Bone and cartilage are derived from the mesoderm layer of the embryo and are specialised forms of connective tissue. Together with other connective tissue and skeletal muscle they constitute the musculoskeletal system. The musculoskeletal system is involved in producing movement and stability of the body by transmitting and resisting forces produced by, for instance, muscle contraction and gravity. Common features of bone and cartilage are that they contain connective tissue fibres and ground substance, forming an extracellular matrix which is relatively rigid compared with the rest of the body and is able to resist mechanical stress. In the extracellular matrix, collagen fibres are important in resisting tensile forces and elastin fibres are capable of deformation under stretching forces (and they recoil when the force is reduced).
Cartilage
Cartilage has cellular and non-cellular components. Cartilage cells (chondrocytes) lie in spaces, known as lacunae, surrounded by cartilage matrix (the non-cellular component) (Fig. 9.1). The matrix is produced by cartilage cells and it is a semi-solid material composed of ground substance comprising large molecules (e.g. glycosaminoglycans and proteoglycans), water and connective tissue fibres. Cartilage in many regions is surrounded by a thin layer of connective tissue known as the perichondrium, which is composed mainly of fibroblasts (Fig. 9.1). However, there are sites where perichondrium is not adjacent to cartilage, e.g. on the articulating surfaces of bone (Fig. 9.2). Blood capillaries are not present in cartilage and the oxygen and nutrients required by chondrocytes diffuse through the matrix from nearby capillaries in the perichondrium.
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| Fig. 9.2 |
There are three types of cartilage defined by the major type of fibre present:
■ hyaline cartilage contains mainly type II collagen fibres and is the most abundant type of cartilage in the body
■ elastic cartilage contains type II collagen and elastin fibres
■ fibrocartilage contains type I collagen fibres.
Hyaline cartilage
Hyaline cartilage is present in a wide range of locations and is important in resisting forces and in supporting soft tissues. It forms the articulating surfaces of bones in synovial joints (Fig. 9.2), where it provides a smooth, friction-free surface at which movements occur, and it acts as a shock absorber. Hyaline cartilage also aids respiration by providing rigidity in the larynx, trachea and bronchi, thus ensuring their patency during inspiration (Chapter 11). It is also present at the anterior ends of ribs where movements of the rib cage occur during respiration. In the embryo, hyaline cartilage forms a temporary skeleton which eventually is largely replaced by bone (see below).
There are two cell types in hyaline cartilage: chondroblasts and chondrocytes. Chondroblasts are able to undergo mitosis and secrete extracellular cartilage matrix. They are very active metabolically and may appear in small clusters. Such clusters (Fig. 9.2) contain new cells produced by mitosis from one initial cell in the region. Chondrocytes are the mature form of chondroblasts and are no longer able to undergo mitosis but can still secrete and modify cartilage matrix. The main chemical components of hyaline cartilage matrix are proteoglycans, glycosaminoglycans and hyaluronic acid, which all attract water molecules. This composition allows hyaline cartilage to resist compressive forces and act as a shock absorber. The type of collagen fibres (type II) and their orientation in hyaline cartilage are important in resisting mechanical forces. The appearance of hyaline cartilage is described as ‘glassy’ and the collagen fibres are not apparent in routine histological preparations. With age, chondrocytes and the lacunae they occupy enlarge (Fig. 9.2) and calcium salts may be deposited in the matrix. These changes may alter the mechanical properties of the cartilage.
Growth in hyaline cartilage occurs by two modes: interstitial and appositional growth. Interstitial growth (growth from within) occurs mainly in the early stage of cartilage development. In interstitial growth, cartilage cells in lacunae undergo mitosis, produce matrix and separate from one another; the matrix occupies increasingly larger regions between the cells. Appositional growth occurs at the periphery of clusters of cartilage cells, adjacent to the perichondrium. Fibroblasts in the innermost part of the perichondrium are specialised and known as chondrogenic cells as they can become chondroblasts and secrete cartilage matrix. Most cartilage in the body grows by appositional growth. However, there are exceptions. For example, articular cartilage is not invested by perichondrium (Fig. 9.2) and grows only by interstitial growth. Furthermore, growth of long bones, which occurs up to the age of about 20years, also occurs via interstitial growth of cartilage (see below).
