Blastogenesis refers to the 16-day early development of the fertilized female egg, the zygote, to the blastocyst. During blastogenesis, the cells, which are still omnipotent at the time, divide continuously and, toward the end of the phase, undergo initial differentiation into an outer sheath of cells (trophoblast) and interior cells (embryoblast), from which the embryo develops.
What is blastogenesis?
Blastogenesis includes the earliest stage of development of the fertilized female egg, the zygote, to the blastocyst. Blastogenesis includes the earliest stage of development of the fertilized female egg, the zygote, to the blastocyst. The total period of blastogenesis is 16 days from the time of fertilization to the blastocyst stage. The fertilized egg goes through several stages during blastogenesis. About 40 hours after fertilization, the four-cell stage is reached after two mitotic divisions, and the 16-cell stage is already reached after 3 days. At this stage, the small cluster of cells is enveloped by a firm skin, the zona pellucida. The skin is so firm that the small cell cluster initially retains its initial volume. From the 16- or 32-cell stage, the small cluster of cells is called a blastomere. The term morula is also common, as the small “cluster of cells” resembles the collection of mulberries. During blastogenesis, the zygote slowly moves from the fallopian tube to the uterus under metamorphosis. Towards the end of blastogenesis, the blastomere reaches the blastocyst stage. A first differentiation of the until then omnipotent cells into an outer shell of cells (trophoblast) and the inner cells (embryoblast) has already taken place. While the outer cells take over functions for implantation in the endometrium, the inner cells serve exclusively for embryonic development. Blastogenesis is followed by embryogenesis, which can be divided into several phases.
Function and task
The main purpose of blastogenesis is to protect the fertilized egg to ensure an unmolested and almost self-sufficient development until implantation in the uterus. The zona pellucida, which hardens immediately after the penetration of a sperm, primarily prevents the penetration of a second sperm (polyspermy), which in most cases would lead to an abortion of development. Another function of the zona pellucida is to prevent the fertilized egg from already settling in the fallopian tube, which would result in a dangerous ectopic pregnancy, with the need for abortion. The solid egg skin also holds together the developing cells, which are still omnipotent at this stage and cannot be distinguished from each other. They are also protected from possible immune attack. Since the female egg has sufficient reserves to be largely self-sufficient in terms of metabolism and energy supply during blastogenesis, there is also good protection during the first five days against infections or problematic substances that could be transferred from the mother. In the meantime, the morula has left the fallopian tube and is in the uterus. The original protective functions of the zona pellucida are no longer necessary, so the blastocyst ruptures the egg membrane with the assistance of enzymatic processes and slips out of the membrane (hatching). The most important task of the trophoblast is now nidation, the complex process of implantation of the blastocyst into the epithelium of the uterine mucosa, with the aim of an early connection to the blood supply. During the first phase of blastogenesis, the cells are omnipotent, they can theoretically differentiate into any tissue cells. This has the advantage that they can take over the function of any other cell in case of division problems, so that errors in division are usually self-corrected. Towards the end of blastogenesis, the embryoblast develops into a two-bladed cotyledon. This means that the cells of the two cotyledons gradually lose their omnipotency, a development that continues during subsequent embryogenesis.
Diseases and ailments
During the first phase of blastogenesis, before nidation, the blastomere is relatively protected from external toxic or hormonal influences.In this almost self-sufficient phase, emerging problems, which are summarized under the term blastopathy, are mostly due to defects in the numerous mitoses that are taking place. At this stage of development, the principle of “all or nothing” applies. Either the blastomere can repair the defect that has occurred itself or the blastomere dies with subsequent rejection. However, in very rare cases, with incomplete separation of the cells, symmetrical double malformations can develop after mitosis, which are neither repaired nor lead to rejection. This may result in the development of conjoined twins. By far the most common problem during blastogenesis is extrauterine or ectopic pregnancy, which in most cases develops in the fallopian tube as an ectopic pregnancy. If the migration of the blastomeres from the fallopian tube to the uterus is delayed, it may become lodged in the fallopian tube and cause an incipient ectopic pregnancy. There are many reasons that may interfere with the transport of the fertilized egg to the uterus. For example, the ciliated epithelium of the fallopian tubes may be impaired in its function due to bacterial infections, or genetic malformations may be present. Normally, tubal pregnancy results in rejection reactions that cause the blastomeres to die and cause an abortion, an early abortion. In many cases, the process goes virtually unnoticed.
