Morphogenetic Movements
The cellular architecture of the blastoderm is that of a simple monolayer. Therefore, embryonic development can, in short, be described as the process of transforming this two-dimensional structure into a three-dimensional structure in which the basic bod
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Chapter 15 Morphogenetic Movements
The cellular architecture of the blastoderm is that of a simple monolayer. Therefore, embryonic development can, in short, be described as the process of transforming this two-dimensional structure into a three-dimensional structure in which the basic body pattern (Sander 1976, 1983) can already be recognized. This topological transformation is achieved by means of morphogenetic movements. Morphogenetic movements may take place either (i) by growth and infolding of certain regions of the monolayer, without alteration of its original arrangement as, for example, occurs in the epidermal anlage; or (ii) by segregation of individual cells or coherent cell arrays from the monolayer into the interior of the embryo, which thereby lose their original contacts with the other regions; examples include the segregation of the neuroblasts and the invagination of the ventral furrow or the midgut. Indeed, most of the elementary morphogenetic movements found in embryos of other animal species, e.g. invagination, involution, ingression, delamination etc., also occur during embryogenesis in Drosophila. Movements associated with cell growth include a variety of processes, e.g. changes in cell shape and size, proliferation, alteration of the position of individual cells relative to each other; these processes may themselves lead to infolding of some areas. Despite these infoldings, the original spatial relationships of neighbouring cells are maintained in this type of movement; that is to say, the cells retain their spatial cohesion throughout the shape changes that occur during later stages. On the other hand, individual cells can separate from the blastodermal monolayer and move into the interior, as is the case with the segregation of neuroblasts. In most instances, however, cells move in groups by means of invaginations; for example, the three germ layers become established by means of invaginations. In any case, unlike the movements associated with growth, this sort of separation implies interruption of the original integrity of the blastoderm, in that cells become
J. A. Campos-Ortega et al., The Embryonic Development of Drosophila melanogaster © Springer-Verlag Berlin Heidelberg 1997
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Chapter 15
Morphogenetic Movements
displaced from their original position to form a new layer. The spatial relationships of these cells in the blastoderm become modified after these movements in such a way that direct translation from the blastoderm frame onto the new coordinates is not immediately possible. During Drosophila embryonic development, several processes, i.e. gastrulation, germ band elongation and germ band shortening, head involution and dorsal closure, have an eminently morphogenetic character. Features of these very complex morphogenetic movements have already been considered in various chapters. Thus in the following description, in order to avoid extensive repetition, only those aspects of the major morphogenetic movements will be emphasized which contribute to the appearance of the bas
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