Neural inducing factors in neuroblastoma and retinoblastoma cell lines. Extraction with acid ethanol

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis

Mammalian embryonic stem cells can be obtained from the inner cell mass of blastocysts or from primordial germ cells. These stem cells are pluripotent and can develop into all three germ cell layers of the embryo. Somatic mammalian stem cells, derived from adult or fetal tissues, are more restricted in their developmental potency. Amphibian ectodermal and endodermal cells lose their pluripotency at the early gastrula stage. The dorsal mesoderm of the marginal zone is determined before the mid-blastula transition by factors located after cortical rotation in the marginal zone, without induction by the endoderm. Secreted maternal factors (BMP, FGF and activins), maternal receptors and maternal nuclear factors (beta-catenin, Smad and Fast proteins), which form multiprotein transcriptional complexes, act together to initiate pattern formation. Following mid-blastula transition in Xenopus laevis (Daudin) embryos, secreted nodal-related (Xnr) factors become important for endoderm and mesoderm differentiation to maintain and enhance mesoderm induction. Endoderm can be induced by high concentrations of activin (vegetalizing factor) or nodal-related factors, especially Xnr5 and Xnr6, which depend on Wnt/beta-catenin signaling and on VegT, a vegetal maternal transcription factor. Together, these and other factors regulate the equilibrium between endoderm and mesoderm development. Many genes are activated and/or repressed by more than one signaling pathway and by regulatory loops to refine the tuning of gene expression. The nodal related factors, BMP, activins and Vg1 belong to the TGF-beta superfamily. The homeogenetic neural induction by the neural plate probably reinforces neural induction and differentiation. Medical and ethical problems of future stem cell therapy are briefly discussed.

Neural induction in embryos

Neural differentiation of the ectoderm is inhibited by bone morphogenetic protein 4 (BMP-4) in amphibia as well as mammalia. This inhibition is released by neural inducing factor(s), which are secreted from the dorsal mesoderm. Masked neuralizing factor(s) are already present in the ectoderm before induction. In homogenates from Xenopus oocytes and embryos neural inducing factors were found in the supernatant (centrifuged at 105000 g), in small vesicles and a ribonucleoprotein fraction. A neuralizing factor, which is a protein of small size, has been partially purified from Xenopus gastrulae. Genes that are expressed in the dorsal mesoderm and involved in the de novo synthesis of neuralizing factor(s) have been cloned. The differentiation of cells with a neuronal fate starts in the neural plate immediately after neural induction. Genes homologous to the Notch and Delta genes of lateral inhibition in insects are involved in this process.

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.