Soon after this, the coordinated movements of gastrulation begin. Na + is pumped across this sheet into the spaces in the (more.) In the outermost regions of the embryo, tight junctions between the blastomeres begin to create an epithelial sheet that isolates the interior of the embryo from the external medium. This event is called the mid- blastula transition. After about 12 cycles of cleavage (7 hours), the cell division rate slows down, the cell cycles begin to follow the standard pattern with G 1 and G 2 phases intervening between the S and M phases, and transcription of the embryo's genome begins. The very high rate of DNA replication and mitosis seems to preclude gene transcription (although protein synthesis occurs), and the cleaving embryo is almost entirely dependent on reserves of RNA, protein, membrane, and other materials that accumulated in the egg while it developed as an oocyte in the mother. These first cell divisions in Xenopus have a cycle time of about 30 minutes, with a direct alternation of S and M phases, as discussed in Chapter 17. The endoderm derives from the most vegetal blastomeres, the ectoderm from the most animal, and the mesoderm from a middle set that (more.) The origins of the three germ layers can be traced back to distinct blastomeres of the embryo in its early cleavage stages. These cells will generate further signals to organize the dorsoventral axis of the body. The subcellular region in which Dishevelled thus becomes concentrated gives rise to cells that behave as though they have received a Wnt signal and express a dorsal-specific set of genes as a result. This leads to a microtubule-based transport of the protein Dishevelled, a downstream component of the Wnt signaling pathway, toward the future dorsal side (see Figure 21-66B). The direction of the cortical rotation is biased according to the point of sperm entry, perhaps through the centrosome that the sperm brings into the egg, and the movement is associated with a reorganization of microtubules in the egg cytoplasm. Treatments that block the rotation allow cleavage to occur normally but produce an embryo with a central gut and no dorsal structures or dorsoventral asymmetry. Following entry of the sperm, the outer, actin-rich cortex of the egg cytoplasm rotates relative to the central core of the egg, so that the animal pole of the cortex is slightly shifted to the future ventral side. Fertilization triggers an intracellular movement that gives the egg an additional asymmetry defining a dorsoventral difference. The animal-vegetal asymmetry of the unfertilized egg is sufficient to define only one of these future body axes-the anteroposterior. In the process, the three principal axes of the body are established: anteroposterior, from head to tail dorsoventral, from back to belly and mediolateral, from the midline outward to the left or to the right. (B) The asymmetric distribution of molecules inside the egg, and how this changes following fertilization so as to define a dorsoventral as well as an (more.)įertilization initiates a complex series of movements that will eventually tuck the vegetal cells and cells from the equatorial (middle) region of the animal-vegetal axis into the interior. (A) Side view of an egg photographed just before fertilization. As a result, the cells that inherit vegetal cytoplasm will produce signals to organize the behavior of adjacent cells and are committed to form the gut-the innermost tissue of the body the cells that inherit animal cytoplasm will form the outer tissues. Near the vegetal pole, for example, there is an accumulation of mRNAs coding for the gene regulatory protein VegT (a DNA-binding protein of the T-box family) and for signal proteins of the TGFβ superfamily, as well as some ready-made protein components of the Wnt signaling pathway ( Figure 21-66B). The animal and vegetal hemispheres contain different selections of mRNA molecules and other cell components, which become allocated to separate cells as the egg cell divides after fertilization. The light-colored lower end of the egg is called the vegetal pole the dark-colored upper end is called the animal pole. The Xenopus egg is a large cell, just over a millimeter in diameter ( Figure 21-66A). The Polarity of the Amphibian Embryo Depends on the Polarity of the Egg
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