Instructive induction or permissive activation? Differentiation of ectodermal cells isolated from the axolotl blastula

Previous observations indicate that the cell differentiations associated with induction in the amphibian embryo may involve a permissive activation rather than an instructive induction. We have studied this process in small explants isolated from the axolotl blastula and cultured in vitro. It was previously found that Li+ and heparan sulphate ensure the differentiation of animal ectodermal cells into neural crest cells. In the present paper we show that this same effect may be achieved by the addition of ouabain, the calcium ionophore A 23187, the potassium ionophore valinomycin, cyclic AMP and cyclic GMP, prostaglandin E1, and glucagon. It was also observed that the substances need to be present for as little as 10 min, and must be added within 6-7 h from the time of isolatoin in order to exert their effect. We report some observations on sulphated polysaccharides, which are strong activators of the differentiation of ectodermal cells. Weak ionic currents are of great importance for various cellular events, including cell differentiations. Inhibitors of these currents have been found to inhibit the differentiation of ectodermal cells initiated by heparan sulphate. If the 'induction' of the ectodermal cells is really an activation, then it follows that they must be induced to form epidermis. Various facts suggest that the tonicity of the medium is a decisive factor in this process, the induction being promoted by low ion concentrations. © 1983.