and often disposed in vertical columns, with intervening radial collagen fibres. As elsewhere, the cells, either singly or in groups, are encapsulated in pericellular matrix which has fine fibrils and contains fibronectin and types II, IX and XI collagen. The deepest layer or calcified layer (zone 4) lies adjacent to the subchondral bone (hypochondral osseous lamina) of the epiphysis. The adjacent surfaces show reciprocal fine ridges, grooves and interdigitations, which, with the confluence of their fibrous arrays, resist shearing stresses produced by postural changes and muscle action. The junction between zones 3 and 4 is called the tidemark. With age, articular cartilage thins and degenerates by advancement of the tidemark zone, and the replacement of calcified cartilage by bone.

Concentrations of GAGs vary according to site and, in particular, with age. The proportion of keratan sulphate increases linearly with depth, mainly in the older matrix between cell nests, whereas chondroitin sulphates are concentrated around lacunae. The turnover rates of GAGs in cartilage are faster than those of collagen, and the smaller, more soluble GAGs turn over fastest. Turnover decreases with age and distance from the cells. The proteoglycan turnover time is estimated at nearly 5 years for adult human articular cartilage.

The sequence of structural features outlined above is also typical of cartilaginous growth plates (see p. 95). During radial epiphysial growth, the extension of endochondral ossification into overlying calcified cartilage starts with the development of isogenous groups followed by the appearance of hypertrophic cells arranged in vertical columns. This ceases in maturity, but the zones persist throughout life. The same terminal mechanism also occurs in bones which lack epiphyses.

Cells of articular cartilage are capable of cell division, but mitoses are rare except in young bones and damage is not repaired in the adult. Superficial cells are lost progressively from normal young joint surfaces, and they are replaced by cells from deeper layers. Degenerating cells may occur in any of the four zones. This probably accounts for the progressive reduction in cellularity of cartilage with advancing age, particularly in superficial layers.

Articular cartilages derive nutrients by diffusion from vessels of the synovial membrane, synovial fluid and hypochondral vessels of an adjacent medullary cavity, some capillaries from which penetrate and occasionally traverse the calcified cartilage. The contributions from these sources are uncertain and may change with age. Small molecules freely traverse articular cartilage, with diffusion coefficients about half those in aqueous solution. Larger molecules have diffusion coefficients inversely related to their molecular size. The permeability of cartilage to large molecules is greatly affected by variations in its GAG, and hence water, content, e.g. a three-fold increase multiplies the diffusion coefficient a hundred-fold.

Fibrocartilage

Fibrocartilage is dense, fasciculated, opaque white fibrous tissue. It contains fibroblasts and small interfascicular groups of chondrocytes. The cells are ovoid and surrounded by concentrically striated matrix (Fig. 5.5). When present in quantity, as in intervertebral discs, fibrocartilage has great tensile strength and appreciable elasticity. In lesser amounts, as in articular discs, the glenoid and acetabular labra, and the cartilaginous lining of bony grooves for tendons and some articular cartilages, it provides strength, elasticity and resistance to repeated pressure and friction. It is resistant to degenerative change.

Fig. 5.5 White fibrocartilage in a late fetal intervertebral disc (human). Chondroblasts lie between coarse collagen type I fibres (blue) derived from the anulus fibrosus. Mallory’s triple stain.

Fibrocartilage is unlike other types of cartilage in that it contains a considerable amount of type I (general connective tissue) collagen which is synthesized by the fibroblasts in its matrix. It is perhaps best regarded as a mingling of the two types of tissue, e.g. where a ligament or tendinous tissue inserts into hyaline cartilage, rather than as a separate type of cartilage. However, fibrocartilage in joints often lacks type II collagen altogether, and so possibly represents a distinct class of connective tissue.

The articular surfaces of bones which ossify in mesenchymal membranes (e.g. squamous temporal, mandible and clavicle) are covered by white fibrocartilage. The deep layers, adjacent to hypochondral bone, resemble calcified regions of the radial zone of hyaline articular cartilage. The superficial zone contains dense parallel bundles of thick collagen fibres, interspersed with typical dense connective tissue fibroblasts and little ground substance. Fibre bundles in adjacent layers alternate in direction, as they do in the cornea. A transitional zone of irregular bundles of coarse collagen and active fibroblasts separates the superficial and deep layers. The fibroblasts are probably involved in elaboration of proteoglycans and collagen, and may also constitute a germinal zone for deeper cartilage. Fibre diameters and types may differ at different sites according to the functional load.

Elastic cartilage

Elastic cartilage occurs in the external ear, corniculate cartilages, epiglottis and apices of the arytenoids. It contains typical chondrocytes, but its matrix is pervaded by yellow elastic fibres, except around lacunae (where it resembles typical hyaline matrix with fine type II collagen fibrils) (Fig. 5.6). Its elastic fibres are irregularly contoured and show no periodic banding. Most sites in which elastic cartilage occurs have vibrational functions, such as laryngeal sound wave production, or the collection and transmission of sound waves in the ear. Elastic cartilage is resistant to degeneration; it can regenerate to a limited degree following traumatic injury, e.g. the distorted repair of a ‘cauliflower ear’.

Fig. 5.6 Elastic cartilage, stained to demonstrate elastin fibres (blue-black). Chondroblasts and larger chondrocytes are embedded in the matrix, which also contains collagen type II fibres.

Bone matrix

Bone matrix is the mineralized extracellular material of bone; like general connective tissues, it consists of a ground substance in which numerous collagen fibres are embedded, usually ordered in parallel branching arrays (Fig. 5.10). In mature bone, the matrix is moderately hydrated, and 10–20% of its mass is water. Of its dry weight, 60–70% is made up of inorganic, mineral salts (mainly microcrystalline calcium and phosphate hydroxides, hydroxyapatite (see below), approximately 30% is collagen and the remainder is non-collagenous protein and carbohydrate, mainly conjugated as glycoproteins. The proportions of these components vary with age, location and metabolic status.

Fig. 5.10 Scanning electron micrograph of collagen fibres on the surface of human trabecular bone. Note branching fibres (female, 2 months, sixth rib).

Osteoblasts

Osteoblasts are derived from osteoprogenitor (stem) cells of mesenchymal origin, which are present in the bone marrow and other connective tissues. They proliferate and differentiate, stimulated by bone morphogenetic proteins (BMPs), into osteoblasts prior to bone formation. Osteoblasts are basophilic, roughly cuboidal mononuclear cells 15–30 μm across. Ultrastructurally, they have features typical of protein-secreting cells. They are found on the forming surfaces of growing or remodelling bone, where they constitute a covering layer (Fig. 5.11). In relatively quiescent adult bones they appear to be present mostly on endosteal rather than periosteal surfaces, but they also occur deep within compact bone where osteons are being remodelled. They are responsible for the synthesis, deposition and mineralization of the bone matrix, which they secrete. Once embedded in the matrix, they become osteocytes.

Fig. 5.11 Osteoblasts (Ob) covering the free surfaces of developing bone in a human fetal hand. Deep to the layer of osteoblasts in the lower field is a layer of osteoid matrix (Os, pale blue) which has yet to be mineralized. Osteocytes (Oc) are shown within lacunae in mineralized matrix (red).

Osteoblasts contain prominent bundles of actin, myosin and other cytoskeletal proteins which are associated with the maintenance of cell shape, attachment and motility. Their plasma membranes display many extensions, some of which contact neighbouring osteoblasts and embedded osteocytes at intercellular gap junctions. This arrangement facilitates coordination of the activities of groups of cells, e.g. in the formation of large domains of parallel collagen fibres.

Osteoblasts synthesize and secrete organic matrix, i.e. type I collagen, small amounts of type V collagen, and numerous other macromolecules involved in bone formation and resorption. Collagen synthesis occurs in the rough endoplasmic reticulum and Golgi apparatus, and type I collagen is secreted as monomers which assemble into the triple helical procollagen extracellularly. Other glycoprotein products include osteocalcin, which is required for bone mineralization, binds hydroxyapatite and calcium, and is used as a marker of new bone formation; osteonectin, a phosphorylated glycoprotein which binds strongly to hydroxyapatite and collagen – it may play a role in initiating hydroxyapatite crystallization, and may also be a cell adhesion factor; RANKL, the cell surface ligand for RANK (receptor for activation of nuclear factor kappa B), which is an osteoclast progenitor receptor (see below); osteoprotegerin (a soluble, high affinity decoy ligand for RANKL) which restricts osteoclast differentiation; the bone proteoglycans biglycan and decorin which bind TGF-β; bone sialoproteins, osteopontin and thrombospondin, which mediate osteoclast adhesion to bone surfaces via binding to osteoclast integrins; latent proteases and growth factors, including BMPs. TGF-β is secreted by osteoblasts as well as osteoclasts: it is activated in the acid conditions of the ruffled border zone of the osteoclast, and may be a coupling factor for stimulating new bone formation at resorption sites.

Extracellular fluid is supersaturated with respect to the basic calcium phosphates, yet mineralization is not a widespread phenomenon. Osteoblasts play a significant role in the mineralization of osteoid, the unmineralized organic matrix. They secrete osteocalcin which binds calcium weakly, but at levels sufficient to concentrate the ion locally. They also contain membrane-bound vesicles, 0.1–0.2 μm in diameter, which contain alkaline phosphatase (which can cleave phosphate ions from various molecules to elevate concentrations locally), and pyrophosphatase (which degrades inhibitory pyrophosphate in the extracellular fluid). The vesicles bud off from the cell surfaces of the osteoblasts into newly formed osteoid and are the sites of initiation of hydroxyapatite crystal formation in newly forming bone (see below). Crystals are then released into the osteoid matrix by an unknown mechanism. Some alkaline phosphatase reaches the blood circulation where it can be detected in conditions of rapid bone formation or turnover.

Osteoblasts play a key role in the hormonal regulation of bone resorption, since they express receptors for parathyroid hormone (PTH), 1,25-dihydroxy vitamin D3 and other promoters of bone resorption. During bone resorption, osteoblasts promote osteoclast differentiation via PTH-activated expression of cell surface RANKL, which binds to RANK on immature osteoclasts, establishes cell–cell contact and triggers contact-dependent osteoclast differentiation. In the presence of PTH, osteoblasts also downregulate secretion of osteoprotegerin, a soluble decoy ligand with higher affinity for RANKL. In conditions favouring bone deposition, secreted osteoprotegerin blocks RANKL binding to RANK and restricts numbers of mature osteoclasts. (For a recent review, see Blair et al 2007.)

Bone-lining cells are flattened epithelioid cells found on the surfaces of adult bone that is not undergoing active deposition or resorption, and are generally considered to be quiescent osteoblasts or osteoprogenitor cells. They form the outer boundary of the marrow tissue on the endosteal surface of marrow cavities, are present on the periosteal surface, and line the system of vascular canals within osteons.

