In Xenopus, the mesodermal precursors exist mostly in the deep layer of cells, while the ectoderm and endoderm arise from the superficial layer on the surface of the embryo. Fate mapping by Løvtrup (1975 Landstrom and Løvtrup 1979) and by Keller (1975, 1976) has shown that cells of the Xenopus blastula have different fates depending on whether they are located in the deep or the superficial layers of the embryo ( Figure 10.5). The movements whereby this is accomplished can be visualized by the technique of vital dye staining (see Chapter 1). 1994a, b), resulting in the obliteration of the blastocoel ( Figure 10.4).Īmphibian blastulae are faced with the same tasks as the invertebrate blastulae we followed in Chapters 8 and 9-namely, to bring inside the embryo those areas destined to form the endodermal organs, to surround the embryo with cells capable of forming the ectoderm, and to place the mesodermal cells in the proper positions between them. If this message is destroyed (by injecting antisense oligonucleotides complementary to this mRNA into the oocyte), the EP-cadherin is not made, and the adhesion between the blastomeres is dramatically reduced ( Heasman et al. 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.
#Depending on the blastopore fate of its members skin
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.
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