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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