Osteocytes

Osteocytes constitute the major cell type of mature bone, and are scattered within its matrix, interconnected by numerous dendritic processes to form a complex cellular network (Fig. 5.12). They are derived from osteoblasts and are enclosed within their matrix but, unlike chondrocytes, they do not divide. Bone growth is appositional: new layers are added only to pre-existing surfaces and so, again unlike chondrocytes, osteocytes enclosed in lacunae do not secrete new matrix. The rigidity of mineralized bone matrix prevents internal expansion, which means that interstitial growth, which is characteristic of most tissues, does not occur in bone. Osteocytes retain contacts with each other and with cells at the surfaces of bone (osteoblasts and bone-lining cells) throughout their lifespan.

Fig. 5.12 Osteocyte lacunae shown at high magnification in a dry ground section of lamellar bone. The territories of three osteocytes are shown. Their branching dendrites contact those of neighbouring cells via the canaliculi seen here within the bone matrix. Several other osteocyte lacunae are present, out of the focal plane in this section, and tangential to the osteon axis.

Mature, relatively inactive, osteocytes possess an ellipsoid cell body with the longest axis (25 μm) parallel to the surrounding bony lamella. The rather narrow rim of cytoplasm is faintly basophilic, contains relatively few organelles and surrounds an oval nucleus. Osteocytes in woven bone are larger and more irregular in shape (Fig. 5.13).

Fig. 5.13 Human parietal bone (male neonate) showing primary osteonal bone (grey) and woven bone (white) containing many connecting osteocyte lacunae (black). Internal resorption of the bone has produced large irregular dark spaces (trabecularization).

Numerous fine dendritic processes emerge from the cell body of each osteocyte and branch a number of times. They contain bundles of microfilaments and some smooth endoplasmic reticulum. At their distal tips they contact the processes of adjacent cells, i.e. other osteocytes and, at surfaces, osteoblasts and bone-lining cells. They form communicating gap junctions with these cells which means that they are in electrical and metabolic continuity.

Bone matrix surrounds the cell bodies and processes. There appears to be a variable space filled with extracellular fluid between each osteocyte and its enclosing wall. Each cell body lies in a lacuna from which many narrow, branched channels extend. These channels or canaliculi are 0.5–0.25 μm wide, and contain the dendritic processes of the osteocytes: they provide a route for the diffusion of nutrients, gases and waste products between the osteocytes and the blood vessels. Canaliculi do not usually extend through and beyond the reversal line surrounding an osteon and so do not communicate with neighbouring systems. The walls of lacunae may be lined with a variable (0.2–2 μm) layer of unmineralized organic matrix.

In well-vascularized bone, osteocytes are long-lived cells which actively maintain the bone matrix. The average lifespan of an osteocyte varies with the metabolic activity of the bone and the likelihood that it will be remodelled, but is measured in years. Old osteocytes may retract their processes from the canaliculi; when they die, their lacunae and canaliculi may become plugged with cell debris and minerals, which hinders diffusion through the bone. Dead osteocytes occur commonly in interstitial bone and the inner regions of trabecular bone which escape surface remodelling, and are particularly noticeable by the second and third decades. Bones which experience little turnover, e.g. the auditory ossicles, are most likely to contain aged osteocytes and low osteocyte viability.

Osteocytes play an essential role in the maintenance of bone: their death leads to the resorption of the matrix by osteoclast activity. They remain responsive to parathyroid hormone and 1,25(OH)₂ vitamin D₃, and it is possible that they are involved in mineral exchange at adjacent bone surfaces. Osteocytes themselves are often mineralized.

Osteoclasts

Osteoclasts are large (40 μm or more) polymorphic cells containing up to 20 oval, closely packed nuclei. They lie in close contact with the bone surface in resorption bays (Howship’s lacunae). Their cytoplasm contains numerous mitochondria and vacuoles, many of which are acid phosphatase-containing lysosomes. The rough endoplasmic reticulum is relatively sparse given the size of cell, but the Golgi complex is extensive. The cytoplasm also contains numerous coated transport vesicles and microtubule arrays involved in the transport of the vesicles between the Golgi stacks and the ruffled membrane, which is the highly infolded cell surface of active osteoclasts at sites of local bone resorption. A well-defined zone of actin filaments and associated proteins occurs beneath the ruffled membrane around the circumference of the resorption bay, in a region termed the sealing zone.

Functionally, osteoclasts are responsible for the local removal of bone during bone growth and subsequent remodelling of osteons and surface bone (see Fig. 5.25). They cause demineralization by proton release, which creates an acidic local environment, and organic matrix destruction by releasing lysosomal (cathepsin K) and non-lysosomal (e.g. collagenase) enzymes. Factors stimulating osteoclasts to resorb bone include osteoblast-derived signals; cytokines from other cells, e.g. macrophages and lymphocytes; blood-borne factors, e.g. parathyroid hormone and 1,25(OH)₂ vitamin D₃ (calcitriol). Calcitonin, produced by C cells of the thyroid follicle, reduces osteoclast activity.

Fig. 5.25 Endochondral ossification in human fetal bone. Spicules of cartilage remnant (pale blue) serve as surfaces for the deposition of osteoid (dark blue), shown in the upper half of the field. Mineralized, woven bone is stained red. Three large multinucleate osteoclasts are seen centre right, further eroding cartilage and remodelling the developing bone. Blood sinusoids and haemopoietic tissue (below) fill the spaces between areas of ossification. Heidenhain’s azan trichrome preparation.

Osteoclasts arise by fusion of monocytes derived from the bone marrow or other haemopoietic tissue. They probably share a common ancestor with macrophages within the granulocyte–macrophage lineage (see Fig. 4.12) but it is thought that they subsequently follow a distinct differentiation pathway.

Osteons

The mechanical properties of bone, particularly its strength and resilience, are dependent on the general composition of its matrix. Woven and lamellar bone display two quite distinct types of organization.

In woven, or bundle, bone, the collagen fibres and bone crystals are irregularly arranged. The diameters of the fibres vary, so that fine and coarse fibres intermingle, producing the appearance of the warp and weft of a woven fabric. Woven bone is typical of young fetal bones, but is also seen in adults during excessively rapid bone remodelling and repair of fractures (Fig. 5.14). It is formed by highly active osteoblasts during development, and is stimulated in the adult by fracture, growth factors, or prostaglandin E₂.

Fig. 5.14 Electron micrograph of woven bone from a failed fracture of human distal tibia. Two osteoblasts (O) lie on the free surface (top). Newly synthesized collagenous osteoid matrix (M) is seen in the centre field, with a mineralization front (electron-dense area) below (arrows).

Lamellar bone makes up almost all of an adult osseous skeleton (Fig. 5.15, Fig. 5.16). The precise arrangement of lamellae varies from site to site, particularly between compact cortical bone and the trabecular bone within. In many bones a few lamellae form continuous circumferential layers at the outer (periosteal) and inner (endosteal) surfaces. However, by far the greatest proportion of lamellae are arranged in concentric cylinders around neurovascular channels called Haversian canals, to form the basic units of bone tissue which are the Haversian systems or osteons. Osteons usually lie parallel with each other (Fig. 5.17) and, in elongated bones such as those of the appendicular skeleton, with the long axis of the bone. They may also spiral, branch or intercommunicate, and some end blindly.

Fig. 5.15 Main features of the microstructure of mature lamellar bone. Areas of compact and trabecular (cancellous) bone are included. Note the general construction of the osteons; distribution of the osteocyte lacunae; Haversian canals and their contents; resorption spaces. Different views of the structural basis of bone lamellation.

Fig. 5.16 A, Osteons in a dry ground transverse section of bone. Concentric lamellae surround the central Haversian canal of each complete osteon; they contain the dark lacunae of osteocytes and the canaliculi which are occupied in life by their dendrites. These canaliculi interconnect with canaliculi of osteocytes in adjacent lamellae. Incomplete (interstitial) lamellae (e.g. centre field) are the remnants of osteons remodelled by osteoclast erosion. B, High-power view of osteocytes within lamellae; a Haversian canal is seen on the right.

(B, Photograph by Sarah-Jane Smith.)

Fig. 5.17 Osteons in a dry ground longitudinal section of bone. The central Haversian canals (H: tubular structures, mainly dark) show transverse nutrient (Volkmann’s) canals (V) which form bridges between adjacent osteons and their blood vessels.

It has been estimated that there are 21 million osteons in the adult skeleton. In transverse section they are round or ellipsoidal, varying from 100 to 400 μm in diameter. A medium-sized osteon contains about 30 lamellae, each approximately 3 μm thick. Each osteon is permeated with the canaliculi of its resident osteocytes, and these form pathways for diffusion of nutrients, gases, etc. between the vascular system and the osteocytes. The maximum diameter of an osteon ensures that no osteocyte is more than 200 μm from a blood vessel, a distance that may be a limiting factor in cellular survival. The spaces between osteons contain interstitial lamellae which are the fragmentary remains of osteons and the partially eroded circumferential lamellae of older bone (see below).

The central Haversian canals of osteons vary in size, with a mean diameter of 50 μm; those near the marrow cavity are somewhat larger. Each canal contains one or two capillaries lined by fenestrated endothelium and surrounded by a basal lamina which also encloses typical pericytes. They usually contain a few unmyelinated and occasional myelinated axons. The bony surfaces of osteonic canals are perforated by the openings of osteocyte canaliculi and are lined by collagen fibres.

Haversian canals communicate with each other and directly or indirectly with the marrow cavity via vascular (nutrient) channels called Volkmann’s canals, which run obliquely or at right angles to the long axes of the osteons (Fig. 5.17). The majority of these channels appear to branch and anastomose, but some join large vascular connections with vessels in the periosteum and the medullary cavity.

Osteons are distinguished from their neighbours by a cement line which contains little or no collagen, and is strongly basophilic because it has a high content of glycoproteins and proteoglycans. Cement lines mark the limit of bone erosion prior to the formation of a new osteon, and are therefore also known as reversal lines. Canaliculi occasionally pass through cement lines, and so provide a route for exchange between interstitial bone lamellae and vascular channels within osteons. Basophilic lines can occur in the absence of erosion: they indicate where bony growth has been interrupted and then resumed and are called resting lines.

Each lamella consists of a sheet of mineralized matrix that contains collagen fibres of similar orientation locally, running in branching bundles 2–3 μm thick, and often extending the full width of a lamella. This interconnecting, three-dimensional construction increases the strength of the bone. The orientation of the collagen fibres and associated mineral crystals differs in adjacent lamellae: the difference varies between 0° and 90°, and is clearly shown by polarized light microscopy. A less perfect packing of collagen fibrils into bundles occurs at the borders of lamellae, where intermediate and random orientations predominate. The main direction of the collagen fibres within osteons of long bone shafts varies: the fibres are more longitudinal at sites which are subjected predominantly to tension, and more oblique at sites subjected mostly to compression. The peripheral lamellae of osteons contain more transverse fibres at any site in a diaphysis.

