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Hyaline cartilage
  1. Occurs fused with bone or as discrete pieces, looking hyaline/translucent (Hyalos - 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, fibroblasta and blood vessels. It merges gradually via a chondrogenic zone with the cartilage proper.
  2. Matrix, apparently amorphous with H/E staining in LM, contains:
    1. Ground substances rich in soluble collagens and the proteoglycan - aggrecan: a core protein with chondroitin and keratan sulphates side chains, and metachromatic staining properties, and bind water and confer resilience.
    2. 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 cartilase.
    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 and glycoproteins.
  3. Growth occurs in two ways:
    1. Appositional/perichondral by the recruitment of fresh cells, chondroblasts, from perichondral stem cells, and the addition of new matrix to the surface;
    2. 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.
  4. Territories Most noticeably in articular cartilage there are:
    1. The chondron - the chondrocyte and the pericellular matrix immediately around it ;
    2. proteoglycan-rich territorial matrix outside the chondron;
    3. 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.
  5. 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.
Elastic/yellow cartilage
  1. 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 Verhoeffs. The matrix is not prone to degeneration and calcification.
  1. 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.
Distribution* of the three cartilage varieties
  1. Hyaline - articular surface of most synovial joints (articular cartilage); costal cartilages; nasal and respiratory tract cartilages; basis of most of the fetal skeleton; fracture callus.
  2. Elastic - external ear (pinna), pharyngotympanic tube (eustachian 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); menisci of knee joint; labra of joints like glenoidal labrum; articular discs of joints like sternoclavicular joint; pubic symphysis; femoral ligamentum teres; many tendon insertions into bone; and the articular surface of some joints, e.g., temporomandibular.

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