The mRNA for this protein is supplied in the oocyte cytoplasm. One of the most important of these molecules is EP-cadherin. While these cells are dividing, numerous cell adhesion molecules keep the blastomeres together. Thus, the blastocoel appears to prevent the contact of the vegetal cells destined to become endoderm with those cells fated to give rise to the skin and nerves. Because mesodermal tissue is normally formed from those animal cells that are adjacent to the vegetal endoderm precursors, it seems plausible that the vegetal cells influence adjacent cells to differentiate into mesodermal tissues. When Nieuwkoop (1973) took embryonic newt cells from the roof of the blastocoel, in the animal hemisphere, and placed them next to the yolky vegetal cells from the base of the blastocoel, these animal cells differentiated into mesodermal tissue instead of ectoderm. The blastocoel probably serves two major functions in frog embryos: (1) it permits cell migration during gastrulation, and (2) it prevents the cells beneath it from interacting prematurely with the cells above it. (B) 8-cell embryo showing a small blastocoel (arrow) at the junction of the three cleavagefurrows. (A) First cleavage furrow, showing a small cleft, which later develops into the blastocoel. As cleavage progresses, the animal region becomes packed with numerous small cells, while the vegetal region contains only a relatively small number of large, yolk-laden macromeres.įormation of the blastocoel in a frog egg. This unequal holoblastic cleavage establishes two major embryonic regions: a rapidly dividing region of micromeres near the animal pole and a more slowly dividing vegetal macromere area ( Figure 10.2C Figure 2.2E). It divides the frog embryo into four small animal blastomeres (micromeres) and four large blastomeres (macromeres) in the vegetal region. However, because of the vegetally placed yolk, this cleavage furrow in amphibian eggs is not actually at the equator, but is displaced toward the animal pole. The third cleavage, as expected, is equatorial. This cleavage is at right angles to the first one and is also meridional. Figure 10.2B shows that while the first cleavage furrow is still cleaving the yolky cytoplasm of the vegetal hemisphere, the second cleavage has already started near the animal pole. One can see the difference in the furrow between the animal and the vegetal hemispheres. (A, B) Because the vegetal yolk impedes cleavage, the second division begins in the animal region of the egg before the first division has divided (more.)įigure 10.2A is a scanning electron micrograph showing the first cleavage in a frog egg. Cleavage furrows, designated by Roman numerals, are numbered in order of appearance. A role for the dorsal lip of the blastopore as the organizer is discussed in relation to the origin of the notochord.Cleavage of a frog egg. These observations suggest organizing or guiding roles for the notochord in the formation of germ layers. From a comparison of the relative locations of tissues in embryos at different stages of development, it was shown that the notochord elongates by a remodeling of the mass of the primitive notochord, and that, as the anteriorly directed translocation of the neural area and the invagination of endoderm occur, these processes keep pace with the elongation of the notochord. Mesodermal cells other than notochord cells were mesenchymal until the neurula stage, when primitive somites appeared on both sides of the notochord. The primitive notochord was also linked to endoderm at its right and left margins, facing the archenteron. Only after the formation of the yolk plug, a narrow strip of primitive notochord, which consisted of columnar cells, established a close contact with the central part of the overlaying presumptive neural plate. In gastrulae, the inner postinvolution layer was not in direct contact with the outer preinvolution layer as a result of the presence of an intervening layer of cells. Three-dimensional relationships between tissues during the formation of germ layers were studied in sections of normally developing embryos of the newt, Cynops pyrrhogaster.
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