COLLAGEN MESSENGER-RNA EXPRESSION DURING TISSUE-DEVELOPMENT - THE TEMPOROSPACIAL ORDER COORDINATES BONE MORPHOGENESIS WITH COLLAGEN FIBER FORMATION

Bone formation of the maxilla and premaxilla of rats was studied by in situ hybridization, using probes for fibrillar collagen mRNAs. Chondr oblasts, osteoblasts, fibroblasts and peripheral bone cells differed i n their expression patterns. Prospective nasal chondroblasts expressed collagen alpha 1(II) and alpha 1(XI) RNA from day 15 post coitum. Bon e formation in the adjacent maxilla and premaxilla started around day 17: groups of osteoblasts, representing ossification centers, expresse d collagen alpha 1(I) RNA strongly, and alpha 1(V), alpha 2(V) and alp ha 1(XI) RNA weakly, but they were deficient in collagen alpha 1(III) RNA. As the centers expanded, osteoblasts in the resulting bone domain s expressed collagen alpha 1(I) RNA in abundance, whereas collagen alp ha 1(III) RNA was absent. Bone domains were surrounded:by fibroblasts containing collagens alpha 1(I), alpha 1(III) and alpha 2(V) RNA. Wide ly separated fibroblasts underwent condensation into densely packed pe riosteum and sutural soft tissues. Cells at the periphery of fast-grow ing bone domains also displayed, apart from collagen alpha 1(I) RNA, c ollagens alpha 2(V) and alpha 1(XI) RNA. Given the continuous recruitm ent of cells from the periosteum, peripheral bone cells represent diff erentiating osteoblasts synthetizing collagens alpha 2(V) and alpha 1( XI) RNA transiently. Thus, gene expression during osteoblast different iation reflects syn thesis of fiber components during bone growth, sin ce collagen V is located in the center of fibers consisting primarily of collagen I.