Monday, August 19, 2019
Embryos :: essays research papers fc
                              Mechanisms of Epiboly of ectoderm in the Xenopus Laevis embryo Introduction      Epiboly is a movement of gastrulation in the amphibian embryo, whereby ectodermal precursors expand to cover the entire embryo. This process occurs in the surface and deep layer cells in the animal and marginal regions. Three rounds of cell division occur in the deep cells, while they also rearrange to form fewer layers. Superficial cells elongate by cell division while flattening, which gives them greater surface area and thinner depth. The ectoderm eventually covers the entire embryo, internalizing the endoderm. This process sets up the correct position for the three germ layers, with the ectoderm on the outside, mesoderm in the middle and endoderm on the inside. Keller (1980) found that superficial cells spread, divide, and undergo rearrangements and a temporary change in shape, which produces an increase in area. The deep cells become thinner and decrease in the number of layers. They do this by a process called radial interdigitation. Radial interdigitation is when the deep cells elongate, extend protrusions between one another along radii of the embryo and interdigitate to form fewer layers with greater area. Once this process is complete, the deep region consists of one layer of columnar cells, which flatten and spread to further increase area. In the dorsal marginal zone the cells also undergo a shape change, which is not seen in the cells of the animal region. The difference may be due to the uniform spreading in the animal region contrasted with extension and convergence that occurs in the dorsal marginal zone. In his work on time-lapse films of exogastrulae, Keller (1980) found that the ectoderm becomes corrugated by rapid constrictions of the apices of superficial cells and by the appearance of holes in the epithelium. From this, he suggested that shrinkage, rather than expansion aids in epiboly of the ectoderm. He presents a model (see Figure 1) in which the superficial layer is under tension and the force for expansion must come from the deep cells. The expansion of the deep region is resisted by the tension in the superficial layer resulting in an outward curling of the bilayer (deep and superficial layers).      An alternative model (see Figure 2) is also proposed, in which the superficial epithelium is stretched by tension at the margin of the blastopore, which initiates the superficial cells to spread passively (Keller, 1980). While the deep cells rearrange themselves to occupy the areas now available that were once occupied by the superficial cells.
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