Epiboly: Function, Tasks, Role & Diseases

Epiboly is a cell movement of gastrulation that corresponds in principle to invagination. In this process, the prospective endoderm is overgrown by the prospective ectoderm. Disturbances of epiboly occur, for example, when there is a loss of function of the molecule fibronectin and can cause miscarriage.

What is epiboly?

Epiboly is a cell movement of gastrulation that is basically equivalent to intussusception. During gastrulation, the blastocyst invaginates. During gastrulation, the blastocyst invaginates. In the course of the process, the three cotyledons are formed, from which the individual anatomical structures of the embryo develop. Immediately after fertilization, the future cells of the embryo are omnipotent. The formation of the three cotyledons corresponds to an initial differentiation of the omnipotent cells. During embryonic development, the formerly omnipotent cells then become organ-specific tissue step by step. In this context, the formation of the three cotyledons during gastrulation is a basic condition. In biology, the cotyledons are called endoderm, mesoderm and ectoderm. All specific tissues of the later individual emerge from them via division processes. Gastrulation proceeds similarly for all multicellular organisms and is characterized by different cell movements. One of these is epiboly, which usually follows the movement of delamination. During epiboly, active overgrowth of the yolk-rich blastulate part occurs. In meroblastic eggs with an extreme amount of yolk, the cotyledons overgrow the unlined yolk, such as in the gastrulation of the bony fish. Thus, epiboly corresponds in principle to invagination, in which the prospective endoderm is overgrown by the prospective ectoderm.

Function and task

Within the early embryonic development (embryogenesis) of multicellular organisms, three cotyledons are formed. The initial material for cotyledon formation is called a blastula in lower mammals and a blastocyst in higher mammals such as humans. The process of cotyledon formation is also known as gastrulation and involves several cell movements that have not yet been conclusively explored or understood. In addition to invagination, involution, ingression, and delamination, epiboly is one such cell movement. In invagination, the future endoderm inverts into the blastocoel of the blastula, forming the endoderm as the inner cell layer and the ectoderm as the outer cell layer. This is followed by involution, in which the endoderm curls up. During the subsequent ingression or immigration, cells of the endoderm migrate into the blastula and are strangulated off during the subsequent delamination of blastula cells into the blastocoel. In yolk-rich eggs, epiboly now takes place, which in principle corresponds to invagination. This cell movement is characterized by an overgrowth of the future endoderm, which is performed by cells of the prospective ectoderm. Epiboly is understood as the first coordinated cell movement and begins during blastula stage completion. All cell layers undergo epiboly. The inner cells of the blastoderm move toward the outer cells and overlap. The blastoderm spreads toward the vegetative embryonic pole until it fully engulfs the yolk cells. The cells of the envelope layer increase their surface area and spread in a similar manner. In the anterior portion, the cells align. The yolk layer again moves toward the vegetative pole during epiboly and spreads along the yolk surface. After the completion of the epiboly, the envelope layer, the yolk layer, and the deeper cells of the blastoderm have completely grown around the yolk cells. The molecule fibronectin is thought to play a major role in epiboly. In addition, signaling pathways such as the Wnt/PCP pathway , the PDGF-PI3K pathway, the Eph-ephrin pathway, Jak-Stat signaling, and the MAP kinase cascade play a role in cell movement.

Diseases and disorders

In the first few days after fertilization of an egg, errors in embryonic development can already occur. If such errors occur, the fertilized egg usually does not implant at all. The result is a miscarriage that does not cause any symptoms and is usually not even noticed by the miscarrying woman.In most cases, this type of miscarriage is not a complication caused by pollutants. The little creature is not particularly susceptible to external pollutants until the cotyledons are formed. However, this changes as soon as the primitive streak forms. From the third week after fertilization, external pollutants can cause disturbances in the development of the embryo and lead to tragic consequences. If the cellular movements of gastrulation are disturbed, the three cotyledons may either fail to form or form in an unpredictable manner. Disorders in epiboly, for example, can result from a loss of function of the molecule fibronectin. Disruptions in the other signaling pathways involved in epiboly can also result in no cell movement at all, inadequate cell movement, or cell movement to a pathological extent. Based on such disturbances, the envelope layer, the yolk layer, and the deeper cells of the blastoderm do not completely surround the yolk cells or do not surround them at all. The result is usually a miscarriage. Unlike the first days and weeks after fertilization, this type of miscarriage is accompanied by symptoms and is noticed by the miscarrying woman.