Context-sensitive cell behaviors during gastrulation

Cell interactions are important during gastrulation in many particularly those of Holtfreter, firmly established systems. Sea urchin gastrulation provides a simple model sys­ that tem for understanding these cell interactions, and many of "the directed movements of embryonic regions the cell behaviors that contribute to gastrulation have been can actually be traced back to basic faculties of the analyzed. These include ingression, migration, and directed single cells and to their specific response to localization of primary mesenchyme cells, invagination of changes in environment. The unitarian character the archenteron, elongation of the archenteron via epithelial of their combined effort is mainly the result of the cell rearrangement, and attachment of the archenteron to the predisposed arrangement of cetts with a locally different kinetic behavior."2 site of the future mouih fly filopodia at the tip of the archen­ teron. These processes illustrate how the intrinsic behavior of For Holtfreter, gastrulation involved the interplay a cell or group of cells can be modulated fly the context in of the behavior of single cells with a particular 'supra­ which the cells find themselves. Contextual influences cellular' context in which the cells perform their include the local mechanical environment, the geometry of the embryo, the juxtaposition of one type of cell or tissue with behavior.2 Gastrulation then can be thought of as a mosaic of region-specific processes which are coordi­ another in space and/or time, the extracellular matrix, and nated in time and space . Individual cells in the gas­ localized regions containing guidance information for mes­ trula comprise sheets of tissues, and the properties of enchyme cells. All of these constraining influences playa role these tissues in turn constrain and regulate the behav­ in sea urchin gastrulation, and serve to emphasize the ior of their constituent cells. As a result, cells are importance of understanding both autonomous cell behav­ influenced in ways that can only be discovered by iors and the regulation of these behaviors fly the embryonic examining both intrinsic cell behaviors and how environment during gastrulation. those behaviors are modulated by the context in