Mechanisms of ectoderm epiboly in the Xenopus Laevis embryoIntroductionEpiboly is a gastrulation movement in the amphibian embryo, by which the ectodermal precursors expand to cover the whole embryo. This process occurs in the cells of the superficial and deep layers of the animal and marginal regions. Three rounds of cell division occur in the deeper cells, while they also reorganize to form fewer layers. Surface cells elongate through cell division while flattening, giving them a greater surface area and finer depth. The ectoderm ultimately 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 middle, and the endoderm on the inside. Keller (1980) found that surface cells spread, divide, and undergo rearrangements and temporary change in shape, which produces an increase in surface 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 them along the rays of the embryo, and interdigitate to form fewer layers with greater surface area. Once this process is complete, the deep region consists of a layer of columnar cells, which flatten and expand to further increase the surface area. In the dorsal marginal zone, cells also undergo a change in shape, which is not visible in cells in the animal region. The difference may be due to the uniform spreading across the animal region, contrasting with the extension and convergence that occurs in the dorsal marginal zone. In his work on time-lapse films of exogastrules, Keller (1980) found that the ectoderm undulates through rapid constrictions. of the apices of the superficial cells and by the appearance of holes in the epithelium. From this, he suggested that shrinkage, rather than expansion, promotes epiboly of the ectoderm. It presents a model (see Figure 1) in which the surface 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 surface layer, resulting in outward rolling of the bilayer (deep and surface layers). An alternative model (see Figure 2) is also proposed, in which the surface epithelium is stretched by tension at the blastopore margin, which initiates passive spreading of surface cells (Keller, 1980). As the deep cells reorganize to occupy the now available areas that were formerly occupied by the superficial cells.