Essay on Chemical and Physical Events that occurs during Fertilization!
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When mammalian egg and sperm come into contact in the female oviduct, a series of steps is set in motion that can lead to fertilization and ultimately to development of new individual. Hence, fertilization is the process of union of an ovum with the sperm. Site of fertilization is fallopian tube (ampulla).
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Fertilization can be internal (inside the body of female) or external (outside the body of female). Fertilization performs two basic functions in the process of embryonic development.
(a) Upon fertilization, the secondary oocyte completes meiotic division and produces a second polar body, which degenerates.
(b) A diploid zygote is formed, which undergoes cleavage to form a morula and then a blastocyst, which implants in the endometrium.
A. Approximation of sperm and ovum:
This can be done by fertilizin-antifertilizin compatibility reaction. Fertilizin (glycoprotein) of egg interacts with antifertilizin (composed of acidic amino acids) of a sperm of the same species. This interaction makes the sperms stick to the egg surface.
B. Penetration of structures surrounding the ovum:
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1. The spermatozoa acrosome aids the sperm in penetrating the layers around the ovum. The acrosome is highly modified lysosomes, derived from the Golgi apparatus during spermatogenesis. It consists of a membrane-bound sac of hydrolytic enzymes and is completely enclosed within the plasma membrane of the sperm cell.
2. As the sperm cells approach the egg, the acrosome reaction occurs to prepare the sperm to fertilize ovum (called capaciation).
a. The acrosomal membrane fuses with the plasma membrane of the sperm cell.
b. The enzymes within the acrosome are released into the milieu surrounding the sperm and egg.
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3. Acrosome contents and their functions
a. Hyaluronidase is a hydrolytic enzyme:
(1) It lyses the glycosaminoglycans in the extracellular matrix holding the cells of the corona radiata together. As the coronal cells become more loosely associated, sperm cells can propel themselves inward, toward the zona pellucida by making a receptive cone or fertilization cone through the cytoplasm of the egg
(2) Hyaluronidase may also be involved in breaking down the zona pellucida.
b. Neuraminidase, also a hydrolytic enzyme, removes neuraminic acid (sialic acid) from glycoproteins. In experimental studies, a neuraminidase-treated zona pellucida cannot be penetrated by sperm cells. Thus, the acrosomal neuraminidase may aid in preventing more than one sperm from entering an ovum (polyspermy).
c. Zona lysins are proteolytic enzymes that are capable of degrading the zona pellucida, perhaps easing the passage of sperm cells through to the ovum.
4. Once the sperm cell has penetrated the corona radiata and zona pellucida, its unit membrane fuses with the unit membrane surrounding the egg cytoplasm. Just after the entry of sperm into the egg, a fertilization membrane is formed (by zona pellucida) in the egg. This fertilization membrane prevents polyspermic fertilization (penetration of many sperms into an ovum simultaneously). A previtelline space is formed between the fertilization membrane and the plasma membrane.
C. Activation of ovum:
As sperm enters the ovum (actually a secondary oocyte) it gets activated and undergoes the second meiotic division. As a result of this the oocyte produces an ovum and a secondary polar body. The polar bodies ultimately degenerate and the ovum contains the haploid number of chromosomes.
Then the ‘chromosome of sperms and egg are set free by the breakdown of their nuclear envelopes. Mixing up the chromosomes of a sperm and an ovum is known as karyogamy or amphimixis. The latter results in the formation of diploid zygote nucleus which contains both the maternal and paternal chromosomes.