Molecular mechanisms of tissue determination and pattern formation in amphibian embryos

Inductive capacity of living eye tissues from adult frogs

The aim of the present work has been to demonstrate the inducing capacity of living homogenous undamaged tissues from adult frogs. Tissues from the eyes of adult frogs Xenopus laevis: retina (R), pigmented epithelium (PE), lens epithelium (LE), and the forebrain (B) as a control, were tested for their inducing capacity using early gastrula ectoderm. To exclude the possibility of ad-mixture of inducing cells in early gastrula ectoderm the transfilter induction technique was used throughout. The results show that the tissues used in the cases of most intense induction tended to induce similar cell types: both R and PE induce R + PE together with adjoining neural cells and secondary lens cells. LE induces lentoids (L) and B cells induce neuroids. In each case epidermis surrounds the explants filled with ectomesenchyme (EM) and melanophores (M). Immunofluorescence reactions clarified the nature of lens cells. This discovery indicates that cells of adult tissues continue throughout life producing substances which are able to promote the appearance of cells of the same type during development. Probably they also serve to mediate interrelations between cells of these tissues, regulating the stability of their differentiation in the adult state, as well.

The Xvent-2 homeobox gene is part of the BMP-4 signalling pathway controlling [correction of controling] dorsoventral patterning of Xenopus mesoderm

We describe a novel Xenopus homeobox gene, Xvent-2, which together with the previously identified homeobox gene Xvent-1, defines a novel class of homeobox genes. vent genes are related by sequence homology, expression pattern and gain-of-function phenotype. Evidence is presented for a role of Xvent-2 in the BMP-4 pathway involved in dorsoventral patterning of mesoderm. (1) Xvent-2 is expressed in regions that also express BMP-4. (2) Xvent-2 and BMP-4 interact in a positive feedback loop. (3) Xvent-2 ventralizes dorsal mesoderm in a dose-dependent manner resulting in phenoytpes ranging from microcephaly to Bauchstuck pieces, as does BMP-4. (4) Like BMP-4 and gsc, Xvent-2 and gsc are able to interact in a crossregulatory loop to suppress each other. (5) Microinjection of Xvent-2 mRNA can rescue dorsalization by a dominant-negative BMP-4 receptor. The results suggest that Xvent-2 functions in the BMP-4 signalling pathway that antagonizes the Spemann organizer.