Development of articular cartilage and the metaphyseal growth plate: the localization of TRAP cells, VEGF, and endostatin.

During long bone development the original cartilaginous model in mammals is replaced by bone, but at the long bone endings the avascular articular cartilage remains. Before the articular cartilage attains structural maturity it undergoes reorganization, and molecules such as vascular endothelial growth factor (VEGF) and endostatin could be involved in this process. VEGF attracts blood vessels, whereas endostatin blocks their formation. The present study therefore focused on the spatio-temporal localization of these two molecules during the development of the articular cartilage. Furthermore, we investigated the distribution of the chondro/osteoclasts at the chondro-osseous junction of the articular cartilage with the subchondral bone. Mice served as our animal model, and we examined several postnatal stages of the femur starting with week (W) 4. Our results indicated that during the formation of the articular cartilage, VEGF and endostatin had an overlapping localization. The former molecule was, however, down-regulated, whereas the latter was uniformly intensely localized until W12. At the chondro-osseous junction, the number of tartrate-resistant acid phosphatase (TRAP)-positive chondro/osteoclasts declined with increasing age. Until W3 the articular cartilage was not well organized but at W8 it appeared structurally mature. At that time only a few TRAP cells were present, indicative of a low resorptive activity at the chondro-osseous junction. Subsequently, we examined the metaphyseal growth plate that is closed when skeletal maturity is attained. Within its hypertrophic zone, localization of endostatin and VEGF was observed until W6 and W8, respectively. At the chondro-osseous junction of the growth plate, chondro/osteoclasts remained numerous until W12 to allow for its complete resorption. According to former findings, VEGF is critical for a normal skeleton development, whereas endostatin has almost no effect on this process. In conclusion, our findings suggest that both VEGF and endostatin play a role in the structural reorganization of the articular cartilage and endostatin may be involved in the maintenance of its avascularity. In the growth plate, however, endostatin does not appear to counteract VEGF, allowing vascular invasion of hypertrophic cartilage and bone growth.