Localization of collagen types I, III and V during tendon development. Changes in collagen types I and III are correlated with changes in fibril diameter.

Collagen types I, III and V were localized at different stages of tendon development: a stage when tendon architecture is established, but immature (14-day), a mature tendon (hatchling) and an intermediate point where there is a rapid growth of tendon fibrils (17-day). The tendon fascicles and their connective tissue investments (endotendenium) were studied. The data show a changing pattern of type III collagen expression in the developing metatarsal tendon. In the immature tendon at 14 days of development, collagen types I and III are codistributed throughout the fascicles and their connective tissue investments. At this stage all of the fibrils regardless of the site are small. With development the regions segregate and become easily recognizable. As this occurs, the fibril diameter distributions diverge; those in the fascicle become larger while those in the endotendenium remain small. During this period, the fascicle loses type III collagen expression, while the endotendenium becomes type III collagen rich. At the same time, the connective tissue investments develop, and the fibrils of the endotendenium remain small during this period, but then become larger in the mature tendon. The increases in diameter are associated with a decrease in type III collagen reactivity. At hatching, both significant collagen type III reactivity and small diameter fibrils are restricted to the outer sheaths. During all stages of tendon development there is a constant small, but detectable amount of type V collagen. However, no correlation between type V reactivity and fibril diameter was observed at any stage of development. These results indicate an inverse relationship between type III collagen reactivity and fibril diameter in the developing tendon.