![]() The first cell movement of fish gastrulation is the epiboly of the blastoderm cells over the yolk. ![]() At this time, cells in specific regions of the embryo give rise to certain tissues in a highly predictable manner, allowing a fate map to be made ( Figure 11.2C see also Figure 1.6 Kimmel et al 1990). The fate of the blastoderm cells appears to be fixed shortly before gastrulation begins. Moreover, any one of these cells can give rise to an unpredictable variety of tissue descendants ( Kimmel and Warga 1987 Helde et al. The fates of the early blastoderm cells are not determined, and cell lineage studies (in which a nondiffusible fluorescent dye is injected into one of the cells so that the descendants of that cell can be followed) show that there is much cell mixing during cleavage. These are the cells that give rise to the embryo proper. Microtubules extend through the yolky cytoplasm and through the external region of the (more.)īetween the EVL and the YSL are the deep cells. The animal surface of the yolk cell is flat and contains the nuclei of the YSL. (A) Prior to gastrulation, the deep cells are surrounded by the EVL. The EVL eventually becomes the periderm, an extraembryonic protective covering that is sloughed off during later development.įish blastula. It is made up of the most superficial cells of the blastoderm, which form an epithelial sheet a single cell layer thick. The second cell population distinguished at the midblastula transition is the enveloping layer ( EVL Figure 11.2A). The YSL will be important for directing some of the cell movements of gastrulation. Later, as the blastoderm expands vegetally to surround the yolk cell, some of the yolk syncytial nuclei will move under the blastoderm to form the internal YSL, and some of the nuclei will move vegetally, staying ahead of the blastoderm margin, to form external YSL ( Figure 11.2A, B). This fusion produces a ring of nuclei within the part of the yolk cell cytoplasm that sits just beneath the blastoderm. The YSL is formed at the ninth or tenth cell cycle, when the cells at the vegetal edge of the blastoderm fuse with the underlying yolk cell. The first of these is the yolk syncytial layer ( YSL). At this time, three distinct cell populations can be distinguished. (F) 64-cell embryo, (more.)īeginning at about the tenth cell division, the onset of the midblastula transition can be detected: zygotic gene transcription begins, cell divisions slow, and cell movement becomes evident ( Kane and Kimmel 1993). (D) 8-cell embryo, wherein two rows of four cells are formed. ![]() The mound atop the cytoplasm is the blastodisc region. Initially, all the cells maintain some open connection with one another and with the underlying yolk cell so that moderately sized (17-kDa) molecules can pass freely from one blastomere to the next ( Kimmel and Law 1985).ĭiscoidal cleavage in a zebrafish egg. The first 12 divisions occur synchronously, forming a mound of cells that sits at the animal pole of a large yolk cell. These divisions are rapid, taking about 15 minutes each. Early cleavage divisions follow a highly reproducible pattern of meridional and equatorial cleavages. This converts the spherical egg into a more pear-shaped structure, with an apical blastodisc ( Leung et al. The calcium waves initiated at fertilization stimulate the contraction of the actin cytoskeleton to squeeze non-yolky cytoplasm into the animal pole of the egg. Scanning electron micrographs show beautifully the incomplete nature of discoidal meroblastic cleavage in fish eggs ( Figure 11.1). Since only the cytoplasm of the blastodisc becomes the embryo, this type of meroblastic cleavage is called discoidal. The cell divisions do not completely divide the egg, so this type of cleavage is called meroblastic (Greek, meros, “part”). In fish eggs, cleavage occurs only in the blastodisc, a thin region of yolk-free cytoplasm at the animal cap of the egg.
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