Seminars in Developmental Biology最新文献

Gastrulation is defined as the phase of morphogenetic movements lying between the blastula stage and the phylotypic (zootype) stage of development. The occurrence of gastrulation is universal among animals undergoing embryonic development from eggs, but there is considerable diversity in the types of movement observed. Gastrulation is also the stage at which the first very important developmental commitments are established, in particular those corresponding to the classical 'germ layers'. From an evolutionary point of view, animal development falls into three phases of which the first and last are variable and the middle, phylotypic, phase is most conservative. Gastrulation belongs to the early phase of development which is inherently variable because it is subject to selective forces operating on reproductive behaviour and life history, principally those controlling the number and size of eggs, and the means for embryonic nutrition.

The sea urchin embryo is a good model system for studying the role of mechanical and cell-cell interactions during epithelial invagination, cell rearrangement and mesenchymal patterning in the gastrula. The mechanisms underlying the initial invagination of the archenteron have been surprisingly elusive; several possible mechanisms are discussed. In contrast to its initial invagination, the cellular basis for the elongation of the archenteron is better understood: both autonomous epithelial cell rearrangement and further rearrangement driven by secondary mesenchyme cells appear to be involved. Experiments indicate that patterning of freely migrating primary mesenchyme cells and secondary mesenchyme cells residing in the tip of the archenteron relies to a large extent on information resident in the ectoderm. Interactions between cells in the early embryo and later cell-cell interactions are both required for the establishment of ectodermal pattern information. Surprisingly, in the case of the oral ectoderm the fixation of pattern information does not occur until immediately prior to gastrulation.

The three germ layers in Drosophila are established by both the invagination of the ventral furrow, which internalizes the anterior midgut and mesoderm primordia, and the invagination of the posterior midgut primordium. The invaginations of these primordia occur by similar cell shape changes. The gene hierarchies responsible for positioning each primordium within the epithelial blastoderm are well understood. By going further down in the hierarchy, we hope to identify the genes whose products are directly involved in the mechanisms that change the cell shape. Presumably these mechanisms are similar in Drosophila and in other organisms.

The mechanisms controlling cell movements during vertebrate gastrulation are not known. Studies using the zebrafish embryo show promise at identifying these mechanisms, combining an embryo that is accessible and optically clear with mutations that affect early development. In this article we describe the movements of cells during the midblastula, early epiboly and gastrulation stages of the zebrafish, correlating 'domains of movement' with embryonic morphology. We suggest that these domains of movement may parallel the 'zones of movement' of Xenopus.

Mesoderm migration across the inner surface of the outer embryonic layer is an essential morphogenetic mechanism in vertebrate gastrulation. Conserved traits of this process are (1) cadherin-dependent cohesion of the mesoderm, and (2) a predominant role for fibronectin in mediating mesoderm cell-substrate interactions. Compared to lower vertebrates, differentiation of the outer substrate-forming cell layer is accelerated in amniotes, providing mesoderm cells with a basement membrane substrate instead of a loose network of extracellular matrix fibrils. Guidance cues which determine the direction of mesoderm migration have been demonstrated in the fibrillar matrix of the amphibian gastrula.

In this article, we describe some of the morphogenetic movements reshaping the Xenopus laevis embryo during gastrulation. We have learned a great deal about these movements in recent years through advances made in explant culture techniques. Here, we will focus on involution, the process by which mesoderm is internalized and placed in between ectoderm and endoderm. Our aim is to present our current view of how involution takes place in the dorsal involuting marginal zone of the Xenopus embryos.

Gastrulation in Caenorhabditis elegans has been described by following the movements of individual nuclei in living embryos by Nomarski microscopy. Gastrulation starts in the 26-cell stage when the two gut precursors, Ea and Ep, move into the blastocoele. The migration of Ea and Ep does not depend on interactions with specific neighboring cells and appears to rely on the earlier fate specification of the E lineage. In particular, the long cell cycle length of Ea and Ep appears important for gastrulation. Later in embryogenesis, the precursors to the germline, muscle and pharynx join the E descendants in the interior. As in other organisms, the movement of gastrulation permit novel cell contacts that are important for the specification of certain cell fates.