Blastula

Abstract: The role of the nucleus in embryonic differentiation has been the subject of investigations dating back to the beginnings of experimental embryology. At first it was supposed by Roux, Weismann and others that differentiation is the result of qualitative nuclear divisions, different blastomeres thereby receiving the different kinds of nuclei which determine their subsequent differentiation. Later on this theory was disproved by numerous experiments showing that, during early cleavage at least, the distribution of the nuclei can be changed at will without altering the pattern of development. The cleavage nuclei have, therefore, been regarded as identical, and differentiation has been ascribed primarily to the well-known localizations in the egg cytoplasm. This evidence, it should be emphasized, relates only to the early phases of development. During this time it is definitely true that the nuclei in the various blastomeres are equivalent. However, whether they remain equivalent or become differentiated as the various parts of the embryo differentiate has never been tested. The possibility that nuclei might differentiate in response to regional differences in the cytoplasm, and that such nuclear changes might have reciprocal effects on the cytoplasm during cell differentiation, was suggested by Morgan.1 More recently Schultz2– 4 has discussed the problem more fully, indicating the known cytogenetical mechanisms that could account for nuclear differentiation, and Weisz5 has reviewed it in relation to ciliate morphogenesis. Obviously this problem can be solved only by the development of a method for testing directly whether nuclei of differentiating embryonic cells are or are not themselves differentiated. This sort of test could be obtained, as suggested to us several years ago by Schultz, if it were possible to transplant nuclei. Ideally, this type of experiment should be carried out by transplanting the nucleus …

Abstract: The organizer is formed in an equatorial sector of the blastula stage amphibian embryo by cells that have responded to two maternal agents: a general mesoendoderm inducer (involving the TFG-beta signaling pathway) and a dorsal modifier (probably involving the Wnt signaling pathway). The meso-endoderm inducer is secreted by most vegetal cells, those containing maternal materials that had been localized in the vegetal hemisphere of the oocyte during oogenesis. As a consequence of the inducer's distribution and action, the competence domains of prospective ectoderm, mesoderm, and endoderm are established in an animal-to-vegetal order in the blastula. The dorsal modifier signal is secreted by a sector of cells of the animal and vegetal hemispheres on one side of the blastula. These cells contain maternal materials transported there in the first cell cycle from the vegetal pole of the egg along microtubules aligned by cortical rotation. The Nieuwkoop center is the region of blastula cells secreting both maternal signals, and hence specifying the organizer in an equatorial sector. Final steps of organizer formation at the late blastula or early gastrula stage may involve locally secreted zygotic signals as well. At the gastrula stage, the organizer secretes a variety of zygotic proteins that act as antagonists to various members of the BMP and Wnt families of ligands, which are secreted by cells of the competence domains surrounding the organizer. BMPs and Wnts favor ventral development, and cells near the organizer are protected from these agents by the organizer's inducers. The nearby cells are derepressed in their inherent capacity for dorsal development, which is apparent in the neural induction of the ectoderm, dorsalization of the mesoderm, and anteriorization of the endoderm. The organizer also engages in extensive specialized morphogenesis, which brings it within range of responsive cell groups. It also self-differentiates to a variety of axial tissues of the body.

Abstract: We have analyzed lineages of cells labeled by intracellular injection of tracer dye during early zebrafish development to learn when cells become allocated to particular fates during development, and how the fate map is organized. The earliest lineage restriction was described previously, and segregates the yolk cell from the blastoderm in the midblastula. After one or two more cell divisions, the lineages of epithelial enveloping layer (EVL) cells become restricted to generate exclusively periderm. Following an additional division in the late blastula, deep layer (DEL) cells generate clones that are restricted to single deep embryonic tissues. The appearance of both the EVL and DEL restrictions could be causally linked to blastoderm morphogenesis during epiboly. A fate map emerges as the DEL cell lineages become restricted in the late blastula. It is similar in organization to that of an amphibian embryo. DEL cells located near the animal pole of the early gastrula give rise to ectodermal fates (including the definitive epidermis). Cells located near the blastoderm margin give rise to mesodermal and endodermal fates. Dorsal cells in the gastrula form dorsal and anterior structures in the embryo, and ventral cells in the gastrula form dorsal, ventral and posterior structures. The exact locations of progenitors of single cell types and of local regions of the embryo cannot be mapped at the stages we examined, because of variable cell rearrangements during gastrulation.