Mesoderm and Neural Inductions on Newt Ectoderm by Activin A. (mesoderm induction/newt embryo/activin/EDF)

Patterns of gene expression in the core of Spemann's organizer and activin-treated ectoderm in Cynops pyrrhogaster

The presumptive pharyngeal endoderm region of the Cynops early gastrula induces head or trunk-tail structures in sandwich culture. Activin-treated ectoderm can mimic this phenomenon at least at the histological level. The patterns of expression of organizer-specific genes were examined to compare these two inductive materials at the molecular level. A chordin cDNA clone from Cynops pyrrhogaster (Cychd) was isolated by reverse transcription-polymerase chain reaction (RT-PCR). Cychd mRNA was first detected in the presumptive pharyngeal endoderm and prechordal plate regions of stage 11 embryos, and was expressed continuously until stage 20. The spatiotemporal expression pattern of Cychd was similar to that of Xenopus chordin. The patterns of expression of organizer-related genes in the pharyngeal endoderm and activin-treated ectoderm were compared by RT-PCR analysis. Expression of Cychd in these two materials peaked at the time when they can induce head structures in sandwich culture. Expression of fork head and goosecoid did not change in the presumptive pharyngeal endoderm over this period. Cychd may play a key role in head formation in the Cynops embryo.

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.

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.

Control of the embryonic body plan by activin during amphibian development

Embryonic induction plays an important role in establishing the fundamental body plan during early amphibian development. The factors mediating this embryonic induction have, however, only recently been discovered. In the mid-1980's, certain peptide growth factors belonging to the FGF and TGF-beta families were found to have a mesoderm-inducing effect on isolated Xenopus blastula ectoderm. The study of embryonic induction subsequently expanded rapidly and knowledge at the molecular level has gradually accumulated. One of these peptide growth factors, activin, a member of the TGF-beta superfamily, is present maternally in the Xenopus early embryo and induces various mesodermal and endodermal tissues in isolated presumptive ectoderm. After exposure of presumptive ectoderm to activin, many genes are expressed in the same manner as in normal embryogenesis. Ectoderm treated with activin can induce a complete secondary embryo, the same as the organizer does in transplantation experiments. These findings suggest that activin is one of the first induction signals responsible for establishing the embryonic body plan in early amphibian development. In this article we shall review to what extent we can control the embryonic body plan in vitro, referring to some significant findings in this field.

Expression of activin subunits, activin receptors and follistatin in postimplantation mouse embryos suggests specific developmental functions for different activins

Using in situ hybridization we have studied the localization of the messenger RNAs encoding the inhibin/activin subunits (alpha, beta A, beta B), the activin-binding protein follistatin and activin receptors (IIA, IIB) in mouse embryos during postimplantation development. From 6.5- to 9.5-days post coitum (p.c.) activin beta A and beta B subunit expression was restricted to the decidua, while activin receptor type IIB messages were exclusively detected in the embryo. Expression of activin receptor type IIA was apparent in the embryo as early as 9.5 days p.c. In contrast, follistatin transcripts were present in both the decidua and the embryo at the early postimplantation stages. In particular, the primitive streak region, specific rhombomeres in the developing hindbrain, somites, paraxial mesoderm and parietal endoderm cells attached to the Reichert's membrane showed strong expression of follistatin. In 10.5- and 12.5-day embryos expression of the beta A subunit message was abundant in mesenchymal tissue, in particular in the developing face, the body wall, the heart, precartilage condensations in the limb and in the mesenchyme of structures that show both epithelial and mesenchymal components, including tissues of the embryonic digestive, respiratory and genital tracts. The distribution of beta B transcripts was quite different from that observed for beta A. beta B is strongly expressed in selected regions of the brain, in particular the fore- and hindbrain, and in the spinal cord. Specific hybridization signals were also present in the epithelium of the stomach and oesophagus. Common sites of beta A and beta B expression are blood vessels, intervertebral disc anlagen, mesenchymal condensations in the flank region and the gonad primordium. The latter organ is the only site in the embryo where the alpha subunit is expressed, and thus where inhibit activity may be present. During the period of organogenesis the sites of expression of activin receptors type IIA and IIB messenger RNA (mRNA) generally coincide with or are adjacent to the sites of beta subunit expression. Differences in the expression patterns of the receptor RNAs are the whisker follicles, where type IIA is expressed, and the metanephros and the forebrain where type IIB transcripts are present. Taken together, the present data suggest that follistatin, but not one of the known activin forms (A,B,AB) is involved in early postimplantation development.(ABSTRACT TRUNCATED AT 400 WORDS)