William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
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:
- (a) Ground substances rich in soluble collagens and the proteoglycan - aggrecan: a core
protein with chondroitin and keratan sulphates side chains, and a link
protein for attachment to hyaluronic acid. The
aggregated proteoglycans impart basophilic and metachromatic staining
properties, and bind water and confer resilience.
- (b) Collagen fibrils visible in EM; and LM after silver impregnation,
digestion of the ground substance, or in polarized light. The
reinforcing fibrils are oriented in some relation to stresses
experienced by the cartilage.
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:
- (a) Appositional/perichondral by the recruitment of fresh cells,
chondroblasts, from perichondral stem cells, and the addition of new matrix
to the surface;
- (b) Interstitial by the mitotic division of, and deposition of more
matrix around, chondrocytes already established in the cartilage.
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.
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.,