Blastogenesis includes the earliest stage of development of the fertilized female ice, the zygote, to the blastocyst. The total period of blastogenesis is 16 days from fertilization to blastula 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 after 16 days the 16-cell stage is reached. The small cell cluster is enveloped in this phase by a firm skin, the zona pellucida. The skin is so firm that the small cell pile initially retains its initial volume. From the 16- or 32-cell stage, the small accumulation of cells is called blastomere. Also common is the term morula, as the small "cell clump" reminds of the accumulation of mulberries.
During blastogenesis, the zygote migrates slowly from the fallopian tube to the uterus. At the end of blastogenesis, the blastomere reaches the stage of the blastocyst. A first differentiation of the hitherto omnipotent cells into an outer envelope of cells (trophoblast) and the internal cells (embryoblast) has already taken place. While the outer cells perform functions for implantation into the lining of the uterus, the inner cells serve exclusively for embryonic development.
Blastogenesis is followed by embryogenesis, which can be subdivided into several phases.
The blastogenesis is mainly used to protect the fertilized ice, to ensure an undisturbed and almost self-sufficient development until implantation in the uterus. The zona pellucida hardening immediately after penetration of a sperm primarily prevents the penetration of a second sperm (polyspermia), which in most cases would lead to a developmental breakdown. Another task of the zona pellucida is that the fertilized egg can not already settle in the fallopian tube, which would result in a dangerous ectopic pregnancy, with the need for a termination of pregnancy. The firm skin also holds together the developing cells, which are still omnipotent in this phase and can not be distinguished from one another. They are also saved from a possible immune attack.
Since the female egg has sufficient reserves to be as self-sufficient as possible in terms of metabolism and energy supply during blastogenesis, there is also good protection against infections or problematic substances that could be transmitted by the mother during the first five days.
Meanwhile, 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 that the blastocyst, with the help of enzymatic processes, breaks up the egg shell and hatches out of the shell (hatching). The most important task of the trophoblast now consists in the nidation, the complex process of implantation of the blastocyst in 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 the case of division problems, so that errors in the division are usually remedied themselves. Towards the end of blastogenesis, the embryoblast develops into a two-leaved cotyledon. This means that the cells of the two cotyledons gradually lose their omnipotence, a development that continues during subsequent embryogenesis.
During the first phase of blastogenesis, even before the nidation, the blastomeres are relatively protected from external toxic or hormonal influences. In this almost autarkic phase, emerging problems, which are summarized under the term blastopathy, usually due to errors of the numerous ongoing mitoses. At this stage of development, the principle of "all or nothing" applies. Either the blastomers can repair the defect themselves or the blastomeres die off with subsequent repulsion.
However, in very rare cases, with incomplete separation of the cells, after mitosis, symmetric double malformations may develop which are neither repaired nor lead to rejection reactions. It can form the plant for Siamese twins.
By far the most common problem during blastogenesis is extrauterine or ectopic pregnancy, which in most cases develops in the fallopian tubes as ectopic pregnancy. If the migration of blastomeres from the fallopian tube into the uterus is delayed, it may become lodged in the fallopian tube and cause incipient ectopic pregnancy. There are many reasons that can hinder the transport of fertilized ice into the uterus. For example, the ciliated epithelium of the fallopian tubes may be impaired in function by bacterial infections or there may be genetic abnormalities. Normally, an ectopic pregnancy causes rejection reactions that cause the blastomeres to die and lead to an abortion, an early abortion. In many cases, the process goes almost unnoticed.