Information transfer during embryonic induction in amphibians.

Neural induction and differentiation has been studied using Concanavalin A, cyclic AMP, tunicamycin and calcium ionophore A 23187. Competent ectoderm of Xenopus laevis treated with Concanavalin A differentiates into neural (archencephalic) structures. Binding studies with gold-labelled ConA indicate that the superficial ectodermal layer contains fewer ConA-sensitive sites (alpha-D-mannoside and alpha-D-glucoside residues of glycoproteins) than the inner ectodermal layer. The small number of ConA-sensitive sites can be correlated with the fact that the isolated superficial ectoderm layer, in contrast to the inner layer, does not differentiate into neural structures. The gold-ConA particles bound to inner ectoderm are quickly (within 30 minutes) internalized, presumably by receptor-mediated endocytosis. However, endocytosis is not a prerequisite for neural induction. On the contrary ConA apparently must be bound to the plasma membrane for a certain period to initiate neural induction. The rapid internalization of ConA could explain why neural inductions are evoked only if ectoderm is incubated in ConA-containing medium for longer than 30 minutes. On the other hand cyclic AMP or calcium ionophore A 23187 does not elicit neural inductions. On the contrary calcium ionophore A 23187 apparently inhibits neural and mesodermal differentiation. This effect could be correlated with an increase of intracellular calcium level of the ectodermal target cells, which could influence the permeability of gap junctions resulting in a loss of cell communication, followed by a change of differentiation and pattern formation.