MINERALIZATION AND THE EXPRESSION OF MATRIX PROTEINS DURING IN-VIVO BONE-DEVELOPMENT

The in vivo expression of fibronectin, type I collagen, and several no n-collagenous proteins was correlated with the development of bone in fetal and early neonatal rat calvariae. Fibronectin was the earliest m atrix protein expressed in calvariae: with a peak expression in fetal 16- and 17-day (d) bones. Fibronectin expression coincided with the co ndensation of preosteoblasts prior to calcification and decreased once bone mineralization commenced. The expression of type I collagen, ost eonectin, bone sialoprotein, and all;aline phosphatase mRNAs was found at 17 d, The increase in type I collagen mRNA levels was correlated w ith a 3.5-fold increase in calcium deposition at 19-20 d. Bone sialopr otein and alkaline phosphatase peaked on fetal 21 d while osteonectin remained at a low level and was localized to the osteoblast layer and the osteocyte lacunae. Osteopontin mRNA levels increased rapidly in ne onatal calvariae, After birth, osteonectin and fibronectin were mainly associated with blood vessels, Thus, fibronectin is one of the earlie st matrix proteins expressed in calvariae and is rapidly followed by t ype I collagen, bone sialoprotein, and alkaline phosphatase. Osteocalc in, osteonectin, and osteopontin mRNAs have similar patterns of expres sion in the developing fetal calvaria, and their synthesis coincided w ith mineralization.