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Mesoderm formation in the amphibian embryo is thought to be induced in the ectoderm of the animal region by signals emanating from the endoderm of the vegetal region after cleavage up to the mid-blastula. During this process the dorsal vegetal zone is thought to stimulate the dorsal animal zone to establish the Spemann organizer, which will in turn trigger the overlaying neuroectoderm during gastrulation resulting in the development of the central nervous system. In this concept it is assumed that the animal hemisphere is an uncommitted area, which receives its instructions from the vegetal region of the embryo. However, the experiments of this paper show that the 4 animal blastomeres of the eight-cell stage will form dorsal mesodermal structures in over 50% of the cases. The results support the view that developmental determinants are distributed in distinct gradients already in the early cleavage stages and that in the embryo the mesoderm is determined by factors prelocalized in the marginal zone. The spatial and temporal activation of certain genes in a distinct pattern is not simply emanating from certain areas or centers, but is a result of complex interactions between the vegetal and animal hemisphere and vice versa.

Suramin, a polyanionic compound, which has previously shown to dissociate platelet derived growth factor (PDGF) from its receptor, prevents the differentiation of neural (brain) structures of recombinants of dorsal blastopore lip (Spemann's organizer) and competent neuroectoderm. Furthermore, the suramin treatment changes the prospective differentiation pattern of isolated blastopore lip. While untreated dorsal blastopore lip will differentiate into dorsal mesodermal structures (notochord and somites), suramin treated dorsal blastopore lip will form ventral mesoderm structures, especially heart structures. The results are discussed in the context of the current opinion about the mode of action of different growth factor superfamilies.

Early amphibian gastrula ectoderm induced with vegetalizing factor for 6 h using the sandwich-method and cultured for up to 12 days, differentiated into mesodermal and endodermal tissues. Explants which were dissociated into single cells after the induction followed by immediate reaggregation and then cultured for 12 days likewise differentiated into mesodermal and endodermal tissues. However, if after induction and dissociation of the tissue, single cells are cultured for 20 h prior to the reaggregation, the reaggregated cell mass mainly differentiated into endoderm (liver and intestine) and irregularly shaped epidermis (formerly called 'atypical epidermis'). Blood cells and, in few cases, heart structures were the only mesodermal structures found in this series. The results suggest that an endodermal anlage is induced first. The differentiation of mesodermal derivatives depends on secondary cell interactions between endodermal induced and non-induced ectoderm. For this process permanent cell-to-cell contacts are necessary.

Factors which induce mesoderm, including endothelium lined cavities and primitive blood cells in omnipotent amphibian ectoderm, have been isolated from different sources. Recently it was shown that angiogenic factors, which belong to the protein families of the heparin binding growth factors (acidic and basic fibroblast growth factor) and the transforming growth factors (TGF-beta 1 and -beta 2), also induce mesodermal tissues in amphibian ectoderm. In triturus ectoderm, capillary like endothelial networks are induced preferentially by the transforming growth factors. The relationship between growth factors and inducing factors is discussed.

The inducing activity of two heparin binding growth factors HBGF-1 (prostate epithelial cell growth factor; acidic pI) and HBGF-2 (fibroblast growth factor; basic pI) from bovine brain has been tested on totipotent ectoderm from early amphibian (Xenopus laevis, Ambystoma mexicanum) embryos. Both factors induced, at high concentrations, mostly compact spheres surrounded by a non-epidermal epithelium. When the concentration or time of incubation was reduced, large muscle inductions frequently organized as somites were formed besides endothelial vesicles, mesenchyme and smaller areas of intestine-like epithelium. Further reduction of the concentrations or the time of incubation led to an increase in size and number of endothelium-lined vesicles and of mesenchyme, whereas the induction of muscle decreased. At still lower concentrations the overall rate of inductions decreased. The relationship of the growth factors to the vegetalizing factor from chicken embryos, dilution of which shows a similar shift in induced organs, is discussed. The present and previous experiments suggest that different mesodermal and endodermal tissues are induced by secondary interactions in which additional factors are involved. The induced organs derive from dorsal as well as from ventral mesoderm.

Using transmission and scanning electron microscopy we were able to show that during primary embryonic induction in amphibians (Triturus alpestris) the interspace between the inducing chorda-mesoderm and the reacting ectoderm (presumptive medullary plate) of mid-gastrula stages is traversed by cell projections starting from cells of both tissue layers. In addition intimate membrane contacts between the main bodies of the ectodermal and chorda-mesodermal cells could be observed. It could be ruled out that cytoplasmic bridges (anastomosis) exist between cells of inducing chorda-mesoderm and reacting ectoderm, which would allow a free transfer of inducing substances without passing through membranes, as Eakin and Lehmann [1] have postulated. The possible role of cell to cell contact for neural induction is emphasized.

The results of this study indicate that the induction of the central nervous system in Xenopus laevis depends on the close juxtaposition of inducing chordamesoderm and reacting ectoderm, which is necessary for the short distance migration of neural inducing factors. The examination of the neuroectoderm-chordamesoderm interface at intervals of 1 h up to 5 h showed that the onset of neural induction is correlated to the degree of contact formation between ectodermal and mesodermal cells. In the ectoderm cells the number of coated pits, a feature of receptor-mediated endocytosis, is increased. Furthermore there exist telophase bridges between some ectoderm cells, which are possibly correlated to secondary cell interactions.