Expression of major bone extracellular matrix proteins during embryonic osteogenesis in rat mandibles

It is not known how bone proteins appear in the matrix before and after cal cification during embryonic osteogenesis. The present study was designed to investigate expressions of the five major bone extracellular matrix protei ns - i.e. type I collagen, osteonectin, osteopontin, bone sialoprotein and osteocalcin - during osteogenesis in rat embryonic mandibles immunohistoche mically, and their involvement in calcification demonstrated by von Kossa s taining. Wistar rat embryos 14 to 18 days post coitum were used. Osteogenes is was not seen in 14-day rat embryonic mandibles. Type I collagen was loca lized in the uncalcifed bone matrix in 15-day mandibles, where no other bon e proteins showed immunoreactivity. Osteonectin, osteopontin, bone sialopro tein and osteocalcin appeared almost simultaneously in the calcified bone m atrix of 16-day mandibles and accumulated continuously in 18-day mandibles. The present study suggested that type I collagen constitutes the basic fra mework of the bone matrix upon which the noncollagenous proteins are orient ed to lead to calcification, whereas the noncollagenous proteins are deposi ted simultaneously by osteoblasts and are involved in calcification coopera tively.