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19.3Morphogenetic Movement

In addition to the mechanism by which the complicated bodies of animals are formed via cleavage of the fertilized egg and repeated cell specification, discussed earlier, another important mechanism in animal development is morphogenetic movement. Morphogenetic movement is caused by large-scale and dynamic movement of embryonic cells. It rearranges the distribution of embryonic cells, thereby allowing the interaction between germ layers that previously existed separately. The first morphogenetic movement triggered is gastrulation, which forms the future digestive tract. Gastrulation is one of the most important morphogenetic movements in the formation of the basic tubular structure of animals. Morphogenetic movement consists of several basic cellular deformations and movements, including the invagination movement, in which contraction of one side of the embryo causes bending and encourages epithelial cells to extend inward into the embryo, the extension movement caused by the rearrangement of epithelial cells, and the ingression movement of epithelial cells migrating into the embryo (Figure 19-10).

Figure 19-10 Morphogenetic movement

(A) Morphogenetic movement in the early embryo takes place mainly through the movement and deformation of the epithelial cells, which constitute the embryo, and includes: a) invagination movement of epithelium, b) extension movement caused by rearrangement of epithelial cells, and c) locomotive movement (migration) of epithelial tissues. (B) Examples of morphogenetic movement in amphibians and mammalian embryos. The cells move in the directions of the red arrows shown in the diagram. a) Gastrulation of the frog embryo: the mesoderm and endoderm move into the interior of the embryo through invagination and extension movements and form an inner lining of the ectoderm. b) Gastrulation of the mammalian embryo: cells separate from the central part of the ectoderm and move into the interior of the embryo, and become endoderm and mesodermal cells. In the mammalian embryo, the archenteron is not actually formed at this time, but this morphogenetic movement is called gastrulation.

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