Abstract
We investigated the spatiotemporal localization of collagen Type I, II, and X mRNAs in the subcutaneously grafted chick periosteum by in situ hybridization. Five days after transplantation, we noted three types of histological findings in the grated tissue. (a) Developing trabecular bone exhibited proliferation of spindle-shaped fibroblastic cells and polygonal osteoblasts with moderate signals for collagen Type I mRNA. (b) Developing cartilage contained ovoid chondrocytes with a moderate level of both collagen Type I and II mRNAs. Differentiating chondrocytes with increased collagen Type X mRNA developed during the course of endochondral ossification. (c) An atypical mass of cartilage weakly stained with alcian blue was composed of a large number of non-hypertrophic chondrocytes exhibiting high signals for collagen Type X mRNA. At Day 9, we observed the typical histological features of both membranous and endochondral ossification. However, sparsely distributed chondrocytes with high signals for collagen Type X mRNA were also demonstrated in osteoid and/or woven bone. The phenotype of chondrocytes showing rapid expression of collagen Type X gene derived from grafted periosteum seems to participate in the important role of endochondral bone formation in the early stage of fracture repair.