Epigenetic mechanisms in the early amphibian embryo: cell differentiation and morphogenetic elements

The terminology of early development: history, concepts, and current usage

The meaning of some terms used in describing the development of embryos is discussed in the light of their historical origin and current usage. Attention is focused on the terminology associated with the early development of animals, particularly that concerned with gastrulation and germ layer theory. It is suggested that terms linked with outdated concepts are not always appropriate for, and unless defined with care may limit, our appreciation of the significance of subsequent observation and experiment. Recommendations are made concerning the use of the terms mesoblast, mesoderm, and mesenchyme.

Ectoderm promotes differentiation of the presumptive ventral mesoderm in early gastrulae of Bufo arenarum

Cultures of both isolated and conjugated explants from early gastrulae of Bufo arenarum were prepared for a study of the development of ventral mesoderm. Only combinations including components of the deep ventral marginal zone and the animal pole successfully differentiated into blood cells (erythrocytes). Histological studies indicated that, while prospective mesodermal cells constituted the only source of such cells, prospective ectodermal cells provided the necessary stimulus for this kind of differentiation. Differentiated cultures, in which the tracer of cell-lineage fluorescein dextran amine was used to label these components, confirmed the above conclusions. These findings are discussed in the context of current concepts about the formation of mesoderm.

Factors involved in the formation and stabilization of cell aggregates obtained from amphibian embryonic explants

The effect of factors influencing the formation and stability of animal and vegetal aggregates from Xenopus laevis and Ambystoma mexicanum was examined in the light and scanning electron microscopes. At extreme values of pH the surface coat covering the vegetal aggregates is dissolved and dissociation may take place. Animal aggregates are more resistant. At high tonicities vegetal aggregates may be dissociated, and in the animal aggregates the epidermal differentiation is suppressed. In the absence of Ca2+ the vegetal aggregates are dissociated, but the animal aggregates are not affected. The results obtained with the inhibitor selenate and from incorporation experiments indicate that sulfated glycosaminoglycans are involved in the formation of aggregates in both species. Corresponding observations with tunicamycin suggest that even glycoproteins may play a role in aggregate formation, particularly in the vegetal aggregates.