Fertilization and implantation are the most critical events in the reproduction process. In this process, both egg and sperm are fused together to form a zygote. Later it gets implanted into the uterus and the development of an organism.
Let us have a look at how fertilization in humans takes place.
“Fertilization in humans refers to the fusion of male and female gametes that facilitates the development of a new organism.”
Fertilization is the natural life process, which is carried out by the fusion of both male and female gametes, which results in the formation of a zygote. In humans, the process of fertilization takes place in the fallopian tube.
During this process, semen comprising thousands of sperms are inseminated into the female vagina during coitus. The sperms move towards the uterus and reach the opening of the fallopian tube. only a few sperms will succeed in reaching the opening of the fallopian tube.
The secondary oocyte releases from the matured Grafian follicle of the ovary and enters into the fallopian tube, where it is fertilized within 24 hours, after which it is released from the ovary.
Though surrounded by several sperms, the oocyte is fertilized by a single sperm. During meiosis-II, the sperm enters the secondary oocyte and completes the meiosis. After this, the secondary oocyte is known as the egg.
Both sperm and egg can show their vitality only to a limited period. Sperm is alive for 48-72 hours in a female reproductive tract, whereas the egg can be fertilized for 24 hours before it is released.
The fertilization process in humans takes place in several stages involving both the chemical and physical events. The different stages of fertilization in humans are mentioned below:
The sperms incapacitation undergo acrosomal reactions and release certain chemicals known as sperm lysins present in the acrosome.
Due to the acrosomal reactions, the plasma membrane of the secondary oocyte and the sperm are fused together so that the contents of the sperms can enter. When the plasma membrane of the sperm binds with that of the secondary oocyte, the plasma membrane of the oocyte depolarizes. This prevents polyspermy.
Calcium ions play a significant role in the acrosomal reaction. The main factors essential for acrosomal reactions are optimum pH, temperature and calcium and magnesium concentration.
Soon after the fusion of the plasma membranes, the oocyte shows cortical reactions. Cortical granules found under the plasma membrane of the oocyte, which fuses with the plasma membrane and releases cortical enzymes between the zona pellucida and plasma membrane. The zona pellucida is hardened by the cortical enzymes that prevent polyspermy.
A projection known as the cone of reception is formed by the secondary oocyte at the point of sperm contact. This cone of reception receives the sperm.
After the entry of the sperm, the suspended second meiotic division is completed by the secondary oocyte. This gives rise to a haploid ovum and a second polar body.
The head of the sperm containing the nucleus detaches from the entire sperm and is known as male pronucleus. The tail and the second polar body degenerates. The nucleus of the ovum is known as female pronuclei.
The male and female pronuclei fuse and their nuclear membranes degenerate. The fusion of the chromosomes of male and female gametes is called karyogamy. The ovum is now fertilized and is known as a zygote.
The entry of sperm triggers the metabolism in the zygote. Consequently, protein synthesis and cellular increase.
After activation the next process is implantation
In humans or in any conceptus ( placental ) animal, the implantation begins after one week, and mostly range from 8 to 10 days. Before the implantation to start, there are certain changes that occur in the endometrium and this is called the implantation window, and the implantation occurs after a peak in the luteinizing hormone.
Endometrial changes occur after the ovulation is predecidualization and if the pregnancy confirms it enters the decidualization stage, where there is an increase in vascularization, and decidual cells become polyhedral shapes with large nucleus and glycogen storage.
The implantation starts with an initial stage called adaptation, in the first phase of adaptation, the blastocyst loosely adheres to the endothelium, and in the second phase, this blastocyst roll to the site of implantation, and firmly attached to the endometrial layer of the uterus, wherein the third phase there is adhesion and interaction of the blastocyst with uterus takes place.
The implantation should occur always in the body of the uterus, sometimes it does not happen it may implant in some loss wrong places like fallopian tubes occur, which is the abnormal implantation, and sometimes even if implantation is proper but placenta may not develop properly.
