INTRO to EMBRYOLOGY 2
As we have finished the fertilization process the last time, we will take the next steps after the fertilization happens. these processes that we will talk about will be happening in the First week of development.
first, we have the cleavage process, which is fundamental in embryonic development where a single fertilized egg (zygote) undergoes a series of rapid mitotic divisions. these cells that will be formed after the mitotic division happens is called blastomeres, cleavage has many stages which are:
1-First Cleavage Division: The zygote undergoes its first mitotic division, resulting in two blastomeres. This typically occurs within 24 hours after fertilization in many organisms.
2-Subsequent Divisions: The two blastomeres divide, leading to four cells, then eight, sixteen, and so on. These divisions occur rapidly and are characterized by the absence of significant growth between divisions. that means that they only increase in number but the size in general will stay the same and the cells here are still enveloped by the zona pellucida.
3-Morula Stage: After several divisions, the embryo forms a solid ball of cells known as the morula. The morula typically consists of 16 to 32 blastomeres. the morula is also still enveloped by the zona pellucida and it gets its nutrition from the uterine tube secretions.
4-Blastula Stage: As cleavage continues, a fluid-filled cavity called the blastocoel forms within the morula, leading to the formation of the blastula. The blastula is characterized by a hollow sphere of cells surrounding the blastocoel.
5- the blastocyst stage: it happens after 5 days from fertilization, and it is located in the uterine cavity, and it will get its nutrition from the uterine glandular secretions and the surrounding blood vessels, and by the end of this stage the blastocyst will have 2 layers, an outer layer called the trophoblast and an inner one called the embryoblast.
Now we go to the next step which is the IMPLANTATION, it is a critical process in early pregnancy where the blastocyst, attaches to the wall of the uterus (endometrium), specifically near the fundus u can see it in the picture below, and between the openings of the glands, the blastocyst needs to be between the glands so it can get the nutrition it needs.
as we said earlier the blastocyst has 2 layers, an outer layer called the trophoblast and an inner one called the embryoblast, both of these 2 layers will develop into more important structures, that will help in the development of the blastocyst. so for the implantation process to begin the trophoblastic cells over the embryonic pole will start penetrating the endometrium by using proteolytic enzymes.
during the stage of implantation, the uterus will undergo some changes, in which The endometrial stromal cells undergo a transformation known as the decidual reaction, where they become larger and more secretory, providing nutritional support to the developing embryo and forming a specialized structure called the decidua.
and by this, we finished the events that happens in the first week of development.
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