Development and ageing of phenotypically distinct fibrocartilages associated with the rat Achilles tendon.

We describe by routine histology and by immunohistochemistry three phenotypically and developmentally distinct fibrocartilages associated with the Achilles tendon of the rat. All the fibrocartilages develop after birth and show significant age-related changes in the composition of their extracellular matrix. Attachment-zone fibrocartilage occurs at the insertion of the tendon on the calcaneus. It derives from the cartilage rudiment of the calcaneus and from the region where the tendon merges with the perichondrium. The extracellular matrix contain type II collagen and chondroitin sulphate. Compressive tendon fibrocartilage occurs in the deep part of the tendon where it presses against the calcaneus, and is derived by metaplasia of tendon cells. The cells label strongly for the intermediate filament vimentin, and the extracellular matrix contains chondroitin and keratan sulphates, but type II collagen only in very old animals (> 2 years). Calcaneal fibrocartilage covered the posterior surface of the calcaneus where it was in contact with the Achilles tendon. It labelled intensely for type II collagen and contained chondroitin and keratan sulphates. The cells were rich in vimentin. This fibrocartilage was derived from the calcaneal perichondrium.