Elastic cartilage
There are similarities between the structure and mode of growth of hyaline and elastic cartilage. However, chondrocytes in elastic cartilage are larger than those in hyaline cartilage and the volume of matrix is less. The matrix of elastic cartilage contains type II collagen fibres, but in addition it has an abundance of elastin fibres which confers on the cartilage a degree of deformability and recoil. The elastin fibres may be readily displayed using special stains (Fig. 9.1). Elastic cartilage is present in, for example, the epiglottis and the external ear.
Fibrocartilage
Fibrocartilage is present in the secondary cartilaginous joints of the body, e.g. in intervertebral discs (Fig. 9.3). Fibrocartilage is not invested by perichondrium and in general it is less rigid than hyaline cartilage. The chondrocytes in fibrocartilage often appear oriented along the lines of stress on the cartilage and there are intervening layers of collagen fibres (type I). Fibrocartilage provides resistance to mechanical forces and sometimes is also present in the dense, regular connective tissue in tendons and ligaments.
Bone
Bone plays a vital physical role in protecting delicate underlying body structures, e.g. the heart and brain. Collectively, bones form the jointed skeleton of the body and, in conjunction with the attachment sites of skeletal muscles and tendons, bones act as levers and enable movements to occur. Although bone is hard and apparently inert, it is able to remodel in response to changes, e.g. in the stresses acting upon it, particularly during growth, exercise and after fracture. In addition, calcium in bone acts as a store for use by the rest of the body if calcium uptake in the diet is inadequate. Bone also provides the framework for body shape (along with fat and muscle) and spaces in bones, filled with bone marrow, are sites where blood cell formation (haematopoiesis) occurs (Chapter 8).
Bone consists of several types of bone cell and associated bone matrix. Most bone cells in adults are osteocytes and they lie in lacunae embedded in extracellular bone matrix. Although bone matrix resembles cartilage matrix (Fig. 9.4) it has organic and inorganic components. The organic components of bone matrix include type I collagen fibres, proteoglycans and glycosaminoglycans. The inorganic component of bone matrix consists largely of calcium hydroxyapatite (Ca10(PO4)6(OH)2). The hydroxyapatite is deposited alongside the collagen fibres and this produces the rigid hardness of bone. In comparison with cartilage matrix, the composition of bone matrix restricts the diffusion of gases, nutrients and waste molecules. However, bone matrix is permeated by small blood vessels (capillaries), an arrangement that ensures osteocytes receive sufficient oxygen and nutrients.
Individual bones are covered by a thin layer of connective tissue (the periosteum) except at some joint surfaces, e.g. in synovial joints the ends of the bones are covered by hyaline cartilage (Fig. 9.2). Periosteum contains collagen fibres, small blood vessels and fibroblasts, which can differentiate and become bone-forming cells. Importantly, collagen fibres surrounding many skeletal muscle cells and those in tendons and ligaments are anchored to periosteum and pass through into the bone matrix as Sharpey’s fibres. Such collagen fibres transfer forces from muscles, tendons and ligaments to bones.
There are four major cell types in bone: osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts.
■ Osteoprogenitor cells are fibroblast-like cells in the inner layer of the periosteum adjacent to the surface of bone: they function as stem cells. In actively growing bone or after fracture they undergo mitosis. Some offspring cells differentiate into osteoblasts whilst others remain as osteoprogenitor cells.
■ Osteoblasts (Fig. 9.5) are present at the surface of bone matrix (and the surface of hyaline cartilage matrix in growing bones; see below). Their appearance depends on their level of activity in synthesising new matrix: inactive osteoblasts have little cytoplasm but active ones may appear polygonal or cuboidal in shape. Active osteoblasts have an abundance of rough endoplasmic reticulum in their cytoplasm, reflecting their activity in secreting the organic components of bone matrix. Newly formed bone matrix, deposited onto the surface of existing matrix, does not contain calcium salts and is known as osteoid. Rapidly, osteoid becomes mineralised by the addition of calcium salts and this forms bone matrix. Osteoblasts are essential in this calcification process, which confers rigidity on bone.
