Ingression (biology)

Ingression is one of the many changes in the location or relative position of cells that takes place during the gastrulation stage of embryonic development. It produces an animal's mesenchymal cells at the onset of gastrulation. During the epithelial–mesenchymal transition (EMT), the primary mesenchyme cells (PMCs) detach from the epithelium and become internalized mesenchyme cells that can migrate freely. While the mechanisms of ingression are not fully understood, studies using the sea urchin as a model organism have begun to shed light on this developmental process, and will be the focus here. There are three main changes that must occur within a cell to enable the process of ingression. The ingressing PMCs must first alter their affinity for the neighboring epithelial cells that will remain in the vegetal pole (vertebrate PMCs ingress from the primitive streak). During this time, these cells must lose their affinity for the hyaline layer to which their apical surface is attached. The ingressing cells will then apically constrict and alter their cellular architecture through a dramatic reorganization of their cytoskeleton. Lastly, these cells will modify their mode of motility and presumably gain affinity for the basal lamina which composes the lining of the blastocoel, the future migration substrate of the PMCs. Changes in the adhesion properties of these cells are the best characterized and understood mechanism of ingression. In sea urchins, epithelial cells adhere to one another as well as the hyaline layer through classic cadherins and adherens junctions. Ingression is a very dynamic process however, and the first sign of an ingressing cell is seen when a future PMC loses its adhesion to hyaline, and cadherin, and increases its adhesion to a basal laminal substrate. These processes occur rapidly, over approximately 30 minutes. It is not understood how the PMCs penetrate the basal lamina. The basal lamina is a loose matrix, therefore it is possible that the ingressing cells squeeze through the matrix.