Mechanisms of ectoderm epiboly in the Xenopus Laevis embryoIntroductionEpiboly is a gastrulation movement in the amphibian embryo, whereby the ectodermal precursors expand to cover the entire embryo. This process occurs in cells of the superficial and deep layers in the animal and marginal regions. In deep cells, three cycles of cell division occur as they reorganize to form fewer layers. Superficial cells elongate by cell division as they flatten, which gives them more surface area and less depth. The ectoderm eventually covers the entire embryo, internalizing the endoderm. This process establishes the correct position of the three germ layers, with the ectoderm on the outside, the mesoderm in the center and the endoderm on the inside. Keller (1980) found that surface cells spread, divide, and undergo rearrangements and a temporary change in shape, which produces an increase in area. Deep cells become thinner and the number of layers decreases. They do this through a process called radial interdigitation. Radial interdigitation occurs when deep cells elongate, extend protrusions between each other along the rays of the embryo, and interdigitate to form fewer layers with a greater area. Once this process is complete, the deep region consists of a layer of columnar cells, which flatten and expand to further increase the area. In the dorsal marginal zone the cells also undergo a change in shape, which is not observed in the cells of the animal region. The difference may be due to uniform diffusion in the animal region as opposed to the extension and convergence that occurs in the dorsal marginal zone. In his work on time-lapse films of the exogastrules, Keller (1980) found that the ectoderm wrinkles due to rapid constriction of the surface cell apices and the appearance of holes in the epithelium. From this, he suggested that shrinkage, rather than expansion, aids epiboly of the ectoderm. It presents a model (see Figure 1) in which the superficial layer is under tension and the expansion force must come from the deep cells. Expansion of the deep region is resisted by tension in the superficial layer resulting in outward curling of the bilayer (deep and superficial layers). An alternative model is also proposed (see Figure 2), in which the surface epithelium is stretched by tension at the edge of the blastopore, which initiates surface cells to spread passively (Keller, 1980). While the deep cells reorganize to occupy the now available areas that were once occupied by the superficial cells.