Trabecular bone

The organization of trabecular (cancellous, spongy) bone is basically lamellar, as shown most clearly under polarized light (Fig. 5.18). It takes the form of branching and anastomosing curved plates, tubes and bars of various widths and lengths which surround marrow cavities and are lined by endosteal tissue (Fig. 5.8, Fig. 5.9). Their thickness ranges from 50 to 400 μm. In general, bone lamellae are oriented parallel with the adjacent bone surface, and the arrangement of cells and matrix is similar to that found in circumferential and osteonic bone. Thick trabeculae and regions close to compact bone may contain small osteons, but blood vessels do not otherwise lie within the bony tissue, and osteocytes therefore rely on canalicular diffusion from adjacent medullary vessels. In young bone, calcified cartilage may occur in the cores of trabeculae, but this is generally replaced by bone during subsequent remodelling.

Fig. 5.18 Trabecular bone in a bone marrow sample taken from the human posterior iliac crest. A, Irregular trabeculae of bone, surrounded by bone marrow haemopoietic and adipose tissue (haematoxylin and eosin stain); B, the same field viewed under polarized light, demonstrating lamellar, non-osteonic bone with lamellae oriented in different directions in different regions. Osteocytes are just visible, embedded in the solid matrix.

Remodelling

Remodelling of the interior of a bone depends upon the balance of resorption and deposition of bone, i.e. on the balanced activities of osteoclasts and osteoblasts. Osteoclasts first excavate a cylindrical tunnel by concerted action. A ‘cutting cone’ is formed by groups of osteoclasts moving at 50 μm/day, followed by osteoblasts which fill in the space so created. The osteoblasts deposit new osteoid matrix concentrically around a centrally ingrowing blood vessel, starting at the peripheral surface of the tunnel. This forms a ‘closing cone’ with 4000 osteoblasts/mm². Deposition of successive, concentric lamellae follows, as cohorts of osteoblasts become embedded in the matrix they secrete, and are succeeded by new osteoblasts which line the free surface thus created, and secrete the next layer. In this way the walls of resorption cavities are lined with new lamellar matrix, and the vascular channels are progressively narrowed (Fig. 5.19). The pattern and extent of remodelling is dictated by the mechanical loads applied to the bone.

Fig. 5.19 Bone remodelling. Longitudinal and cross-sections of a time line illustrating the formation of an osteon. Osteoclasts cut a cylindrical channel through bone. Osteoblasts follow, laying down bone on the surface of the channel until matrix surrounds the central blood vessel of the newly formed osteon (closing cone of a new osteon).

A hypermineralized basophilic cement line marks a site of reversal from resorption to deposition. Formation of osteons does not end with growth but continues variably throughout life. Remnants of circumferential lamellae of old osteons form interstitial lamellae between newer osteons (Fig. 5.15, Fig. 5.16A).

It has been estimated that approximately 10% of the adult bony skeleton turns over each year by the process of remodelling. The degree of remodelling varies with age and the number of osteons and osteon fragments have therefore been used in attempts to estimate the age of skeletal material at death.

Periosteum, endosteum and bone marrow

The outer surface of bone is covered by a condensed, fibrocollagenous layer, the periosteum. The inner surface is lined by a thinner, more cellular, endosteum. Osteoprogenitor cells, osteoblasts, osteoclasts and other cells important in the turnover and homeostasis of bone tissue lie in these layers.

The periosteal layer is tethered to the underlying bone by extrinsic collagen fibres, Sharpey’s fibres, which penetrate deep into the outer cortical bone tissue. It is absent from articular surfaces, and from the points of insertion of tendons and ligaments (entheses) (see Fig. 5.46). The periosteum is highly active during fetal development, when it generates osteoblasts for the appositional growth of bone. These cells form a layer, two to three cells deep, between the fibrous periosteum and new woven bone matrix. Osteoprogenitor cells within the mature periosteum are indistinguishable morphologically from fibroblasts. Periosteum is important in the repair of fractures: where it is absent, e.g. within the joint capsule of the femoral neck, fractures are slow to heal.

Fig. 5.46 The microstructure of bone at entheses. A, B, The cortical shell of bone (short arrows) is very thin at fibrocartilaginous attachment sites. In these examples showing the attachment of the tendons of triceps brachii (TB) and of fibularis longus (FL), it is approximately the same thickness as the underlying trabeculae (T). Note that in A, the superficial trabeculae (long arrows) are aligned along the direction of pull of the tendon of triceps. C, In marked contrast, the layer of cortical bone (CB) at the fibrous attachment site of pronator teres (PT) to the mid-shaft of the radius, is much thicker. D, Higher power view of the cortical calcified shell of tissue at a fibrocartilaginous attachment site (the Achilles tendon), which consists of bone (B) and calcified fibrocartilage (CF). In this specimen, there are two tidemarks, TM1 and TM2, associated with the cortical shell of calcified tissue. TM1 is adjacent to the zone of uncalcified fibrocartilage (UF), and marks the mechanical boundary between hard and soft tissues. TM2 lies nearer the bone and indicates an earlier phase of calcification. Note the relative straightness of the tidemarks but the highly irregular interface between calcified fibrocartilage and bone (arrows), which is important in anchoring the tendon to the bone. Sections of human cadaveric bone stained with Masson’s trichrome.

(Photographs provided by courtesy of Professor Michael Benjamin from sections cut and stained by S. Redman.)

In resting adult bone, quiescent osteoblasts and osteoprogenitor cells are present chiefly on the endosteal surfaces, which act as the principal reservoir of new bone-forming cells for remodelling or repair. The endosteum provides a surface of approximately 7.5 m², thought to be important in calcium homeostasis. It is formed by flattened osteoblast precursor cells and reticular (type III collagen) fibres, and lines all the internal cavities of bone, including the Haversian canals. It overlies the endosteal circumferential lamellae, and encloses the medullary cavity.

Vascular supply and lymphatic drainage

The osseous circulation supplies bone tissue, marrow, perichondrium, epiphysial cartilages in young bones, and, in part, articular cartilages. The vascular supply of a long bone depends on several points of inflow which feed complex and regionally variable sinusoidal networks within the bone. The sinusoids drain to venous channels which leave through all surfaces that are not covered by articular cartilage. The flow of blood through cortical bone in the shafts of long bones is mainly centrifugal (Fig. 5.20).

Fig. 5.20 The main features of the blood supply of a long bone. Note the contrasting supplies of the diaphysis, metaphysis and epiphysis, and their connections with periosteal, endosteal, muscular and periarticular vessels. The expansion shows part of the diaphysis in more detail. The marrow cavity contains a large central venous sinus, a dense network of medullary sinusoids, and longitudinal medullary arteries and their circumferential rami. Longitudinally oblique transcortical capillaries emerge through minute ‘cornet-shaped’ foramina to become confluent with the periosteal capillaries and venules. The obliquity of the cortical capillaries is emphasized for clarity. Not to scale.

One or two main diaphysial nutrient arteries enter the shaft obliquely through nutrient foramina which lead into nutrient canals. Their sites of entry and angulation are almost constant and characteristically directed away from the dominant growing epiphysis. Nutrient arteries do not branch in their canals, but divide into ascending and descending branches in the medullary cavity which approach the epiphyses, dividing repeatedly into smaller helical branches close to the endosteal surface. The endosteal vessels are vulnerable during operations which involve passing metal implants into the medullary canal, e.g. intramedullary nailing for fractures. Near the epiphyses they are joined by terminal branches of numerous metaphysial and epiphysial arteries. The former are direct branches of neighbouring systemic vessels, the latter come from periarticular vascular arcades formed on non-articular bone surfaces. Numerous vascular foramina penetrate bones near their ends, often at fairly specific sites; some are occupied by arteries, but most contain thin-walled veins. Within bone, the arteries are unusual in consisting of endothelium with only a thin layer of supportive connective tissue. The epiphysial and metaphysial arterial supply is richer than the diaphysial supply.

Medullary arteries in the shaft give off centripetal branches which feed a hexagonal mesh of medullary sinusoids that drain into a wide, thin-walled central venous sinus. They also possess cortical branches which pass through endosteal canals to feed fenestrated capillaries in osteons (Haversian systems). The central sinus drains into veins which retrace the paths of nutrient arteries, sometimes piercing the shaft elsewhere as independent emissary veins. Cortical capillaries follow the pattern of Haversian canals, and are mainly longitudinal with oblique connections via Volkmann’s canals (Fig. 5.17). At bone surfaces, cortical capillaries make capillary and venous connections with periosteal plexuses (Fig. 5.20). The latter are formed by arteries from neighbouring muscles which contribute vascular arcades with longitudinal links to the fibrous periosteum. From this external plexus a capillary network permeates the deeper, osteogenic periosteum. At muscular attachments, periosteal and muscular plexuses are confluent and the cortical capillaries then drain into interfascicular venules.

In addition to the centrifugal supply of cortical bone, there is an appreciable centripetal arterial flow to outer cortical zones from periosteal vessels. The large nutrient arteries of epiphyses form many intraosseous anastomoses, their branches passing towards the articular surfaces within the trabecular spaces of the bone. Near the articular cartilages these form serial anastomotic arcades (e.g. three or four in the femoral head), which give off end-arterial loops. These often pierce the thin hypochondral compact bone to enter, and sometimes traverse, the calcified zone of articular cartilage, before returning to the epiphysial venous sinusoids.

In immature long bones the supply is similar, but the epiphysis is a discrete vascular zone. Epiphysial and metaphysial arteries enter on both sides of the growth cartilage; there are few, if any, anastomoses between them. Growth cartilages probably receive a supply from both sources, and from an anastomotic collar in the adjoining periosteum. Occasionally, cartilage canals are incorporated into a growth plate. Metaphysial bone is nourished by terminal branches of metaphysial arteries and by primary nutrient arteries of the shaft which form terminal blind-ended sprouts or sinusoidal loops in the zone of advancing ossification. Young periosteum is more vascular, its vessels communicate more freely with those of the shaft than their adult counterparts, and they give off more metaphysial branches.

Large irregular bones, e.g. the scapula and innominate, receive a periosteal supply. In addition, they often have large nutrient arteries which penetrate directly into their cancellous bone: the two systems anastomose freely. Short bones receive numerous fine vessels which supply their compact and cancellous bone and medullary cavities from the periosteum. Arteries enter vertebrae close to the bases of their transverse processes. Each vertebral medullary cavity drains to two large basivertebral veins which converge to a foramen on the posterior surface of the vertebral body. Flatter cranial bones are supplied by numerous periosteal or mucoperiosteal vessels. Large thin-walled veins run tortuously in cancellous bone. Lymphatic vessels accompany periosteal plexuses but have not been convincingly demonstrated in bone.

