Fertilization and Implantation

FERTILIZATION AND IMPLANTATION
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Explain where the process of fertilization happens, describing the key changes that take place in the development of the embryo during three, four, and eight weeks. How is the embryo implanted in the uterus endometrium? (330 words)
Fertilization describes the process through which the male gamete (sperm) and the female gamete (ovum) combine to form a cell called a zygote during the sexual reproduction process (Moore et al. 2018). Internal fertilization occurs in the first part of the oviduct in a female’s body. During the fertilization process, the sperm’s nuclei and egg’s nuclei fuse to form a single nucleus referred to as a zygote, which contains both the nuclear part of sperm from the male and egg from the female.
After fertilization, the zygote and its associated derivatives (conceptus) are continuously projected towards the uterus by peristalsis and beating cilia. During the process, the zygote goes through several mitotic cell divisions, as shown in the diagram. Cell divisions result in more cells. However, cell division does not increase the total volume of the embryo. Roughly three days after conception, a 16-cell morula reaches the womb, as shown in the diagram. The conceptus floats freely in the uterus as it continuously sub-divides into more than 100 cells after four days. At approximately six days, the tightly bound cells (blastocyst) begin secreting fluid and position themselves around a fluid-filled environment (blastocoel). The inner cell mass inflates to become the embryo. In contrast, the outer cells develop to form the placenta's chronic sac and fetal part, useful in feeding and nourishing the embryo.
During the third week of development of the embryo, conception takes place. Conception forms a zygote, which has 46 chromosomes (23 from the female parent and another 23 from the male parent), which help determine the sex of the baby and their personal features (Schoenwolf et al. 2020). Fertilization of two different eggs by two separate sperms results in fraternal twins. However, if one fertilized egg splits, it results in the development of two fetuses that result in identical twins.
The splitting of the embryo characterizes the fourth week into two parts. One part develops into the placenta, which nourishes the embryo. At the same time, the other half continues to grow to become the embryo by creating a neural tube through which the brain, spinal cord, and backbone of the baby eventually develop.
During the eighth week, the embryo's growth accelerates. Delicate facial features such as the ears, nose, and upper lip become visible. The baby's eyelids also form during this week as his heart continues getting stronger as the embryo develops.
Explain why it is important that several spermatocytes must be present for the successful fertilization of an oocyte. How does the structure of an ovum ensure that polyspermy is avoided? (200 words)
Fertilization is a process rather than a single isolated occurrence. Interruption of any step in the fertilization process causes fertilization failure. Severa...