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PubMed1978

Microheterogeneities, non-equivalance, and embryonic induction.

The thrust of this report is to stress the importance of microheterogeneities in the microenvironment of differentiating tissues as a possible inducer or regulator of differentiation. During chondrogenesis both qualitative and quantitative changes occur in the proteoglycan population. Using molecular sieve chromatography, these changes can be characterized and used as indices of differentiation. Microheterogeneities of the extracellular matrix may be an example of "non-equivalence" as a regulatory device for differentiation.

Animals↗
PubMed1985

Embryonic induction and cation concentrations in amphibian embryos.

Explanted ectoderm from early gastrulae of Triturus alpestris was treated with the Na-K ionophore gramicidin (10(-9) to 10(-5) M) and the Ca-ionophore A 23187 (10(-7) to 10(-5) M). The ectoderm developed almost exclusively to atypical epidermis as in the control explants. When the ectoderm was treated with ouabain (10(-4) M), intracellular Na+ increased about 4.4-fold and K+ was reduced by half. Mesenchyme cells in small number differentiated in about 40% of the ouabain-treated explants. The time course of total Na+ and K+ ion concentrations was measured over a period of 72 h in ectoderm of T. alpestris after induction with vegetalizing factor and in control explants. In the first 15 h after explantation, no significant differences between control and induced explants were found. Thereafter, the steady state concentration of K+ decreased in the induced explants, whereas the steady-state concentration of Na+ slightly increased. The membrane resting potential recorded intracellularly of ectoderm sandwiches from early gastrula stages was found to be -41.3 mV in control and -59.3 mV in induced explants. From the specific conductances and permeabilities of non-induced and induced cells it is concluded that the induction process leads to a differentiation of the cell membrane, which acquires the characteristics of ionic selectivity. Ectoderm from Ambystoma mexicanum forms neural or neuroid tissue, mesenchyme and melanophores after explantation in salt solution in up to 50% of the explants without any additions. Isolated Ambystoma ectoderm is therefore not suitable for test experiments.

Ambystoma↗
PubMed1992

The role of growth factors in embryonic induction in Xenopus laevis.

Establishment of the body pattern in all animals, and especially in vertebrate embryos, depends on cell interactions. During the cleavage and blastula stages in amphibians, signal(s) from the vegetal region induce the equatorial region to become mesoderm. Two types of peptide growth factors have been shown by explant culture experiments to be active in mesoderm induction. First, there are several isoforms of fibroblast growth factor (FGF), including aFGF, bFGF, and hst/kFGF. FGF induces ventral, but not the most dorsal, levels of mesodermal tissue; bFGF and its mRNA, and an FGF receptor and its mRNA, are present in the embryo. Thus, FGF probably has a role in mesoderm induction, but is unlikely to be the sole inducing agent in vivo. Second, members of the transforming growth factor-beta (TGF-beta) family. TGF-beta 2 and TGF-beta 3 are active in induction, but the most powerful inducing factors are the distant relatives of TGF-beta named activin A and activin B, which are capable of inducing all types of mesoderm. An important question relates to the establishment of polarity during the induction of mesoderm. While all regions of the animal hemisphere of frog embryos are competent to respond to activins by mesoderm differentiation, only explants that include cells close to the equator form structures with some organization along dorsoventral and anteroposterior axes. These observations suggest that cells in the blastula animal hemisphere are already polarized to some extent, although inducers are required to make this polarity explicit.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Expression of cell-adhesion molecules in embryonic induction. II. Morphogenesis of adult feathers.

The developmental appearance of cell-adhesion molecules (CAMs) was mapped during the morphogenesis of the adult chicken feather. Neural CAM (N-CAM), liver CAM (L-CAM), and neuron-glia CAM (Ng-CAM), as well as substrate molecules (laminin and fibronectin), were compared in newborn chicken skin by immunohistochemical means. N-CAM was found to be enriched in the dermal papilla, which was closely apposed to L-CAM-positive papillar ectoderm. The two CAMs were then co-expressed in cells of the collar epithelium. Subsequently generated barb epithelia expressed only L-CAM, but N-CAM reappeared periodically on cells between developing barbs and barbules. N-CAM first appeared on a single L-CAM-positive basilar cell located in each valley flanked by two adjacent barb ridges. Subsequently, the expression of N-CAM extended one cell after another to include the whole basilar layer. N-CAM also appeared in the L-CAM-positive axial-plate epithelia, beginning in a single cell located at the ridge base. The two collectives of N-CAM-positive epithelia constituting the marginal and axial plates then disintegrated, leaving interdigitating spaces between keratinized structures that had previously expressed L-CAM. The morphological transformation from an epithelial cylinder to a three-level branched feather pattern is thus achieved by coupling alternating CAM expression in linked cell collectives with specific differentiation events, such as keratinization. During all of these morphogenetic processes, laminin and fibronectin formed a continuous basement membrane separating pulp from feather epithelia, and were excluded from the sites involved in periodic appearances of N-CAM. The same staining pattern described for developing chickens persisted in the feather follicles of adult chicken tissue that have gone through several cycles of molting. Cyclic expression of the two different CAMs underlies each of the different morphological events that are generated epigenetically during feather morphogenesis.

Age Factors↗
PubMed1983

Transferrin as a fetal growth factor: acquisition of responsiveness related to embryonic induction.

Differentiation of the metanephric mesenchyme, which is triggered by an inductive tissue interaction, has been shown to proceed in a chemically defined medium containing transferrin. Here, we report that neither transferrin-depleted serum nor a chemically defined medium devoid of transferrin promote differentiation and that activity can be restored by the addition of transferrin. It thus appears that we have identified the serum factor required for kidney differentiation. Transferrin seems to affect differentiation by stimulating cell proliferation. We show by using an organ-culture model system that only mesenchymes induced to differentiate by the 24-hr tissue interaction respond to transferrin by proliferation and differentiation, whereas uninduced mesenchymes remain unresponsive. The inductor tissue used is not responsive to transferrin. Thus, the data suggest that the short-range cell-mediated tissue interaction acts by making the nephrogenic mesenchyme responsive to the long-range mediator, which is transferrin. Transferrin is suggested to be an important circulating growth factor required for proliferation during embryogenesis.

Animals↗
PubMed1984

Analysis of embryonic induction by using cell lineage markers.

Three distinct inductive interactions have been demonstrated in early embryos of Xenopus laevis: mesoderm induction, dorsalization and neural induction. The experiments were done with grafts from embryos uniformly labelled with passive cell lineage markers, either FITC-lysine-dextran (FLDx) or horseradish peroxidase (HRP), which allow the provenance of regions to be determined down to the single cell level. In each case the fate of the target tissue in the presence of the appropriate inductor was quite different from the fate in normal development.

Animals↗
PubMed1981

A rapid experimental method to study primary embryonic induction.

The technique described here is a combination of the sandwich-method and cell culture. It allows one to know quickly whether induction occurred or not (48 h if cell spreading is considered, 4-5 days if morphological differentiation is observed). The dissociation of the explants soon after induction does not disturb their inductive pattern. Moreover, this method allows a daily observation of the morphological events and study at the cellular and/or molecular level.

Animals↗