CONTROL OF CHONDROGENESIS IN THE EMBRYO.

Control of chondrogenesis in the embryo involves mechanisms which induce certain cells to synthesize chondroitin sulfate, as well as mechanisms which regulate the continued production of these molecules in differentiated chondrocytes. The embryonic spinal cord and notochord induce somite cells to chondrify. Interaction between these inducers and somite cells, however, is not sufficient in itself to transform the latter into chondrocytes; there must be additional reactions. The notion that induction simply involves the transmission of information-rich molecules like RNA or protein from inducers to somite cells is thought unlikely. When differentiated 10-day chondrocytes with all the enzymes and genetic information for the synthesis of chondroitin sulfate are removed from their mucopolysaccharide matrix and cultured as isolated cells in vitro, they rapidly transform from spherical cells to stellate ones. Correlated with this change in shape and increase in surface area of cell membrane is the cessation of the production of chondroitin sulfate. The stellate cells are induced to synthesize DNA and multiply; their progeny, though permitted to resume their originally spherical shape, do not differentiate into recognizable chondrocytes again. Experiments of this kind suggest the presence of metabolic controls located in the cytoplasm and/or cell membrane.