Activation of masked neural determinants in amphibian eggs and embryos and their release from the inducing tissue

The neural-archencephalic (forehead) inducing activity of the microsomes and the high speed supernatant from Xenopus laevis ovaries and eggs was tested on gastrula ectoderm of Triturus alpestris. Both fractions have a very small inducing activity which increases considerably after autolysis or treatment with dissociating agents. A comparison with the inducing activity of the high speed supernatant from gastrulae suggests that neural inducing factors are synthesized in the ovary, stored in a masked form and activated, in part, during gastrulation. When the supernatant proteins from eggs and gastrulae were subjected to size-exclusion HPLC the neural inducing activity was eluted in different size classes, suggesting a limited proteolysis of a precursor during early embryogenesis. Further experiments have shown that treatment of the blastoporal lip from early Triturus gastrulae with actinomycin D or cycloheximide which inhibit the synthesis of RNA and protein respectively, diminishes the neural-archencephalic (forehead) inducing activity of the blastoporal lip. Treatment with actinomycin D or cycloheximide followed by ethanol, which, besides other structures, impairs the plasma membrane, does, however, not reduce the inducing activity. This may suggest that components which are needed for the transport of the neuralizing factor are diminished, when the synthesis of mRNA and protein is inhibited. Such components are probably needed for the release of the neuralizing factor from the blastoporal lip, the inducer tissue.