Nonuniformity within Embryonic Somites: Differental Response to Retinoic Acid in Vitro

Recent studies have shown that in the developing chick embryo, at physiological level retinoic acid (RA) causes mirror-image duplication of limb skeletal elements. This has led to the suggestion that RA could be the endogenous morphogen or isgnal substance. In this study, in order to explore the effect of RA on somite chondrogenesis, we have standardized a serum-free chemically defined medium that supports the growth of somite explants in vitro. The results indicate that in somites RA at 10 ng/ml level induces cell proliferation, DNA and sulfated proteoglycan synthesis, and at higher concentrations is toxic. The results further show that RA induced stimulation of somite chondrogenesis is sclerotomal specific and the dermamyotemal portion of the somites does not exihibit a similar response. Retinoic acid also increases heparan sulfate synthesis and aggregation of isolated sclerotomal cells in culture. These results thus suggest that in amplifying chondrogenesis, RA acts at all phases such as cell proliferation (may increase cell viability) and aggregation, increased DNA synthesis and increased synthesis of matrix components. In otherwords, RA seems to initiate a chain of inter-related events.