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).

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 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.

Most bones in the body contain compact (dense) and cancellous (spongy) bone. Compact bone is very dense compared with cancellous bone (Fig. 9.8). In a typical adult long bone, e.g. the humerus, compact bone forms an outer collar along the shaft (diaphysis), whereas cancellous bone constitutes the less dense interior. Cancellous bone forms most of the ends of long bones (the epiphyses) in adults although compact bone may form a thin rim. (Hyaline cartilage is also a component of epiphyses of bones involved in synovial joints and it covers the joint surface.) Spaces within cancellous bone (Fig. 9.8) are filled by bone marrow, which is involved in blood cell formation, and fat cells; the number of fat cells increases with age.

Compact bone is based on an arrangement of bone cells and matrix known as the Haversian system. This arrangement consists of many thin layers (lamellae) of bone (Fig. 9.6) arranged concentrically as cylinders which lie along the length of the shaft of long bones. This arrangement gives maximal resistance to forces acting along the bone and is particularly important in resisting compression due to weight bearing. Osteocytes, in lacunae, are embedded within the lamellae and lie in concentric circles which are apparent if the cylinders are sectioned across their length, i.e. transversely (Figs 9.6 and 9.9). At the centre of each cylinder of lamellae are small blood and lymphatic vessels and nerves in a space known as a Haversian canal (Figs 9.6 and 9.9). Cytoplasmic processes of osteocytes extend through very small channels (canaliculi) in the bone matrix (Fig. 9.6) and via this route exchange gases and molecules with blood in vessels in a nearby Haversian canal. The whole unit of a Haversian canal plus lamellae is known as an osteon. Although Haversian systems have such a highly regular arrangement of osteons, in many regions the cylinders are packed together with irregular lamellae of bone (Fig. 9.9). The regularity of the cylinders of osteons is also apparent when they are sectioned along their length (Fig. 9.10). In addition, canals (Volkmann’s) carry vessels and nerves perpendicular to Haversian canals. The presence of blood vessels in bone matrix ensures all osteocytes have adequate oxygen and nutrients, without which they die and the bone matrix degenerates.

Cancellous bone consists of thin spicules (trabeculae) of bone that project as a meshwork from compact bone (Fig. 9.8). This type of bone has a random lamellar structure with osteocytes located in lacunae but it does not have Haversian or Volkmann’s canals. The osteocytes are nourished via diffusion from blood vessels in the marrow.

Bone formation (ossification) and growth of bones begins in utero and continues up to the age of 20 or so. After this time, ossification and bone growth stops but the components of matrix are gradually removed and/or replaced throughout life. In addition, at any stage, the shape of bones may be remodelled in response to various factors including dietary changes, strenuous exercise or physical damage. Bone formation and growth occurs by two methods: intramembranous and endochondral ossification.