Innervation

Nerves are most numerous in the articular extremities of long bones, vertebrae and larger flat bones, and in periosteum. Fine myelinated and unmyelinated axons accompany nutrient vessels into bone and marrow and lie in the perivascular spaces of Haversian canals. Bone has a complex autonomic and sensory innervation; osteoblasts possess receptors for several neuropeptides that are found in the nerves which supply bone, e.g. neuropeptide Y, calcitonin gene-related peptide, vasoactive intestinal peptide and substance P.

Intramembranous ossification

Intramembranous ossification is the direct formation of bone (membrane bone) within highly vascular sheets or ‘membranes’ of condensed primitive mesenchyme (Fig. 5.21). At centres of ossification, mesenchymal stem cells differentiate into osteoprogenitor cells which proliferate around the branches of a capillary network, forming incomplete layers of osteoblasts in contact with the primitive bone matrix. The cells are polarized because they secrete a fine mesh of collagen fibres and ground substance, osteoid, from the surface which faces away from the blood vessels. The earliest crystals appear in association with extracellular matrix vesicles produced by the osteoblasts; crystal formation subsequently extends into collagen fibrils in the surrounding matrix, producing an early labyrinth of woven bone, the primary spongiosa. As layers of calcifying matrix are added to these early trabeculae, the osteoblasts enclosed by matrix come to lie within primitive lacunae. New osteocytes retain intercellular contact by means of their fine cytoplasmic processes (dendrites) and, as these elongate, matrix condenses around them to form canaliculi.

Fig. 5.21 Intramembranous ossification forming the nasal bones of a 7-month human fetus. Islands of bone (solid pink matrix [M], enclosing osteocytes) enlarge through the deposition of new matrix by osteoblasts (arrows). They subsequently fuse and are remodelled by osteoclasts to form mature lamellar bone.

As matrix secretion, calcification and enclosure of osteoblasts proceed, the trabeculae thicken and the intervening vascular spaces become narrower. Where bone remains trabecular, the process slows and the spaces between trabeculae become occupied by haemopoietic tissue. Where compact bone is forming, trabeculae continue to thicken and vascular spaces continue to narrow. Meanwhile the collagen fibres of the matrix, secreted on the walls of the narrowing spaces between trabeculae, become organized as parallel, longitudinal or spiral bundles, and the cells they enclose occupy concentric sequential rows. These irregular, interconnected masses of compact bone each have a central canal, and are called primary osteons (primary Haversian systems). They are later eroded, together with the intervening woven bone, and replaced by generations of mature (secondary) osteons.

While these changes are occurring, mesenchyme condenses on the outer surface to form a fibrovascular periosteum. Bone is laid down increasingly by new osteoblasts which differentiate from osteoprogenitor cells in the deeper layers of the periosteum. Modelling of the growing bone is achieved by varying rates of resorption and deposition at different sites.

Endochondral ossification

The hyaline cartilage model which forms during embryogenesis is a miniature template of the bone (cartilage bone) that will subsequently develop. It becomes surrounded by a condensed, vascular mesenchyme or perichondrium, which resembles the mesenchymal ‘membrane’ in which intramembranous ossification occurs. Its deeper layers contain osteoprogenitor cells.

The first appearance of a centre of primary ossification (Fig. 5.22) occurs when chondroblasts deep in the centre of the primitive shaft enlarge greatly, and their cytoplasm becomes vacuolated and accumulates glycogen. Their intervening matrix is compressed into thin, often perforated, septa. The cells degenerate and may die, leaving enlarged and sometimes confluent lacunae (primary areolae) whose thin walls become calcified during the final stages (Fig. 5.23). Type X collagen is produced in the hypertrophic zone of cartilage. Matrix vesicles originating from chondrocytes in the proliferation zone are most evident in the intercolumnar regions, where they appear to initiate crystal formation. At the same time, cells in the deep layer of perichondrium around the centre of the cartilage model differentiate into osteoblasts and form a peripheral layer of bone. Initially, this periosteal collar, formed by intramembranous ossification within the perichondrium, is a thin-walled tube which encloses and supports the central shaft (Fig. 5.22, Fig. 5.23). As it increases in diameter it also extends towards both ends of the shaft.

Fig. 5.22 A, Section of a human fetal hand showing cartilaginous models of the carpal bones, and the primary ossification centres, which display varying stages of maturity, in the metacarpals and phalanges. Note that none of the carpal elements shows any evidence of ossification. B, Higher power view of an early primary ossification centre. The cartilage cells in the shaft have hypertrophied and this region is surrounded by a delicate tube or collar of subperiosteal bone (red).

(Photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)

Fig. 5.23 The sequence of cellular events in endochondral ossification. This low magnification micrograph shows the primary ossification centre in a human fetal bone. See Fig. 5.26 for further details. G, growth zone; H, hypertrophic zone; O, ossification zone; R, remodelling zone.

The periosteal collar which overlies the calcified cartilaginous walls of degenerate chondrocyte lacunae is invaded from the deep layers of the periosteum (formerly perichondrium) by osteogenic buds. These are blind-ended capillary sprouts and are accompanied by osteoprogenitor cells and osteoclasts. The latter excavate newly formed bone to reach adjacent calcified cartilage where they continue to erode the walls of primary chondrocyte lacunae (Fig. 5.24, Fig. 5.25). This process leads to their fusion into larger and irregular communicating spaces, secondary areolae, which fill with embryonic medullary tissue (vascular mesenchyme, osteoblasts and osteoclasts, haemopoietic and marrow stromal cells, etc.). Osteoblasts attach themselves to the delicate residual walls of calcified cartilage and lay down osteoid which rapidly becomes confluent, forming a continuous lining of bone. Further layers of bone are added, enclosing young osteocytes in lacunae, and narrowing the perivascular spaces. Bone deposition on the more central calcified cartilage ceases as the formation of subperiosteal bone continues.

Fig. 5.24 Section showing the hypertrophy and palisading of cartilage cells as the ossifying (mineralising) front of an early primary centre of ossification is approached (below). Lacunae are enlarged, and matrix partitions are reduced in width and exhibit increased staining density following cartilage calcification. Alcian-blue PAS stained section of human fetal femur.

Osteoclastic erosion of the early bone spicules then creates a primitive medullary cavity in which only a few trabeculae, composed of bone with central cores of calcified cartilage (Fig. 5.24), remain to support the developing marrow tissues. These trabeculae soon become remodelled and replaced by more mature bone or by marrow. Meanwhile new, adjacent, cartilaginous regions undergo similar changes. Since these are most advanced centrally, and the epiphyses remain cartilaginous, the intermediate zones exhibit a temporospatial sequence of changes when viewed in longitudinal section (Fig. 5.26F). This region of dynamic change from cartilage to bone persists until longitudinal growth of the bone ceases.

Fig. 5.26 The stages of endochondral ossification in a long bone.

Expansion of the cartilaginous extremity (usually an epiphysis) keeps pace with the growth of the rest of the bone both by appositional and interstitial growth. The growth zone expands in all dimensions. Growth in thickness of a developing long bone is caused by occasional transverse mitoses in its chondrocytes, and by appositional growth as a result of matrix deposition by cells from the perichondrial collar or ring at this level. The future growth plate therefore expands in concert with the shaft and adjacent future epiphysis. A zone of relatively quiescent chondrocytes (the resting zone) lies on the side of the plate closest to the epiphysis. An actively mitotic zone of cells faces towards the shaft of the bone: the more frequent divisions in the long axis of the bone soon create numerous longitudinal columns (palisades) of disc-shaped chondrocytes, each in a flattened lacuna (Fig. 5.26). Proliferation and column formation occurs in this zone of cartilage growth (the proliferative zone), and its continued longitudinal interstitial expansion provides the basic mode of elongation of a bone.

The columns of cells show increasing maturity towards the centre of the shaft, as their chondrocytes increase in size and accumulate glycogen. In the hypertrophic zone, energy metabolism is depressed at the level of the mineralizing front (Fig. 5.24). The lacunae are now separated by transverse and longitudinal walls, and the latter are impregnated with apatite crystals (the zone of calcified cartilage). The calcified partitions enter the zone of bone formation and are invaded by vascular mesenchyme containing osteoblasts, osteoclasts, etc. The partitions, especially the transverse ones, are then partly eroded while osteoid deposition, bone formation and osteocyte enclosure occur on the surfaces of the longitudinal walls. Lysis of calcified partitions is mediated by osteoclast (chondroclast) action, aided by cells associated with the terminal buds of vascular sinusoids which occupy, and come into close contact with, each incomplete columnar trabecular framework.

Continuing cell division in the growth zone adds to the epiphysial ends of cell columns, and the bone grows in length as this sequence of changes proceeds away from the diaphysial centre. The bone also grows in diameter as further subperiosteal bone deposition occurs near the epiphyses, and its medullary cavity enlarges transversely and longitudinally. Internal erosion and remodelling of the newly formed bone tissue continues.

Growth continues in this way for many months or years in different bones but eventually one or more secondary centres of ossification usually appear in the cartilaginous extremities. These epiphysial centres (or the ends of bones which lack epiphyses) do not at first display cell columns. Instead, isogenous cell groups hypertrophy, with matrix calcification, and are then invaded by osteogenic vascular mesenchyme, sometimes from cartilage canals. Bone is formed on calcified cartilage, as described above. As an epiphysis enlarges, its cartilaginous periphery also forms a zone of proliferation in which cell columns are organized radially; hypertrophy, calcification, erosion and ossification occur at increasing depths from the surface. The early osseous epiphysis is thus surrounded by a superficial growth cartilage, and the growth plate adjacent to the metaphysis soon becomes the most active region.

As a bone reaches maturity, epiphysial and metaphysial ossification processes gradually encroach upon the growth plate from either side, eventually meeting when bony fusion of the epiphysis occurs and longitudinal growth of the bone ceases. The events which take place during fusion are broadly as follows. As growth ceases, the cartilaginous plate becomes quiescent and gradually thins, proliferation, palisading and hypertrophy of chondrocytes stop, and the cells form short, irregular conical masses. Patchy calcification is accompanied by resorption of calcified cartilage and some of the adjacent metaphysial bone, to form resorption channels which are invaded by vascular mesenchyme. Some endothelial sprouts pierce the thin plate of cartilage, and the metaphysial and epiphysial vessels unite. Ossification around these vessels spreads into the intervening zones and results in fusion of epiphysis and metaphysis.

This bone is visible in radiographs as a radiodense epiphysial line (a term which is also used to describe the level of the perichondrial collar or ring around the growth cartilage of immature bones, or the surface junction between epiphysis and metaphysis in a mature bone). In smaller epiphyses, which unite earlier, there is usually one initial eccentric area of fusion, and thinning of the residual cartilaginous plate. The original sites of fusion are subsequently resorbed and replaced by new bone. Medullary tissue extends into the whole cartilaginous plate until union is complete and no epiphysial ‘scar’ persists. In larger epiphyses, which unite later, similar processes also involve multiple perforations in growth plates, and islands of epiphysial bone often persist as epiphysial scars. Calcified cartilage coated by bone forms the epiphysial scar, and is also found below articular cartilage. It has been called metaplastic bone, a term also applied to sites of attachments of tendons, ligaments and other dense connective tissues to bone.

