Collagen and alkaline phosphatase gene expression during bone morphogenetic protein (BMP)-induced cartilage and bone differentiation.

Endochondral bone formation induced by bone morphogenetic protein (BMP) involves expression of collagen Types I and II and alkaline phosphatase (ALP) genes. Expression of these genes was studied in mice after implantation of BMP. The amount of Type I collagen mRNA increased from Day 3 to Day 7, when mesenchymal cell aggregation was observed. On Day 17, Type I collagen mRNA expression was correlated with an increased number of osteoblasts. Type II collagen mRNA increased from Day 7 and coincided with chondroblast appearance. This increase was suppressed by Day 17, although hypertrophic and degenerative chondrocytes were present. Alkaline phosphatase mRNA increased markedly from Day 7 with the appearance of chondroblasts. The high level of ALP mRNA continued until Day 11, during chondrogenesis. Mineral deposition was first observed roentgenographically on Day 11. Thus, BMP-induced bone formation occurs with the expression of collagen Types I and II and ALP genes.