The significance of cell contacts for the differentiation of the skeletal blastema.

Isolated limb bud cells from day-11 and day-12 mouse embryos served to investigate the significance of adhesion and contact formation for triggering cartilage differentiation in the blastema. In monolayer culture at low cell density, fibroblastlike cells developed which produced collagen type I and pro III as well as fibronectin. In mass culture at high cell density, however, cartilage tissue formed whose matrix contained collagen type II and cartilage-specific proteoglycans. Other experiments showed that the existence of an overgrowth phenomenon and a cartilage-inducing factor is not the reason for differentiation differences. An increased adhesion tendency and the occurrence of gap junctions speak for changes in the cell membrane during blastema formation. These notions are supported by investigations using fluorochrome-labelled lectins which show that sugar chains with terminal galactosyls only exist during the blastemal stage. It is assumed that adhesion gives the signal for chondrogenesis. The differentiation signal is then stabilized and passed on through cellular communication via gap junctions. After the onset of chondrogenesis, the cell membrane changes again and the gap junction-containing segments are incorporated and disaggregated.