The cartilaginous surfaces of epiphyses that form synovial joints remain unossified, but the typical sequence of cartilaginous zones persists in them throughout life. A similar developmental sequence occurs at synchondroses, except that the proliferative rates of chondrocytes and the replacement of cartilage by bone are similar, although not identical, on either side of the synchondrosis.

Postnatal growth and maintenance

Modelling, by which is meant changes in general shape, occurs in all growing bones. The process has been studied mainly in cranial and long bones with expanded extremities. A bone such as the parietal thickens and expands during growth, but decreases in curvature. Accretion continues at its edges by proliferation of osteoprogenitor cells at sutures, and periosteal bone is mainly added externally and eroded internally, but not at uniform rates or at all times. The rate of formation increases with radial distance from the centre of ossification (in this case, the future parietal eminence), and formation may also occur endocranially as well as ectocranially, changing the curvature of the bone. As the skull bones thicken and grow at the sutures, the relative positions of the original centres of ossification change in three dimensions as the vault of the skull expands with growth of the brain. The development of diploë (trabeculae of spongy bone) and marrow space internally produces outer and inner cortical plates.

Long bones elongate mainly by extension of endochondral ossification into calcified zones of adjacent growth cartilages, which are continually replaced by the longitudinal interstitial growth of their proliferative zones, with minor additions by radial epiphysial growth. Simultaneously, diametric increases of growth cartilages and shafts occur by continuing subperiosteal deposition and endosteal erosion. However, in many bones growth occurs at different rates, and is even reversed, at different places. A bone which is initially tubular may thus become triangular in section, e.g. the tibia. Similarly, the waisted contours of metaphyses are preserved by differential rates of periosteal erosion and endosteal deposition, as metaphysial bone becomes diaphysial in position. The junction between a field of resorption and one of deposition on the surface of a bone during its growth is called a surface reversal line. The relative position of such a line may remain stable over long periods of cortical drift.

Lamellar bone forms at variable rates which reflect the slow turnover of osteons throughout adult life. A resorption canal (cutting cone) is typically 2 mm long and takes 1–3 months to form; a new osteon (closing cone) forms in a similar period (Fig. 5.19). Internal remodelling continuously supplies young osteons with labile calcium reserves, and provides a malleable bony architecture that is responsive to changing patterns of stress. The remodelling unit in cancellous bone, equivalent to the secondary osteon of compact bone, is the bone structural unit: it has an average thickness of 40–70 μm and an average length of 100 μm, but may be more extensive and irregular in shape.

The normal development and maintenance of bone requires adequate intake and absorption of calcium, phosphorus, vitamins A, C and D, and a balance between growth hormone (GH, somatotropin), thyroid hormones, oestrogens and androgens. Various other factors, including different prostaglandins and glucocorticoids, may also play important roles in the maintenance and turnover of bone. Prolonged deficiency of calcium causes loss of bone mineral via a loss of bone tissue (osteoporosis) and consequent bone fragility. Vitamin D influences intestinal transport of calcium and phosphate, and therefore affects circulatory calcium levels. In adults, prolonged deficiency (with or without low intake) produces bones which contain regions of deformable, uncalcified osteoid (osteomalacia). During growth, similar deficiencies lead to severe disturbance of growth cartilages and ossification, e.g. reductions of regular columnar organization in growth plates, and failure of cartilage calcification even though chondrocytes proliferate. Growth plates also become thicker and less regular than normal, as exemplified in classic rickets or juvenile osteomalacia. In rickets, the uncalcified or poorly calcified cartilage trabeculae are only partially eroded: osteoblasts secrete layers of osteoid, but these fail to ossify in the metaphysial region, and ultimately gravity deforms these softened bones.

Vitamin C is essential for the adequate synthesis of collagen and matrix proteoglycans in connective tissues. When vitamin C is deficient, growth plates become thin, ossification almost stops, and metaphysial trabeculae and cortical bone are reduced in thickness. This causes fragility and delayed healing of fractures. Vitamin A is necessary for normal growth, and for a correct balance of deposition and removal of bone. Deficiency retards growth as a result of the failure of internal erosion and remodelling, particularly in the cranial base. Foramina are narrowed, sometimes causing pressure atrophy of contained nerves, and the cranial cavity and spinal canal may fail to expand with the central nervous system, which impairs nervous function. Conversely, excess vitamin A stimulates vascular erosion of growth cartilages, which become thin or totally lost, and longitudinal growth ceases. Retinoic acid, a vitamin A derivative, is involved in pattern formation in limb buds (see Ch. 51), and in the differentiation of osteoblasts.

Balanced endocrine functions are also essential to normal bone maturation, and disturbances in this balance may have profound effects. In addition to its role in calcium metabolism, excess parathyroid hormone (primary hyperparathyroidism) stimulates unchecked osteoclastic erosion of bone, particularly subperiosteally and later endosteally (osteitis fibrosa cystica). Growth hormone is required for normal interstitial proliferation in growth cartilages, and hence increase in stature. Termination of normal growth is imperfectly understood, but may involve a fall in hormone production or in the sensitivity of chondroblasts to insulin-like growth factors regulated by GH. Reduction of GH production in the young leads to quiescence and thinning of growth plates and hence pituitary dwarfism. Conversely, continued hypersecretion in the immature leads to gigantism, and in the adult results in thickening of bones by subperiosteal deposition; the mandible, hands and feet are the most affected, a condition known as acromegaly.

While continued longitudinal growth of bones depends on adequate levels of GH, effective remodelling to achieve a mature shape also requires the action of the thyroid hormones. Moreover, growth and skeletal maturity are closely related to endocrine activities of the ovaries, testes and suprarenal cortices. High oestrogen levels increase deposition of endosteal and trabecular bone, conversely, osteoporosis in postmenopausal women reflects reduced ovarian function. Fluctuations in the rate of growth and the timing of skeletal maturation are a function of circulating levels of suprarenal and testicular androgens. In hypogonadism, growth plate fusion is delayed and the limbs therefore elongate excessively, conversely, in hypergonadism, premature fusion of the epiphyses results in diminished stature.

Although the morphogenetic factors that determine the shape of a bone have yet to be fully defined, responses to strain are believed to play a major role. Bone resorption typically occurs when gravitational or other mechanical stresses are reduced, as occurs in bed rest, or in zero gravity conditions in space. Bone subjected to constant pressure also tends to resorb, a response that underpins much orthodontic treatment, since teeth can be made to migrate slowly through alveolar bone by the application of steady lateral or medial pressure. Conversely, with constant tension, bone is deposited, e.g. the bones in the racket arm of tennis players are more robust than in the contralateral limb.

Growth of individual bones

Ossification centres appear over a long period during bone growth, many in embryonic life, some in prenatal life, and others well into the postnatal growing period. Ossification centres are initially microscopic but soon become macroscopic, which means that their growth can then be followed by radiological and other scanning techniques.

Some bones, including carpal, tarsal, lacrimal, nasal, and zygomatic bones, inferior nasal conchae and auditory ossicles, ossify from a single centre which may appear between the eighth intrauterine week and the tenth year, a wide sequence for studying growth or estimating age. Most bones ossify from several centres, one of which appears in the centre of the future bone in late embryonic or early fetal life (seventh week to fourth month). Ossification progresses from the centres towards the ends, which are still cartilaginous at birth (Fig. 5.27). These terminal regions ossify from separate centres, which are sometimes multiple, and which appear between birth and the late teens: they are therefore secondary to the earlier primary centre from which much of the bone ossifies. This is the pattern in long bones, as well as in some shorter bones such as the metacarpals and metatarsals, and in the ribs and clavicles.

Fig. 5.27 A, Radiograph of a neonatal arm. Ossification from primary centres is well advanced in all of the limb bones except the carpals, which are still wholly cartilaginous. The gaps by which individual elements appear to be separated are filled by radiolucent hyaline cartilage, in which epiphysial or carpal ossification will subsequently occur. Note the flaring contours, narrow midshaft and relatively expanded metaphyses of the long bones, and the proportions of the limb segments, in particular the relatively large hand, which are characteristic of this age. B, The bones and cartilages of a neonatal left arm. Compare the radiolucent areas in the radiograph (A) with the preserved cartilaginous epiphyses and carpal elements in this specimen.

(Part B prepared by Michael C.E. Hutchinson; photographed by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)

At birth a bone such as the tibia is typically ossified throughout its diaphysis from a primary centre which appears in the seventh intrauterine week, whereas its cartilaginous epiphyses ossify from secondary centres. As the epiphyses enlarge almost all the cartilage is replaced by bone, except for a specialized layer of articular hyaline cartilage which persists at the joint surface, and a thicker zone between the diaphysis and epiphysis. Persistence of this epiphysial plate or disc (growth plate or growth cartilage) allows increase in bone length until the usual dimensions are reached, by which time the epiphysial plate has ossified. The bone has then reached maturity. Coalescence of the epiphysis and diaphysis is fusion, the amalgamation of separate osseous units into one.

Many long bones have epiphyses at both their proximal and distal extremities. Metacarpals, metatarsals and phalanges have only one epiphysis. Typical ribs have epiphyses for the head and articular tubercle and one for the non-articular area. The costal cartilages represent the unossified hyaline cartilage of the developing rib and therefore do not display epiphyses. Epiphysial ossification is sometimes complex, e.g. the proximal end of the humerus is wholly cartilaginous at birth, and subsequently develops three centres during childhood which coalesce into a single mass before they fuse with the diaphysis; only one of these centres forms an articular surface, the others form the greater and lesser tubercles which give muscular attachments. Similar composite epiphyses occur at the distal end of the humerus and in the femur, ribs and vertebrae.

Many cranial bones ossify from multiple centres. The sphenoid, temporal and occipital bones are almost certainly composites of multiple elements in their evolutionary history. Some show evidence of fusion between membrane and cartilage bones which unite during growth.

If the growth rate was uniform, ossification centres would appear in a strict descending order of bone size. However, disparate rates of ossification occur at different sites and do not appear to be related to bone size. The appearance of primary centres for bones of such different sizes as the phalanges and femora are separated by, at most, a week of embryonic life. Those for carpal and tarsal bones show some correlation between size and order of ossification, from largest (calcaneus in the fifth fetal month) to smallest (pisiform in the ninth to twelfth postnatal year). In individual bones, succession of centres is related to the volume of bone which each centre produces. The largest epiphyses, e.g. the adjacent ends of the femur and tibia, are the earliest to begin to ossify (immediately before or after birth, and of forensic interest). At epiphysial plates, the rate of growth is initially equal at both ends of those bones which possess two epiphyses. However, experimental observations in other species have revealed that one generally grows faster than the other after birth. Since the faster-growing end also usually fuses later with the diaphysis, its contribution to length is greater. Though faster rate has not been measured directly in human bones, later fusion has been documented radiologically.

