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The inducing capacity of the vegetal hemisphere of early amphibian blastulae was studied by placing a Nucleopore filter (pore size 0.4 μm) between isolated presumptive endoderm and animal (ectodermal) caps. The inducing effect was shown to traverse the Nucleopore membrane. The reacting ectoderm differentiated into mainly ventral mesodermal derivatives. Expiants consisting of five animal caps also formed dorsal mesodermal and neural structures. Those results together with data published elsewhere suggest that, in addition to a vegetalizing factor, different mesodermal factors must be taken into consideration for the induction of either the ventral or the dorsal mesodermal derivatives. The neural structures are thought to be induced by the primarily induced dorsal mesodermal tissue. Electron microscopic (TEM) examination did not reveal any cell processes in the pores of the filter. The results indicate that transmissible factors rather than signals via cytoplasmic contacts or gap junctions are responsible for the mesodermal induction of ectodermal cells. The data support the view that in normogenesis the mesoderm is determined by the transfer of inducing factors from vegetal blastomeres to cells of the marginal zone (presumptive mesodermal cells).

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Isolated competent amphibian ectoderm differentiates into neural (archencephalic) structures when treated with the plant lectin concanavalin A (Con A). While the inner ectoderm layer ofXenopus laevis forms brain structures after incubation with Con A, the outer ectoderm layer differentiates into ciliated epidermis only. This difference can be correlated with the pattern of Con A bound to the plasma membrane. With gold-labelled Con A it could be shown by transmission electron microscopy (TEM) that the outer ectoderm binds substantially less lectin than the inner layer. Furthermore we observed characteristic differences at the apical and basal surfaces of the cells of the same layer, i.e. on the apical cell surface of the superficial layer almost no Con A-gold could be found. In contrast, we observed a lot of gold particles on the basal cell side of the superficial layer. However, the number on both surfaces (apical and basal side of the cell) of the inner ectoderm layer was essentially higher, which could explain its biological reaction to the Con A stimulus and the differentiation into neural structures. The data presented in this paper indicate that early and late gastrula ectoderm bind similar amounts of Con A and support the view that the decrease in competence is not correlated with a loss of receptors for inducing factors. Furthermore, we describe the binding and the internalization of Con A via receptor-mediated endocytosis and the further fate of the Con A-gold-receptor complex inside the target cell.

We have compared the chemical properties and biological activities of the mesoderm-inducing factor that is secreted by the Xenopus XTC cell line with the vegetalizing factor from chicken embryos. The inducing activity of the factors was tested in different concentrations on totipotent ectoderm either by implantation into early gastrulae of Triturm alpestris or by application of solutions to isolated ectoderm of early gastrulae of Xenopus laevis. Both factors have similar properties. They are not irreversibly inactivated after treatment with 6 M urea or with phenol at 60° C. Reduction with thioglycolic acid inactivates the factors completely. The inducing activity of XTC-conditioned medium decreases only slightly after treatment with 50% formic acid. The apparent molecular mass and the isoelectric point of the factors are similar. The XTC factor was partially purified by size-exclusion and reversed-phase high-pressure liquid chromatography and by isoelectric focusing. The possible relationship of these factors to transforming growth factor β is discussed.