NEW ASPECTS OF ENDOCHONDRAL OSSIFICATION IN THE CHICK - CHONDROCYTE APOPTOSIS, BONE-FORMATION BY FORMER CHONDROCYTES, AND ACID-PHOSPHATASE-ACTIVITY IN THE ENDOCHONDRAL BONE-MATRIX

A detailed histological study of the growth plates from 9- to 20-day-o ld embryonic chick long bones was carried out with the aim of clarifyi ng the long-debated question of the fate of the hypertrophic chondrocy tes, Since resorption in chick bones does not occur synchronously acro ss the plate as it does in mammals, specialized regions develop and th e fate of the chondrocyte depends on its location within the growth pl ate, Where resorption took place, as at the sites of primary vascular invasion or at the main cartilage/marrow interface, chondrocytes under went apoptosis before the lacunae were opened, In addition, spontaneou s apoptosis of chondrocytes occurred at apparently random sites throug hout all stages of chondrocyte differentiation, In older chick bones, a thick layer of endochondral bone matrix covered the cartilage edge, This consisted of type I collagen and the typical noncollagenous bone proteins but, in addition, contained tartrate-resistant acid phosphata se in the mineralized matrix, Where such matrix temporarily protected the subjacent cartilage from resorption, chondrocytes differentiated t o bone-forming cells and deposited bone matrix inside their lacunae, A t sites of first endochondral bone formation, some chondrocytes underw ent an asymmetric cell division resulting in one daughter cell which u nderwent apoptosis, while the other cell remained viable and re-entere d the cell cycle, This provided further support for the notion that ch ondrocytes as well as marrow stromal cells give rise to endochondral o steoblasts.