The more active end of a long limb bone is often termed the growing end, but this is a misnomer. The rate of increase in stature, which is rapid in infancy and again at puberty, demonstrates that rates of growth at epiphyses vary. The spurt at puberty, or slightly before, decreases as epiphyses fuse in post-adolescent years, and has been the subject of much study.

Growth cartilages do not grow uniformly at all points, which presumably accounts for changes such as the alteration in angle between the humeral shaft and its neck. The junctions between epiphysis and diaphysis at growth plates are not uniformly flat on either surface. Osseous surfaces usually become reciprocally curved by differential growth, and the epiphysis forms a shallow cup over the convex end of the shaft, with cartilage intervening, an arrangement that may resist shearing forces at this relatively weak region. Reciprocity of bone surfaces is augmented by small nodules and ridges, as can be seen when the surfaces are stripped of cartilage. These adaptations emphasize the formation of many immature bones from several elements held together by epiphysial cartilages. Most human bones exhibit these complex junctions, at which bone is bonded to bone through cartilage throughout the active years of childhood and adolescence.

Forces at growth cartilages are largely compressive, but with an element of shear. Interference with epiphysial growth may occur as a result of trauma, but more frequently follows disease: the resulting changes in trabecular patterns of bone are visible radiographically as dense transverse lines of arrested growth (Harris’s growth lines). Several such lines may appear in the limb bones of children afflicted by successive illnesses.

Variation in skeletal development occurs between individuals, sexes and possibly also races. The timing rather than the sequence of events varies, and females antedate males in all groups studied: differences which are perhaps insignificant before birth may be as great as two years in adolescence.

Suture

Sutures are restricted to the skull (see Ch. 26 for descriptions of individual sutures). In a suture, the two bones are separated by a layer of membrane-derived connective tissue. The sutural aspect of each bone is covered by a layer of osteogenic cells (cambial layer) overlaid by a capsular lamella of fibrous tissue which is continuous with the periosteum on both the endo- and ectocranial surfaces. The area between the capsular coverings contains loose fibrous connective tissue and decreases with age so that the osteogenic surfaces become apposed. On completion of growth, many sutures synostose and are obliterated. Synostosis occurs normally as the skull ages: it can begin in the early twenties and continues into advanced age. A schindylesis is a specialized suture in which a ridged bone fits into a groove on a neighboring element, e.g. where the cleft between the alae of the vomer receives the rostrum of the sphenoid.

Gomphosis

A gomphosis is a peg-and-socket junction between a tooth and its socket, where the two components are maintained in intimate contact by the collagen of the periodontium connecting the dental cement to the alveolar bone. Strictly speaking, a gomphosis is not an articulation between two skeletal structures.

Syndesmosis

A syndesmosis is a truly fibrous connection between bones. It may be represented by an interosseous ligament (inferior tibiofibular joint), a slender fibrous cord, or a more dense aponeurotic membrane (posterior part of sacroiliac joint).

Primary cartilaginous joints

Primary cartilaginous joints or synchondroses occur where advancing centres of ossification are separated by an area of hyaline (non-articular) cartilage. They are present in all postcranial bones that form from more than one centre of ossification. Since hyaline cartilage retains the capability to ossify with age, synchondroses tend to synostose when growth is complete. Primary cartilaginous joints are almost exclusively associated with growth plates (see above): the detailed anatomy of specific joints is described in the appropriate topographical chapter. The location and growth of the spheno-occipital synchondrosis is discussed in Chapter 26.

Secondary cartilaginous joints

Secondary cartilaginous joints or symphyses are largely defined by the presence of an intervening pad or disc of fibrocartilage interposed between the articular hyaline cartilage that covers the ends of two bones. The pad or disc varies from a few millimetres to over a centimetre in width, and the whole region is generally bound by strong, tightly adherent, dense connective tissues. Collagenous ligaments extend from the periostea of the articulating bones across the symphysis and blend with the hyaline and fibrocartilaginous perichondria: they do not form a complete capsule and contain plexuses of afferent nerve terminals which also penetrate the periphery of the fibrocartilage. The combined strength of the ligaments and of the hyaline and fibrocartilage exceeds that of the associated bones, and is designed to withstand a range of stresses (compression, tension, shear and torsion). The range of movement in a symphysis is limited by the physical nature of the articulation and also by the restrictions imposed by other bones associated with the complex. Tears are usually the result of sudden, massive stresses that occur when the body is in an inappropriate posture.

All symphyses occur in the midline (mandibular, manubriosternal, pubic and intervertebral). All except the mandibular symphysis occur in the postcranial skeleton and resist synostosis. The mandibular symphysis (symphysis menti) is histologically different from the other symphyses, however, the widespread use of this descriptive term ensures that it remains, albeit probably inappropriately, within this category.

The concept that synchondroses are temporary and concerned with growth, whereas symphyses are permanent and concerned with movement, is an oversimplification and only partly correct. Both types of joint are concerned with strength and the ability to withstand and transmit considerable stresses, both are sites at which growth occurs, and both contribute either directly or indirectly to the total movement patterns of the parts involved. The strength and mechanical properties of cartilaginous joints are acknowledged, perhaps less attention is paid to the fact that the rigidity of synchrondroses also increases the efficiency of positive movements at related syndesmoses, symphyses and particularly at synovial joints. The movements that occur at a symphysis are not simple extrapolations based on the mechanical properties of a fibrocartilaginous pad or disc. For example, movement of a vertebra relative to its neighbours is a three-dimensional summation of the events that occur in all of the relevant intervertebral joints (syndesmoses, synovial joints, symphyses), each of which is subject to a particular array of stresses.

The prominent role of synchondroses in skeletal growth is widely recognized, whereas growth of symphyses has received less attention. Symphysial growth may, for convenience, be considered from two interrelated aspects, namely intrinsic growth of the fibrocartilaginous disc, and growth of the hyaline cartilaginous plates into which endochondral ossification progresses.

Articular surfaces

Articular surfaces are mostly formed by a specialization of hyaline cartilage, reflecting their preformation as parts of cartilaginous models in embryonic life. Exceptionally, surfaces of the sternoclavicular and acromioclavicular joints and both temporomandibular surfaces are covered by dense fibrous tissue which contains isolated groups of chondrocytes and little surrounding matrix – perhaps a legacy of their formation by intramembranous ossification. Articular cartilage has a wear-resistant, low-frictional, lubricated surface, which is slightly compressible and elastic and is thus ideally constructed for easy movement. It is also able to resist and distribute large forces of compression and shear generated by movement, body weight transfer and muscle contractions.

The thickness of articular cartilage may reach 5–7 mm in larger joints of young individuals but may be reduced to 1–2 mm in the elderly. Young cartilages are typically white, smooth, glistening and compressible, whereas ageing cartilages are thinner, less cellular, firmer and more brittle, with a less regular surface and a yellowish opacity.

Articular cartilages are moulded to bone (see Fig. 5.4), and variations in thickness often accentuate subjacent osseous surface shape. Typically, convex surfaces are thickest centrally, thinning peripherally, and concave surfaces are the reverse. The precise configuration, degree of congruence in various positions, and the dispositions of the surrounding capsule and ligaments, are all related to the types and ranges of movement permitted at a joint. Articular cartilage has no penetrating nerves or blood vessels (except occasional vascular loops). Its nutrition and maintenance therefore largely depend on diffusion from a peripheral vascular plexus in the synovial membrane (circulus vasculosus articuli), blood vessels in adjacent marrow spaces and synovial fluid: the relative importance of these contributions is uncertain.

The zone of articular cartilage next to the joint cavity is mainly a layer of collagen fibres arranged in various planes with small, oval chondrocytes lying deep to it in the matrix. Transmission electron microscopy of heavy metal stained preparations shows an interrupted electron-dense surface coat of a particulate or filamentous material, generally 0.03–0.1 μm thick, covering the cartilage. Synovial fluid and membranous debris, the product of chondrocytic necrosis, may contribute to this surface coat, which is transient in nature; the stable, permanent, articular surface is bounded by the most superficial collagen fibres. The ‘lamina splendens’, a structure that appears as a bright line at the free surface of articular cartilage when oblique sections are examined by negative phase contrast microscopy, is an artefact arising at the border between regions of different refractive index: it is not an anatomically distinct surface layer. The deeper zones of articular cartilage contain a highly complex, three-dimensional reticulum of interconnected fibrils, which have obvious functional implications.

With advancing age, undulations of articular surfaces deepen and develop minute, ragged projections, perhaps as a consequence of wear and tear. These changes are extremely slow in healthy joints: erosion occurs in pathologically ‘dry’ joints and where synovial fluid viscosity is altered. Mitosis is not observed in adult articular chondrocytes.

Fibrous capsule

A fibrous capsule completely encloses a joint except where it is interrupted by synovial protrusions (see descriptions of individual joints for specific details). It is composed of interlacing bundles of parallel fibres of white collagen and is attached continuously round the ends of the articulating bones. In small bones this attachment is usually near the periphery of the articular surfaces, but in long bones it varies considerably, and part or all of the attachment may be a significant distance from the articular surface. The capsule is perforated by vessels and nerves and may contain apertures through which synovial membrane protrudes as bursae. It is lined by a synovial membrane that also covers all non-articular surfaces (non-articular osseous surfaces, tendons and ligaments) that lie partly or wholly within the fibrous capsule, e.g. at the shoulder and knee. Where a tendon is attached inside a joint, an extension of synovial membrane usually accompanies it beyond the capsule. Some extracapsular tendons are separated from the capsule by a synovial bursa continuous with the interior of the joint. These protrusions are potential routes for the spread of infection into joints.

A fibrous capsule usually exhibits local thickenings of parallel bundles of collagen fibres, called capsular (intrinsic) ligaments, that are named by their attachments. Some capsules are reinforced or replaced by tendons of nearby muscles or expansions from them. Accessory ligaments are separate from capsules and may be extracapsular or intracapsular in position.

All ligaments, although yielding little to tension, are pliant. They are slightly elastic and protected from excessive tension by reflex contraction of appropriate muscles. They are taut at the normal limit of a particular movement but do not resist normal actions, since they are designed to check excessive or abnormal movements.

Synovial membrane

Synovial membrane lines fibrous capsules and covers exposed osseous surfaces, intracapsular ligaments, bursae and tendon sheaths (Fig. 5.32). It does not cover intra-articular discs or menisci and stops at the margins of articular cartilages in a transitional zone that occupies the peripheral few millimetres of the cartilage. Synovial membrane secretes and absorbs a fluid that lubricates the movement between the articulating surfaces.

