HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 6 CARTILAGE

A specialized CT to resist compression, and provide modest rigidity with flexibility, by having its cells, chondrocytes, produce a firm resilient matrix of ground substances, and fibres or fibrils. The rapid growth of cartilage is used to assist the growth of bones and the repair of fractures. Based on the composition of the matrix, three kinds are distinguished: hyaline, elastic, and fibrocartilage.

A HYALINE CARTILAGE

l Occurs fused with bone or as discrete pieces, looking hyaline/translucent (glass-like) to the unaided eye. Most surfaces, except joint/articular ones, are covered by a nutritive CT perichondrium/capsule with collagen and elastic fibres, fibroblasts and blood vessels. It merges gradually via a chondrogenic zone with the cartilage proper. Powerpoint.

2 Matrix, apparently amorphous with HE staining in LM, contains:

3 Chondrocytes or cartilage cells are large and rounded, each lying in a space - lacuna - enclosed by matrix. Cells often are grouped in nests of 2, 4, or 6 as a result of mitoses and restricted cellular movement. EM reveals cells to have short stubby processes, fat droplets, glycogen and the GER and Golgi complex appropriate for secretion of the matrix components: proteoglycans, type II collagen [with the homotrimeric molecule a1(II)3], and glycoproteins.

4 Growth occurs in two ways:

Growth is vulnerable to X-rays, poor nutrition, and disturbed blood supply, for example, from fractures at the growth plate.

5 Territories Most noticeably in articular cartilage there are:
.. (i) the chondron - the chondrocyte and the pericellular matrix immediately around it;
.. (ii) proteoglycan-rich territorial matrix outside the chondron;
.. (iii) interterritorial matrix, lying between the territorial matrices.
The matrix of the chondron has its own profile of special collagens, proteoglycans, and cartilage glycoproteins, whereas the differences between territorial and interterritorial matrices are more quantitative, and related to collagen fibril thickness and orientation.

6 Nutrition - cartilage is avascular and no blood vessels serve the matrix directly, but cartilage canals may carry vessels through the matrix to non-cartilaginous regions, e.g., secondary ossification centres. Therefore, nutriment and wastes must diffuse through the matrix for the cells to stay alive and perform their slow turnover of the matrix macromolecules. The diffusion may break down and various degenerations then occur, e.g., calcification. This last is prompted, organized and made use of in the process of endochondral ossification.

B ELASTIC/YELLOW CARTILAGE

l Is more opaque and flexible than the hyaline kind, but the cells are similar in appearance and distribution; and it occurs as separate pieces with a perichondrium.
2 Matrix is permeated by many elastic fibres that can be selectively stained by stains such as orcein or Verhoeff's. The matrix is not prone to degeneration and calcification.

C FIBROCARTILAGE

l In the intervertebral (IV) disc, fibrocartilage at first appears to have a rather disorderly matrix with many thick collagen fibres, amongst which are dispersed only a few chondrocytes in lacunae. However, the fibres are orderly in their alternating orientations and layering, like the burst-resisting fibres of an old-style bias-ply car tyre.
2 The matrix gives the staining reaction of collagen, mostly type I, except for close around the cells where proteoglycans are abundant.
3 Lacks a perichondrium and is not seen as discrete pieces; rather it is a strong tension-resistant, but flexible transitional tissue located between tendon and bone, bone and bone, hyaline cartilage and hyaline cartilage.
4 In the IV disc, the enclosed central nucleus pulposus is not cartilage, but nevertheless has collagen type II, which diminishes in the innermost layers of the annulus fibrosus as it is replaced by type I.

D DISTRIBUTION OF THE THREE CARTILAGE VARIETIES

l Hyaline - articular surface of most synovial joints; costal cartilages; nasal and respiratory tract cartilages; basis of most of the fetal skeleton; fracture callus, Chapter 31.E.1
2 Elastic - external ear, pharyngotympanic tube, epiglottis, and some laryngeal and bronchiolar cartilages.
3 Fibrocartilage - intervertebral disc's annulus fibrosus (around a nucleus pulposus of notochordal origin, present until late in life); pubic symphysis; femoral ligamentum teres; many tendon insertions into bone; and the articular surface of some joints, e.g., temporomandibular.
William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159