Basic fibroblast growth factor can induce exclusively neural tissue in Triturus ectoderm explants

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.

Mesoderm formation in Eleutherodactylus coqui: body patterning in a frog with a large egg

The direct developing frog, Eleutherodactylus coqui, develops from a large egg (diameter 3.5 mm). To investigate the effect of egg size on germ-layer formation, we studied mesoderm formation in E. coqui and compared it to that of Xenopus laevis (diameter 1.3 mm). First, we identified the position of prospective mesoderm in the 16-cell E. coqui embryo by cell-lineage tracing. Although the animal blastomeres are small, they form most of the blastocoel roof and make extensive contributions to some mesodermal tissues. Second, we performed recombinant analysis with X. laevis animal caps to define the distribution of mesoderm-inducing activity. Mesoderm-inducing activity in E. coqui was restricted around the marginal zone with strong activity in the superficial cells. Neither the vegetal pole nor the blastocoel floor had activity, although these same regions from X. laevis induced mesoderm. Third, we cloned Ecbra, a homologue of Xbra, an early mesoderm marker in X. laevis. Ecbra was expressed in the marginal ring close to the surface, similar to X. laevis, but E. coqui had weaker expression on the dorsal side. Our results suggest that mesoderm formation is shifted more animally and superficially in E. coqui compared to X. laevis.

Cell proliferation and protein synthesis as initial factors in determination of axial polarity

The rate of cell proliferation relative to that of protein synthesis appears to have an initial role in establishment of axial polarities in developing animal embryos. An increase in this ratio leads to anterior or dorsal differentiation, while reduction allows posterior or ventral differentiation in a number of organisms. The role that various growth factors play in the regulation of proliferation and protein synthesis is examined.

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.

Hierarchy of inductive events

Examination of data on the inducing capacity of mesoderm-inducing factors shows that none of them induces mesoderm alone, but that they also induce endoderm and neural tissues, while in the mesoderm itself, they induce tissues belonging to its main levels. Tests on the inducing capacity of living retina have shown that it does not induce mesoderm, but does induce a spectrum of tissues, including retina, pigmented epithelium, lens and a piece of brain. This situation seems to be indispensable because if it evokes only one differentiation, an increase of diversity in development would be impossible. Selectivity occurs only at the end of the sequence of inductions, as it does in the induction of lens tissue by lens epithelium. Such mechanisms, however, are insufficient for the development of different kinds of cells into homogeneous tissues. This is achieved through the aggregation or separation of initially induced cells, their own products determining (or not) their further fate and leading to homogeneity of tissues. These mechanisms of the first two levels of inductive interactions overlap with events that allow or prevent the access of inducing factors and are, therefore, also involved in the manifestation of competence of potentially reactive cells.

Fibroblast growth factor receptors and regeneration of the eye lens

If the eye lens of the adult newt, Notophthalmus viridescens, is removed, a new lens will regenerate and only from the dorsal, not the ventral, iris. The source, pigmented epithelial cells, would normally no longer divide, but upon lentectomy they do re-enter the cell cycle and form lens. The cause for this capability is unknown, but the mitogenic Fibroblast Growth Factors and their receptors may be involved. We have demonstrated that FGF receptors are present and operative in lens regeneration, since receptor-directed mitotoxins inhibit regeneration; heterogeneity and differential density in FGF-binding and receptor localization in iris sectors is also present. We propose that the spatial distribution of FGF receptors, especially the amphibian homolog of FGFR-3, is important in initiation of regeneration of eye lens.

Molecular mechanisms of tissue determination and pattern formation in amphibian embryos

Factors of the TGF-beta superfamily (activin, vegetalizing factor) and the FGF family determine endoderm and mesoderm. The dorsoventral polarity of the mesoderm depends on additional factors (BMP-4, Wnt-8, noggin). Activin can directly activate gene transcription by signal transduction. Mesoderm is determined by factors prelocalized in the marginal zone. Its differentiation depends also on the animal ectoderm. Neural inducing factors have been partially purified. A masked neuralizing factor in the ectoderm is activated by induction of the ectoderm to the nervous system. Phorbolester can evoke neuralization signaling.