Normally, a human embryo will take four days to travel down the fallopian tube and into the uterine lining. Successful in vitro fertilization (IVF) transfer rates are about 37.1% for women under the age of 35, and lessen with age. Implantation rates also decline with female age due to a rising chance of chromosomal abnormality. For many women afflicted with infertility, this step is compromised. Successful implantation of the blastocyst in the uterus is necessary for the fetus to grow. Once they adhere, the fertilized egg and uterine lining can no longer be flushed out during the menstrual cycle. The microvilli on the surface of the trophoblast cells will adhere to the epithelial cells of the uterus through glycoproteins. Since the egg has to shed off the zona pellucida, it can now implant itself into the inner lining of the uterus called the endometrium. This is made possible with the help of cilia (little “hairs”) that contract, and move the egg into the uterus. This all takes place on the fifth day post-fertilization, after which the blastocyst travels down the fallopian tube. Upon hatching at the end of day five, the blastocyst is ready to find its new place. The zona pellucida will split on the side opposite the embryonic pole and “hatch,” or release, the blastocyst. This is when a sort of “mini birth” occurs. The growth and division of the blastula stretch and contract the rigid zona pellucida. Meanwhile, the zona pellucida begins to break down. As its Greek roots imply, the trophoblast layer will “nourish” the growing embryo when it becomes the placenta. The trophoblast forms right under the zona pellucida. The fluid is pushed inward by the outer cell layer, and junctions between the cells are formed to pull the cells together to keep the fluid inside the cavity. The official name for the process that forms the internal cavity of the blastocyst is cavitation. This starts a rearrangement of cells on the outside and inside of the egg, and the eventual blastocyst forms. Therefore, the egg sheds off the zona pellucida between days five and seven as it begins to differentiate. While this shield is helpful, it is not compatible with the growth of the new embryo. For reference, for any female egg there are between 40 million to 1.2 billion sperm cells competing for it in a single ejaculation. This is a protective mechanism against multiple sperms penetrating it. Once a sperm enters an egg, a tough membrane called the zona pellucida forms around it that is impenetrable to other sperm. Some drastic steps are taken in making this journey. It takes about seven days for the blastocyst to find its way to the mammalian womb. 1 refers to the blastocoel, 2 to the trophoblast, 3 to the zona pellucida, and 4 to the inner cell mass that will differentiate into the fetal tissues. But as it grows, the little organism will need more food and a safe space to expand. The pre-implanted embryo is a sphere made up of the outer trophoblast, the blastocoel cavity, and a cluster of cells in the center called the inner cell mass. Perhaps one of the most important features of blastocyst development is its journey to the uterine wall.
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