CHANGES IN THE EXPRESSION OF TYPE-X COLLAGEN IN THE FIBROCARTILAGE OFRAT ACHILLES-TENDON ATTACHMENT DURING DEVELOPMENT

This study histologically and immunohistochemically demonstrated devel opmental changes in cell morphology and expression of type-X collagen in the attachment of the Achilles tendon to the calcaneus in the rat. Although the site of attachment in the mature rat showed a well organi zed, direct insertion that was composed of tendon, fibrocartilage, cal cified fibrocartilage, and bone: this four-zone structure was not obse rved in the immature 1-week-old rat. Formation of fibrocartilage was o bserved at 2 weeks, together with the hypertrophy of chondrocytes and the appearance of the secondary center of ossification. Type-X collage n was not detected either in chondrocytes in the attachment area at 1 week or in hypertrophic chondrocytes at the attachment at 2 weeks. In the 3-week-old rat, the secondary center of ossification extended to t he area of attachment and type-X collagen was detected both in cartila ge spicules within the secondary center of ossification and in cells f ound at the attachment adjacent to the secondary center of ossificatio n. A four-zone structure had been established by 6 weeks and remained through 20 weeks. After 6 weeks, type-X collagen was identified both i n the attachment of the tendon and beneath the calcaneal fibrocartilag e. Type-X collagen is produced by cells in transitional zones between calcified and noncalcified tissue, such as the interface between artic ular cartilage and subchondral bone. In these areas, the expression of this protein persists through maturity and is not transient.