Pink, smooth and shining, the internal synovial surface displays a few small synovial villi that increase in size and number with age. Folds and fringes of membrane may also project into a joint cavity; some are sufficiently constant to be named, e.g. the alar folds and ligamentum mucosum of the knee. Synovial villi are more numerous near articular margins and on the surfaces of folds and fringes, and become prominent in some pathological states.

Accumulations of adipose tissue (articular fat pads) occur in the synovial membrane in many joints. These pads, folds and fringes are flexible, elastic and deformable cushions that occupy the potential spaces and irregularities in joints that are not wholly filled by synovial fluid. During movement they accommodate to the changing shape and volume of the irregularities, they also increase synovial area and may promote the distribution of lubricant over articular surfaces (cf. intra-articular discs and menisci).

Synovial membrane is composed of a cellular intima resting on a fibrovascular subintimal layer (subsynovial tissue). The intima consists of pleomorphic synovial cells embedded in a granular, amorphous, fibre-free extracellular matrix. There is considerable regional variation in cell morphology and numbers. Human synovial cells are generally elliptical, and have numerous cytoplasmic processes. At least two morphologically distinct populations, type A and type B synovial cells, are responsible for synthesizing some of the components of the synovial fluid.

Type A cells are macrophage-like cells characterized by surface ruffles or lamellipodia, plasma membrane invaginations and associated pinocytotic vesicles, a prominent Golgi apparatus but little rough endoplasmic reticulum. Type B synovial cells, which predominate, resemble fibroblasts and have abundant rough endoplasmic reticulum but fewer vacuoles and vesicles, and have a less ruffled plasma membrane. It is thought that some of the hyaluronan and glycoproteins of synovial fluid are synthesized by synovial type B cells, whereas the fluid component is a transudate from synovial capillaries. Type A synovial cells synthesize and release lytic enzymes and phagocytose joint debris: potential damage to joint tissues is limited by the secretion of enzyme inhibitors by type B synovial cells. Synovial cells do not divide actively in normal synovial membranes, but their division rate increases dramatically in response to acute trauma and haemarthrosis. In such conditions the type B synovial cells divide in situ, while the type A cell population is increased by immigration of bone marrow-derived precursors.

The synovial cells of normal human joints form an interlacing, discontinuous layer, one to three cells and 20–40 μm deep, between the subintima and the joint cavity. They are not separated from the subintima by a basal lamina, and are distinguished from the subintimal cells only because they associate to form a superficial layer. In many locations, but particularly over loose subintimal tissue, areas are commonly found that are free from synovial cells. Over fibrous subintimal tissue the synovial cells may be flattened and closely packed, forming endothelioid sheets. Neighbouring cells are often separated by distinct gaps but their processes may interdigitate where they lie closer together.

The subintima is often composed of loose, irregular connective tissue, but also contains organized lamellae of collagen and elastin fibres lying parallel to the membrane surface, interspersed with occasional fibroblasts, macrophages, mast cells and fat cells. The elastic component may prevent formation of redundant folds during joint movement. Subintimal adipose cells form compact lobules surrounded by highly vascular fibroelastic interlobular septa that provide firmness, deformability and elastic recoil. The subintima merges with the synovial membrane where it covers the adjacent capsule, intracapsular ligament or tendon.

Synovial fluid

Synovial fluid occupies synovial joints, bursae and tendon sheaths. In synovial joints it is clear or pale yellow, viscous, slightly alkaline at rest (the pH lowers during activity), and contains a small mixed population of cells and metachromatic amorphous particles. Fluid volume is low: usually less than 0.5 ml can be aspirated from a large joint such as the knee.

The composition of synovial fluid is consistent with it being mainly a dialysate of blood plasma. It contains protein (approximately 0.9 mg/100 ml) derived from the blood. It also contains hyaluronan, which is thought to be a significant determinant of the viscoelastic and thixotropic (flow rate dependent) properties of synovial fluid. A small proportion (approximately 2%) of synovial protein differs from plasma protein and is probably produced by synovial type B cells, and a very small proportion (approximately 0.5%) of synovial protein appears to be a specialized lubricating glycoprotein. Synovial fluid contains a few cells (approximately 60 per ml in resting human joints), including monocytes, lymphocytes, macrophages, synovial intimal cells and polymorphonuclear leukocytes: higher counts are found in young individuals. The amorphous metachromatic particles and fragments of cells and fibrous tissue found in synovial fluid are presumed to be produced by wear and tear.

Intra-articular menisci, discs and fat pads

An articular disc or meniscus occurs between articular surfaces where congruity is low. It consists of fibrocartilage where the fibrous element usually predominates, and is not covered by synovial membrane. The term meniscus should be reserved for incomplete discs, like those in the knee joint and, occasionally, in the acromioclavicular joint. Complete discs, such as those in the sternoclavicular and inferior radio-ulnar joints, extend across a synovial joint, dividing it structurally into two synovial cavities; they often have small perforations. The disc in the temporomandibular joint may be complete or incomplete.

The main part of a disc is relatively acellular, but the surface may be covered by an incomplete stratum of flat cells, continuous at the periphery with adjacent synovial membrane. Discs are usually connected to their fibrous capsule by vascularized connective tissue, so that they become invaded by vessels and afferent and vasomotor sympathetic nerves. Sometimes the union between disc and capsule is closer and stronger, as occurs in the knee and temporomandibular joints.

The function of intra-articular fibrocartilages is uncertain. Deductions have been made from structural or phylogenetic data, aided by mechanical analogies; suggestions include shock absorption, improvement of fit between surfaces, facilitation of combined movements, checking of translation at joints such as the knee, deployment of weight over larger surface areas, protection of articular margins, facilitation of rolling movements, and spread of lubricant. The temporomandibular disc has attracted particular attention because of its exceptional, perhaps unique, design and biomechanical properties (see Ch. 31).

The functions of labra and fat pads, two other quite common types of intra-articular structure, are also uncertain. A labrum is a fibrocartilaginous annular lip, usually triangular in cross-section, attached to an articular margin (e.g. glenoid fossa and acetabulum). It deepens the socket and increases the area of contact between articulating surfaces, and may act as a lubricant spreader. Like menisci, labra may reduce the synovial space to capillary dimensions, thus limiting drag, but unlike menisci, labra are not compressed between articular surfaces. Small fibrous labra (connective tissue rims) have been described along the ventral or dorsal margins of the zygapophysial joints at lumbar levels, as have meniscus-shaped fibroadipose meniscoids at the superior or inferior poles of the same joints. Fat pads are soft and change shape to fill joint recesses that vary in dimension according to joint position.

Neurovascular supply

Classification

Synovial joints may be classified (Fig. 5.33) according to their shape. While this has some practical value, it should be remembered that they are merely variations, sometimes extreme, of two basic forms. Articular surfaces are never truly flat, or complete spheres, cylinders, cones or ellipsoids. They are better described as parts of a single ovoid surface or a complex construction of more than one such surface.

Fig. 5.33 Types of synovial joint, with selected examples.

Factors influencing movement

Movements at synovial joints depend upon a number of factors including the complexity and number of articulating surfaces and the number and position of the principal axes of movement.

Degrees of freedom

Joint motion can be described by rotation and translation about three orthogonal axes. There are three possible rotations (axial, abduction–adduction, flexion–extension) and three possible translations (proximo-distal, mediolateral, anteroposterior). Each is a degree of freedom. For most joints, translations are negligible and do not need consideration (Fig. 5.34). A few joints have minor pure translatory movements, but most joint motion is by rotation.

Fig. 5.34 The shoulder joint is multiaxial, and possesses three degrees of freedom. A–C show the three mutually perpendicular axes around which the principal movements of flexion–extension (A), abduction–adduction (B) and medial and lateral rotation (C) occur. Note that these axes are referred to the plane of the scapula and not to the coronal and sagittal planes of the erect body. Although an infinite variety of additional movements may occur at such a joint, e.g. movements involving intermediate planes or combinations, they can always be resolved mathematically into components related to the three axes illustrated.

When movement is practically limited to rotation about one axis, e.g. the elbow, a joint is termed uniaxial and has one degree of freedom. If independent movements can occur around two axes, e.g. the knee (flexion–extension and axial rotation), the joint is biaxial, and has two degrees of freedom. Since there are three axes for independent rotation, joints may have up to three degrees of freedom. This apparently simple classification is complicated by the complexity of joint structure, and has consequent effects on motion. Even though a true ‘ball and socket’ joint can rotate about many chosen axes, i.e. it is multiaxial, for each position there is a maximum of three orthogonal planes, which means that it can have, as a maximum, three degrees of freedom.

For a uniaxial hinge joint with a single degree of freedom, a single unchanging axis of rotation would be predicted. However, because the shapes of joint surfaces are complex, there is a variable radius of curvature (Fig. 5.35) and consequently the axis of rotation will vary as joint movement progresses. When the variation is minor, e.g. the elbow, it is often appropriate to describe a mean position for the axis. In others, e.g. the knee, the situation is more complex.

Fig. 5.35 Profile of a section through an ovoid surface showing that it may be considered as a series of segments of circles of changing radius. The radius of curvature of joint surfaces often changes from one location to another.

Simple movements are rarely such. Often motion in one direction is linked to motion in another in an obligatory fashion. There are two varieties of rotation: conjunct (coupled), which is an integral and inevitable accompaniment of the main movement, and adjunct, which can occur independently and may or may not accompany the principal movement.

Striated muscle

Fibre type transformation

The fibre type proportions in a named muscle may vary between individuals of different age or athletic ability. Fibre type grouping, where fibres with similar metabolic and contractile properties aggregate, increases after nerve damage and with age. It occurs as a result of reinnervation episodes, where denervated fibres are ‘taken over’ by a sprouting motor neurone and their type properties transformed under direction of the new motor neurone. If the nerves to fast white and slow red muscles are cut and cross-anastomosed in experimental animals, so that each muscle is reinnervated by the other’s nerve, the fast muscle becomes slower-contracting, and the slow muscle faster-contracting (Buller et al 1960). There is evidence that fibre type transformation may be a response to the patterns of impulse traffic in the nerves innervating the muscles. If fast muscles are stimulated continuously for several weeks at 10 Hz, a pattern similar to that normally experienced by slow muscles, they develop slow contractile characteristics and acquire a red appearance and a resistance to fatigue even greater than that of slow muscles.

The initial phase of slowing can be explained by less rapid cycling of calcium, the result of a reduction in the extent of the sarcoplasmic reticulum and changes in the amount and molecular type of proteins involved in calcium transport and binding. Chronic stimulation also triggers the synthesis of myosin heavy and light chain isoforms of the slow muscle type: the associated changes in cross-bridge kinetics result in a lower intrinsic speed of shortening. The muscle becomes more resistant to fatigue through changes in the metabolic pathways responsible for the generation of ATP and a reduced dependence on anaerobic glycolysis. There is a switch to oxidative pathways, particularly those involved in the breakdown of fat and fatty acids, and an associated increase in capillary density and in the fraction of the intracellular volume occupied by mitochondria. If stimulation is discontinued, the sequence of events is reversed and the muscle regains, over a period of weeks, all of its original characteristics. The reversibility of transformation is one of several lines of evidence that the changes take place within existing fibres, and not by a process of degeneration and regeneration.

