In situ expression of collagen and proteoglycan genes during development of fibrocartilage in bovine deep flexor tendon

A region of fibrocartilage develops in bovine deep flexor tendon where the tissue wraps around bone and is subjected to compressive and shear forces i n addition to tension. There is no fibrocartilage at this location in fetal tendon or in adjacent adult tendon that is subjected to tensional load onl y. We investigated the development of fibrocartilage in tendon using in sit u hybridization to localize cells that express collagen and proteoglycan ge nes typical of either tendon or cartilage. The signal for type I collagen a nd decorin was high in cells throughout fetal and newborn tendon, as is exp ected in a growing tissue composed predominantly of type I collagen. No sig nal for aggrecan was seen in either fetal or newborn tendon. No hybridizati on with any of the probes for collagen or proteoglycan was detected in cell s in the tensional region of adult tendon, indicating that the cells in thi s tissue are normally quiescent. However, the cells in the fibrocartilage o f adult tendon displayed a high level of expression for types I and II coll agen, decorin, biglycan, and aggrecan. This suggests that the fibrocartilag e in adult tendon is a dynamic tissue. Expression of type IIA collagen is c onsidered a marker of prechondrocytes. Type IIA collagen gene expression wa s present throughout both the tensional and compressed regions of fetal and newborn tendon but was absent in cartilage and adult tendon. This suggests that cells located throughout fetal tendon may have the capacity to develo p as chondrocytes. Fibrocartilage signal was detected for type I collagen i n 75% of the cells and for type II collagen in 50% of the cells at one loca tion in adult tendon, suggesting that some cells in this tissue could have expressed mRNA for both type I and type II collagen.