It is an exciting day to be with you all from far and wide in one community called StemSocial. It is a beautiful day today, and I hope you are enjoying your day, anyways, mine is a sunny day. I have been discussing the reproductive system and anatomy. I started with the male reproductive system, I explained Gametogenesis, then I went further to explain the anatomy of the female reproductive system, and then moved further to look into Oogenesis and Folliculogenesis. If you missed any of those posts, you can just slide down to my blog, pick any of them and read at your pace to satisfaction. You can have fun and learn at the same time. Today, I will be continuing on the reproductive system, and as I concluded last time that I will be discussing Zygotic Cleavage in this post,but before I get there, let me quickly do a quick run on fertilization pathway.
Remember that I mentioned that egg ready for fertilization is a secondary Oocyte, so fertilization is when the sperm cell penetrates the secondary Oocyte but how does the spermatozoa gets to the egg. Fertilization occurs at the ampulla of the fallopian tube and remember I mentioned that the fallopian tube is divied into the isthmus which comes out of the uterus, then the ampulla which is an enlarged region of the fallopian tube, followed by the infundibulum which is the most distal part of the fallopian tube that branches to the fimbriae which is the fingerlike structure. I said in my post that the overies are not in direct contact with the fimbriae during the female cycle even when they appear like that and they only make contact with the fimbriae during ovulation. I explained that during ovulation, the egg moves into the fallopian tube and from my last post I said that the egg is a secondary oocyte and it is matured but not yet a zygote. The secondary Oocyte is formed in the Ovaries and picked up by the Fimbriae, where they move the secondary Oocyte into the fallopian tube where they get into the infundibulum, then to the ampulla. Remember, unlike the sperm cells where there are billions of cells, for females there is only one or oocytes produced in a month ready for fertilization, as others die during the process of meiosis and Mitosis. Also, both ovaries might not produce Oocyte at the same time in a month, it is usually one at a time.
The sperm cell enters the vaginal oriffice in billions and by the time they get to the fallopian tube, it might be less than 10 that will get to the Oocyte as weaker swimmers would have died during the process of going through the vaginal, the cervix, and the Uterus. In the vaginal, so many spermatozoa would die as it is acidic in nature and although the semen is alkaline, the acidity of the vagina down to the fallopian tube is more than enough to kill a lot of spermatozoa. Also, the host immune antibodies that would see the sperm cell as foreign. IgA antibodies are found in the vagina would attack the sperm cell, just like it would attack a microorganism that enters into the vaginal. Also enzymes in the vagina would destroy the sperm cell at the vagina. By the time it gets to the cervix there is a significant amount of mucus with mucin, and the density of the mucus caused by the mucin determines the passage of the sperm cell through the cervix. If the sperm cell cannot move to the cervix and then the fallopian tube when the mucus is thick, but during ovulation, the mucus is very thin that the sperm can penetrate through them. In the Uterus, the environment is acidic and there are antibodies that attack the sperm cells. When they get pass the Uterus, they need to find the correct fallopian tube where the egg is, as both fallopian tubes open at the same time and since only one ovary is ovulating, the sperm might reach the wrong fallopian tube which is inactive. In the fallopian tube, the sperm cells get nutrients from proteins providing them with energy to penetrate the egg by removing the protein that covers the sperm cell preventing it from penetrating the egg known as Capacitation. After capacitation, only one sperm cell can fertilize the secondary Oocyte while others degenerates.
When the sperm cell reaches the secondary Oocyte, it penetrates then there is dumping of the DNA (pronucleus), after which the secondary Oocyte finishes meiosis 2 to become an Ootide, then an Ovum, then when the pronuclei fuses to become a nucleus, then a zygote is created. The zygote will divide through a process called cleavage which is similar to mitosis but different. The division takes a zygote to a blastocyst. Remember I said mitosis and zygotic cleavage are similar, so what's different. Mitosis causes the division into two identical daughter cells to the parent cells. For the parent cells to divide, it has to grow and then divide into two identical cell replicating everything including the organelle. With cleavage, there is no growth required rather there is division with two daughter cells half the size of the parent cell, and for each divide, the cell gets smaller as it is not growing. In the fallopian tube,a cleavage will lead to a division into equal amount of cell increase. The first cleavage gives the 2 cell embryo known as blastomere, then it divides to give 4 cell embryo (blastomere), another cleavage will give give a 8 cell blastomere, then to a 16 cell blastomere which is known as morula between day 4 and day 5. The morula continue to cleave but not in synchronization. This cleavage continues until a blastocyst is formed which is a spherical wall of cells that is hallow inside known as blastocoel. The bastocyst also poccess the germ cells (endoderm, ectoderm, and mesoderm) in a region called Inner cell mass. The blastocyst the exits the fallopian tube into the Uterus where it sheads the zona pellucida in a process known as hatching were it removes its outer shell. The blastocyst then implant in the endometrium of the uterus wall.
https://www.khanacademy.org/test-prep/mcat/cells/embryology/a/egg-meets-sperm
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2846064/
https://academic.oup.com/biolreprod/article/99/1/134/4862467
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3720105/
https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology
https://academic.oup.com/molehr/article/22/10/691/2355285?login=false