Reexamination of the 'regulative development' of amphibian embryos

Establishment of spatial pattern

An overview and perspective are presented of mechanisms for the development of spatial pattern in animal embryos. It is intended both for new entrants to developmental biology and for specialists in other fields, with only a basic knowledge of animal life cycles being required. The first event of pattern formation is normally the localization of a cytoplasmic determinant in the egg, either during oogenesis or post-fertilization. Following cleavage to a multicellular stage, some cells contain the determinant and others do not. The determinant confers a specific developmental pathway on the cells that contain it, often making them the source of the first extracellular signal, or inducing factor. Inducing factors often form concentration gradients to which cells respond by up or downregulating genes at various concentration thresholds. This enables an initial situation consisting of two cell states (with or without the determinant) to generate a multistate pattern. Multiple rounds of gradient signaling, interspersed with phases of morphogenetic movements, can generate a complex pattern using a small number of signals and responding genes. Development proceeds in a hierarchical manner, with broad body subdivisions being specified initially, and becoming successively subdivided to give individual organs and tissues composed of multiple cell types in a characteristic arrangement. Double gradient models can account for embryonic regulation, whereby a similarly proportioned body pattern is formed following removal of material. Processes that are involved at the later stages include the formation of repeating structures by the combination of an oscillator with a gradient, and the formation of tissues with one cell type scattered in a background of another through a process called lateral inhibition. This set of processes make up a 'developmental toolkit' which can be deployed in various sequences and combinations to generate a very wide variety of structures and cell types.

Pre-MBT patterning of early gene regulation in Xenopus: the role of the cortical rotation and mesoderm induction

Patterning events that occur before the mid-blastula transition (MBT) and that organize the spatial pattern of gene expression in the animal hemisphere have been analyzed in Xenopus embryos. We present evidence that genes that play a role in dorsoventral specification display different modes of activation. Using early blastomere explants (16-128-cell stage) cultured until gastrula stages, we demonstrate by RT-PCR analysis that the expression of goosecoid (gsc), wnt-8 and brachyury (bra) is dependent on mesoderm induction. In contrast, nodal-related 3 (nr3) and siamois (sia) are expressed in a manner that is independent of mesoderm induction, however their spatially correct activation does require cortical rotation. The pattern of sia and nr3 expression reveals that the animal half of the 16-cell embryo is already distinctly polarized along the dorsoventral axis as a result of rearrangement of the egg structure during cortical rotation. Similar to the antagonistic activity between the ventral and the dorsal mesoderm, the ventral animal blastomeres can attenuate the expression of nr3 and sia in dorsal animal blastomeres. Our data suggest that no Nieuwkoop center activity at the blastula stage is required for the activation of nr3 and sia in vivo.

Microtubules and specification of the dorsoventral axis in frog embryos

The body plan of the frog is set-up by a rearrangement of the egg cytoplasm shortly after fertilization. Microtubules play several roles in this critical developmental event.