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PubMed2001

Vertebrate cranial placodes I. Embryonic induction.

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

Animals↗
PubMed1997

The origins of the neural crest. Part I: embryonic induction.

Neural crest cells form at the border between the neural plate and the epidermis. The tissue interactions that underlie neural crest cell induction have been investigated primarily by heterotopic grafting experiments in vivo and by conjugating different tissues in vitro. Three models have been proposed to explain the induction of neural crest cells at the neural plate border, i.e. (1) the influence of signals from the mesoderm, (2) changes in ectodermal competence and (3) local interactions between neural and non-neural ectoderm. The weight of the evidence supports the last model, although there are data that suggest a role for signals from the mesoderm. FGFs seem to be necessary but not sufficient for neural crest cell induction. BMP-4 is sufficient to induce neural crest cells from chick neural explants in vitro and intermediate levels of BMP-4-signalling induce neural crest cell markers in Xenopus animal cap assays. These data suggest a gradient model in which neural crest cells are induced by a particular range of BMP-4 activity, although a single-signal model may be too simplistic. Neural crest cell induction may be an ongoing process, in which an initial induction at the neural plate border is followed by further induction within the dorsal neural tube.

Animals↗
PubMed1985

Peanut lectin receptors in the early amphibian embryo: regional markers for the study of embryonic induction.

The regional and temporal specificity of peanut agglutinin binding was determined for early amphibian embryos. With the onset of neurulation, a receptor appears on the epidermis, but remains absent from the neural plate. A second type of receptor, largely masked by sialic acid, appears throughout the extracellular matrix. In the axolotl, the epidermal receptor is epimucin and the matrix receptor is fibronectin plus other components. Both receptors are autonomously expressed, on schedule, by appropriate explants of gastrula tissue. Expression of the epidermal receptor is suppressed after exposure to a neural inducing signal. This shows that the epidermal PNA receptor is a reliable marker of epidermal character and that neural induction affects the program of macromolecular synthesis within hours of the graft.

Ambystoma↗
PubMed1985

Neural induction: embryonic determination elicits full expression of specific neuronal traits.

In Pleurodeles waltl, the early neuronal differentiation of precursor cells from late gastrula stage has been studied by culture in vitro from either isolated neural plate (NP) or isolated neural fold (NF). The aim of this study was to delineate the information acquired by ectodermal target cells during neural induction. By culturing these cells in vitro either with or without the underlying chordamesoderm, we showed that in the absence of chordamesodermal influence such NP or NF cells exhibited a high degree of biochemical and morphological differentiation as revealed by the synthesis and the storage of neurotransmitters, the activity of specific enzymes, as well as by the expression of neuronal markers: specific changes in cell surface carbohydrates, tetanus toxin binding sites and neurofilament polypeptides. Remarkable changes in the cell adhesive properties were the first events observed in the different central (NP) and peripheral (NF) types. In cocultures the chordamesodermal cells exert a beneficial influence on this differentiation, specially increasing acetylcholine synthesis. There are some differences between central (NP) or peripheral (NF) neuroblast response to this further notochord or mesodermal influence.

Acetylcholine↗
PubMed1996

[Embryonic induction and role of activins during early amphibian development].

Growth factors are known to act for the formation of the animal tissues and organs. We showed that activins, members of the TGF-beta family growth factors, exist in early amphibian embryos and induce mesoderm tissues and organs in undifferentiated blastomeres (animal cap). Activins are characteristic in that they can establish embryonic body axis, by inducing different mesoderm and endoderm organs depending on their dose. The action of activin seems to be a primary important phenomenon which commonly create the body pattern of vertebrate embryos.

Activins↗
PubMed1975

Embryonic induction.

Inductive interactions between tissue components in proximity constitute a universal guiding principle for synchronized development during embryogenesis. Such sequential morphogenetic events involve both specific, determinative "instructions" and less specific, supporting or "permissive" influences acting upon predetermined target cells. Transmission of these intercellular messages may be mediated by diffusible signal substances, by morphogenetically active interfacial materials,or via actual cell contacts. Inductive interactions can be upset experimentally by exposure to various exogenous agents known to be potential teratogens, and several mutant strains of animals are known in which a genetic defect is manifested as a malformation through a defective interactive process. Hence, such inductive interactions should be considered likely targets for both genetic and exogenous factors in teratogenesis.

Abnormalities, Drug-Induced↗
PubMedDate not supplied

Embryonic induction: is the Nieuwkoop centre a useful concept?

Regionalisation of the amphibian embryo is classically thought to involve induction by the Spemann organiser, itself induced by the Nieuwkoop centre. This model has now been extended to teleosts, with the identification of a gene that appears to define the zebrafish equivalent of the Nieuwkoop centre.

Animals↗
PubMed1996

Early embryonic induction in C. elegans can be inhibited with polysulfated hydrocarbon dyes.

During embryogenesis of Caenorhabditis elegans cellular interactions are necessary to determine the fate of blastomeres. In one of these interactions, taking place in the 4-cell stage, the germline cell P2 induces longitudinal orientation of the cleavage spindle in the neighboring EMS cell, its asymmetric division, and the establishment of a gut lineage. Application of several polysulfated hydrocarbon dyes (e.g., trypan blue, TB) in the 1- to 4-cell stages inhibits induction of the gut precursor cell. However, dye application from the late 4-cell stage onward does not interfere with gut induction, supporting the earlier finding of a short time window for this interaction. We also tested the effect of TB on the induction of pharyngeal muscle cells by the MS blastomere, which appears to involve a surface receptor-ligand interaction. We found that this process is inhibited as well. These and additional data indicate that polysulfated hydrocarbon dyes are suitable tools to generally interfere with cell-cell interactions in the nematode embryo.

Animals↗
PubMed1985

Information transfer during embryonic induction in amphibians.

Neural induction and differentiation has been studied using Concanavalin A, cyclic AMP, tunicamycin and calcium ionophore A 23187. Competent ectoderm of Xenopus laevis treated with Concanavalin A differentiates into neural (archencephalic) structures. Binding studies with gold-labelled ConA indicate that the superficial ectodermal layer contains fewer ConA-sensitive sites (alpha-D-mannoside and alpha-D-glucoside residues of glycoproteins) than the inner ectodermal layer. The small number of ConA-sensitive sites can be correlated with the fact that the isolated superficial ectoderm layer, in contrast to the inner layer, does not differentiate into neural structures. The gold-ConA particles bound to inner ectoderm are quickly (within 30 minutes) internalized, presumably by receptor-mediated endocytosis. However, endocytosis is not a prerequisite for neural induction. On the contrary ConA apparently must be bound to the plasma membrane for a certain period to initiate neural induction. The rapid internalization of ConA could explain why neural inductions are evoked only if ectoderm is incubated in ConA-containing medium for longer than 30 minutes. On the other hand cyclic AMP or calcium ionophore A 23187 does not elicit neural inductions. On the contrary calcium ionophore A 23187 apparently inhibits neural and mesodermal differentiation. This effect could be correlated with an increase of intracellular calcium level of the ectodermal target cells, which could influence the permeability of gap junctions resulting in a loss of cell communication, followed by a change of differentiation and pattern formation.

Amphibians↗