Many of the changes in the protein profile of a muscle that are induced by stimulation are now known to be the result of transcriptional regulation. For example, analysis of the messenger RNA species encoding myosin heavy chain isoforms shows that expression of the fast myosin heavy chain mRNA is downregulated within a few days of the onset of chronic stimulation, while the slow myosin heavy chain mRNA is upregulated. Although myosin isoform expression is responsive to the increase in use induced by chronic stimulation, it tends to be stable under physiological conditions unless these involve a sustained departure from normal postural or locomotor behaviour.

Tendons

Tendons (Fig. 5.45) take the form of cords or straps of round or oval cross-section, and consist of dense, regular connective tissue. They contain fascicles of type I collagen, orientated mainly parallel to the long axis, but are to some extent interwoven. The fasciculi may be conspicuous enough to give tendons a longitudinally striated appearance to the unaided eye. Tendons generally have smooth surfaces, although large tendons may be ridged longitudinally by coarse fasciculi (e.g. the osseous aspect of the angulated tendon of obturator internus). Loose connective tissue between fascicles provides a conduit for small vessels and nerves, and condenses on the surface as a sheath or epitendineum, which may contain elastin and irregularly arranged collagen fibres. The loose attachments between this sheath and the surrounding tissue present little resistance to movements of the tendon, but in situations where greater freedom of movement is required, a tendon is separated from adjacent structures by a synovial sheath.

Fig. 5.45 Attachment of a tendon (orange) to skeletal muscle (pink). The regular dense connective tissue of the tendon consists of parallel bundles of type I collagen fibres which are orientated in the long axis of the tendon and the muscle to which it is attached. A few elongated fibroblast nuclei are visible in the tendon.

Tendons are strongly attached to bones, both at the periosteum and through fasciculi (extrinsic collagen fibres). Tendinous attachments (entheses or osteotendinous junctions) have been broadly categorized as either fibrocartilagenous or fibrous. In fibrocartilagenous entheses, four zones of tissue have been identified: pure dense fibrous connective tissue (continuous with and indistinguishable from the tendon), uncalcified fibrocartilage, calcified fibrocartilage and bone (continuous with and indistinguishable from the rest of the bone). There are no sharp boundaries between the zones, and the proportions of each component vary between entheses (Fig. 5.46A,B,D). At fibrous entheses, which are characteristic of the shafts of long bones, the tendon is attached to bone by dense fibrous connective tissue either directly or indirectly via the periosteum (Fig. 5.46C). It has been suggested that the greater area of the skeleton to which many fibrous entheses (e.g. pronator teres, deltoid) are attached compared with fibrocartilagenous entheses (e.g. rotator cuff tendons) is important in dissipating stress. (For a review of entheses and the concept of the ‘enthesis organ’, see Benjamin et al 2006.)

Tendons are slightly elastic and may be stretched by 6–10% of their length without damage. Recovery of the elastic ‘strain’ energy stored in tendons can make movement more economical. Although they resist extension, tendons are flexible and can therefore be diverted around osseous surfaces or deflected under retinacula to redirect the angle of pull. Since tendons are composed of collagen and their vascular supply is sparse, they appear white. However, their blood supply is not unimportant: small arterioles from adjacent muscle tissue pass longitudinally between the fascicles, branching and anastomosing freely, and accompanied by venae comitantes and lymphatic vessels. This longitudinal plexus is augmented by small vessels from adjacent loose connective tissue or synovial sheaths. Vessels rarely pass between bone and tendon at osseous attachments, and the junctional surfaces are usually devoid of foramina. A notable exception is the calcaneal tendon (Achilles tendon), which receives a blood supply across the osseotendinous junction. During postnatal development, tendons enlarge by interstitial growth, particularly at myotendinous junctions, where there are high concentrations of fibroblasts. Growth decreases along the tendon from the muscle to the osseous attachments. The thickness finally attained by a tendon depends on the size and strength of the associated muscle, but appears to be influenced by additional factors such as the degree of pennation of the muscle. The metabolic rate of tendons is very low but increases during infection or injury. Repair involves an initial proliferation of fibroblasts followed by interstitial deposition of new collagen fibres.

The nerve supply to tendons is largely sensory, and there is no evidence of any capacity for vasomotor control. Golgi tendon organs, specialized endings that are sensitive to force, are found near myotendinous junctions; their large myelinated afferent axons run within branches of muscular nerves or in small rami of adjacent peripheral nerves.

Actions of muscles

Conventionally, the action of a muscle is defined as the movement that takes place when it contracts. However, this is an operational definition: equating ‘contraction’ with shortening, and ‘relaxation’ with lengthening is too simple in the context of whole muscles and real movements. Whether a muscle approximates its attachments on contraction depends on the degree to which it is activated, and the forces against which it has to act. The latter are generated by numerous factors: gravity and inertia, any external contact or impact, actively by opposing muscles, and passively by the elastic and viscous resistance of all the structures which undergo extension and deformation, some within the muscle itself, others in joints, inactive muscles and soft tissues. Depending on the conditions, an active muscle may therefore maintain its original length or shorten or lengthen, and during this time its tension may increase, decrease or not change. Movements that involve shortening of an active muscle are termed concentric, e.g. contraction of biceps/brachialis while raising a weight and flexing the elbow. Movements in which the active muscle undergoes lengthening are termed eccentric, e.g. in lowering the weight previously mentioned, biceps/brachialis ‘pays out’ length as the elbow extends. Eccentric contractions are associated with increased risk of muscle tears, especially in the hamstrings. Muscle contraction that does not involve change in muscle length is isometric.

Natural movements are accomplished by groups of muscles. Each muscle may be classified, according to its role in the movement, as a prime mover, antagonist, fixator or synergist. It is usually possible to identify one or more muscles which are consistently active in initiating and maintaining a movement: they are its prime movers. Muscles that wholly oppose the movement, or initiate and maintain the opposite movement, are antagonists, e.g. brachialis has the role of prime mover in elbow flexion, and triceps is the antagonist. To initiate a movement, a prime mover must overcome passive and active resistance and impart an angular acceleration to a limb segment until the required angular velocity is reached; it must then maintain a level of activity sufficient to complete the movement. Antagonists may be transiently active at the beginning of a movement, and thereafter they remain electrically quiescent until the deceleration phase, when units are activated to arrest motion. During the movement, the active prime movers are not completely unrestrained, and are balanced against the passive, inertial and gravitational forces mentioned above.

When prime movers and antagonists contract together they behave as fixators, stabilizing the corresponding joint by increased transarticular compression, and creating an immobile base on which other prime movers may act, e.g. flexors and extensors of the wrist co-contract to stabilize the wrist when an object is grasped tightly in the fingers. In some cases, sufficient joint stability can be afforded by gravity, acting either on its own, e.g. knee and hip joints when they are in or near the close-packed position in the erect posture, or in conjunction with a single prime mover, e.g. the shoulder joint when it is stabilized by supraspinatus with the arm pendent. In other cases, and whenever strong external forces are encountered, prime movers and antagonists contract together, holding the joint in any required position.

Acting across a uniaxial joint, a prime mover produces a simple movement. Acting at multiaxial joints, or across more than one joint, prime movers may produce more complex movements which contain elements that have to be eliminated by contraction of other muscles. The latter assist in accomplishing the movement, and are considered to be synergists, although they may act as fixators, or even as partial antagonists of the prime mover. For example, flexion of the fingers at the interphalangeal and metacarpophalangeal joints is brought about primarily by the long flexors, superficial and deep. However, these also cross intercarpal and radiocarpal joints, and if movement at these joints was unrestrained, finger flexion would be less efficient. Synergistic contraction of the carpal extensors eliminates this movement, and even produces some carpal extension, which increases the efficiency of the desired movement at the fingers.

In the context of different movements, a given muscle may act differently, as a prime mover, antagonist, fixator or synergist. Even the same movement may involve a muscle in different ways if it is assisted or opposed by gravity. For example, in thrusting out the hand, triceps is the prime mover responsible for extending the forearm at the elbow, and the flexor antagonists are largely inactive. However, when the hand lowers a heavy object, the extensor action of the triceps is replaced by gravity, and the movement is controlled by active lengthening, i.e. eccentric contraction, of the flexors. It is important to remember that all movements take place against the background of gravity, and its influence must not be overlooked.

The development of fibre types

Developing myotubes express an embryonic isoform of myosin which is subsequently replaced by fetal and adult myosin isoforms. The major isoform of sarcomeric actin in fetal skeletal muscle is cardiac α-actin; only later is this replaced by skeletal α-actin. The significance of these developmental sequences is not known.

The pattern of expression is fibre-specific as well as stage-specific. In primary myotubes, embryonic myosin is replaced by adult slow myosin from about 9 weeks onwards. In secondary and higher order myotubes the embryonic myosin isoform is superseded first by fetal and then by adult fast myosin, and a proportion go on to express adult slow myosin. Other fibre-specific, tissue-specific and species-specific patterns of myosin expression have been described in mammalian limb muscles and jaw muscles.

The origin of this diversity in the temporal patterns of expression of different fibres, even within the same muscle, is far from clear. It has been suggested that intrinsically different lineages of myoblast emerge at different stages of myogenesis or in response to different extracellular cues. If this is the case, their internal programmes may be retained or overridden when they fuse with other myoblasts or with fibres that have already formed. The fibres that emerge from this process go on to acquire a phenotype that will depend on the further influence of hormones and neural activity.

In man, unlike many smaller mammals, muscles are histologically mature at birth, but fibre type differentiation is far from complete. In postural muscles, the expression of type I myosin increases significantly over the first few years of life; during this period the fibre type proportions in other muscles become more divergent. The presence in adult muscles of a small proportion of fibres with an apparently transitional combination of protein isoforms reinforces the view that changes in fibre type continue to some extent in all muscles and throughout adult life. Fibre type transitions also occur in relation to damage or neuromuscular disease; under these conditions, the developmental sequence of myosins may be recapitulated in regenerating fibres.

Growth and regulation of fibre length

Muscle fibres grow in length by addition of sarcomeres to the ends of the myofibrils. It is important that the number of sarcomeres is regulated throughout life, so that the mean sarcomere length, and hence filament overlap, is optimized for maximum force. This is achieved by addition or removal of sarcomeres in response to any prolonged change of length. For example, if a limb is immobilized in a plaster cast, the fibres of muscles that have been fixed in a shortened position lose sarcomeres, while those that have been fixed in a lengthened position add sarcomeres; the reverse process occurs after the cast has been removed.