Ingression or immigration is a cell movement of gastrulation and thus constitutes an embryonic developmental step. In this process, cells of the prospective endoderm, i.e., those of the outer cotyledon, migrate into the blastula. Errors in cell movements during the early development of the cotyledons lead to miscarriage in most cases.
What is ingression?
Ingression is a cell movement of gastrulation, making it an embryonic developmental stage. Gastrulation is a phase of embryogenesis. In humans, the phase involves the invagination of the blastocyst and the formation of three germ layers. In principle, gastrulation of all four-celled organisms follows the same basic principles, but may differ somewhat species-specifically. Several cell movements characterize gastrulation. In addition to invagination, involution, delamination, and epiboly, ingression is a crucial process for the formation of cotyledons and thus early embryonic development. Ingression is also referred to as immigration. During cell movement, cells of the future endoderm migrate into the blastula and are subsequently strangulated into the blastocoel during delamination. The cell movements of gastrulation have not yet been conclusively studied.
Function and task
During gastrulation, the blastocyst gives rise to a two-layered structure consisting of inner and outer cotyledons. These cotyledons are known as the entoderm and ectoderm. Through initial cell division processes, the omnipotent tissue of the blastocyst thus becomes increasingly specific collections of cells from which the individual organs and tissues of the embryo are eventually formed. Through processes of migration and displacement, the so-called mesoderm pushes itself between the endoderm and the ectoderm. This results in three germ layers that contain organ-specific tissues for the development of the individual body structures. The first gastrulation process is characterized by the cell movement of invagination. In this process, the future entoderm invaginates into the blastocoel of the blastula. This is followed by the cell movement of involution, in which the future endoderm curls up. These processes are followed by the so-called ingression or immigration. In this cell movement, cells of the entoderm migrate in. Thus, the cells change their position or relative position. The mesenchymal cells of the embryo are the focus of this process. In EMT (epithelial-mesenchymal transition), the primary mesenchymal cells detach from the epithelium and become freely migrating mesenchymal cells. The mechanism of ingression has not been fully understood. Studies exist, for example, on the sea urchin. According to the studies, three different processes take place to enable the ingression of a cell: The primary mesenchymal cells of the epithelium change their affinity for the neighboring epithelial cells that remain in the primitive streak. In addition, during ingression, the cells apparently change their affinity for the hyaline layer facing their apical side. At the apical side, the cells become constricted, whereupon they alter their intracellular structure by drastically restructuring the cytoskeleton. As a result, the motility of the cells changes. In addition, the affinity for the basal lamina lining the blastocoel is increased. The immigration of the cells into the blastocoel is the final goal. Meanwhile, the adhesion properties of the cells have been characterized. While a future primary mesenchymal cell loses affinity for the hyaline layer, its affinity for the basal substrate increases. However, how the cells penetrate the basement membrane during ingression remains unclear. The basement membrane represents a loose matrix, so the cells presumably squeeze through the matrix. Speculation also suggests the use of a proteinase by the cells. During ingression, numerous transcription factors are activated, most notably β-catenin and the growth factor receptor VEGFR. Ingression is probably facilitated for individual cells by the fact that their neighboring cells undergo ingression at the same time. Ingression is followed by delamination, in which cells of the blastula sequester cells of the endoderm into the blastocoel.
Diseases and disorders
Embryonic developmental disorders can be triggered by internal factors and external factors such as pollutants.Such an error is not further noticed by the pregnant woman in the first days after fertilization of an egg. Often, an unnoticed miscarriage occurs shortly after fertilization. In this scenario, the egg does not even nest. The affected person does not experience any symptoms. From the beginning of the third week of development, this changes. From this point on, the unborn child is susceptible to harmful substances from the outside. Especially during the development of the three germ layers, harmful substances such as chemical or organic noxae can cause numerous errors that can have serious consequences. For example, errors in cell migration such as ingression can result in an abnormal amount of cells being available for each cotyledon. β-Catenin plays an essential role in ingression and its failure-free progression. If β-catenin is blocked in its function by damaging influences or processes internal to the body, a pathological result is shown for cell migration. In this case, the cotyledons cannot continue to form. A miscarriage is the result. Disturbances also occur when there is an oversupply of β-catenin. In this case, too many cells undergo the cell migration of ingression. An oversupply of cells of the prospective ectoderm is thus available during delamination. Depending on the amount of oversupply, pregnancy may determine or progress further and lead to embryonic malformations. Dysfunction and formation defects of the growth factor receptor VEGFR may also be responsible for ingression defects.
