THE PHENOTYPIC SWITCH FROM CHONDROCYTES TO BONE-FORMING CELLS INVOLVES ASYMMETRIC CELL-DIVISION AND APOPTOSIS

We have investigated the early cellular events that take place during the phenotypic switch from hypertrophic chondrocytes to bone-forming c ells in a) chondrocytes located inside intact lacunae after embryonic chick femurs had been cut through the hypertrophic cartilage and cultu red for 1-15 days; and b) at the cartilage/marrow interface of femurs after short-term culture. Ultrastructural studies were combined with i n situ methods localizing proliferating and apoptotic cells, and 3D-re constructions of confocal images of the cartilage/marrow edge, The cru cial event in the phenotypic switch was an asymmetric cell division wh ich resulted in one daughter cell which underwent apoptosis and anothe r viable daughter cell which subsequently differentiated to an osteoge nic cell, i.e to a smaller basophilic cell that was positive for alkal ine phosphatase, type I collagen, osteonectin, osteopontin, bone sialo protein and osteocalcin and that, after 12-15 days in culture, could s ynthesize a mineralized bone matrix within intact lacunae, The present results suggest a mechanism whereby differentiated cells can change t heir phenotype, At least one mitotic division seems to be required to fix the commitment to the new phenotype.