The process of fertilization involves a sperm fusing with an ovum—usually following ejaculation during copulation. It is possible, but less probable, for fertilization to occur without copulation, artificial insemination, or In vitro fertilization. Upon encountering the ovum, the acrosome of the sperm produces enzymes which allow it to burrow through the outer jelly coat of the egg. The sperm plasma then fuses with the egg's plasma membrane, the sperm head disconnects from its flagellum and the egg travels down the Fallopian tube to reach the uterus.
In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitro.
Anatomy
Corona radiata
The egg and the sperm bind through the corona radiata, a layer of follicle cells on the outside of the secondary oocyte. Fertilization occurs when the nuclei of a sperm and an egg fuse. The successful fusion of gametes form a new organism.
Cone of attraction and Vitelline membrane
Where the spermatozoon is about to pierce, the yolk (ooplasm) is drawn out into a conical elevation, termed the cone of attraction. Once the spermatozoon has entered, the peripheral portion of the yolk changes into a membrane, the vitelline membrane, which prevents the passage of additional spermatozoa.
Acrosome reaction
The acrosome reaction must occur to mobilise enzymes within the head of the spermatozoon to degrade the zona pellucida. example: hyaluronidase.
Zona pellucida
After binding to the corona radiata the sperm reaches the zona pellucida, which is an extra-cellular matrix of glycoproteins. A special complementary molecule on the surface of the sperm head binds to a ZP2 glycoprotein in the zona pellucida. This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida.
Some sperm cells consume their acrosome prematurely on the surface of the egg cell, facilitating the penetration by other sperm cells. As a population, sperm cells have on average 50% genome similarity so the premature acrosomal reactions aid fertilization by a member of the same cohort. It may be regarded as a mechanism of kin selection.
Recent studies have shown that the egg is not passive during this process.
Cortical reaction
Once the sperm cells find their way past the zona pellucida, the cortical reaction occurs: cortical granules inside the secondary oocyte fuse with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glyco-proteins in the zona pellucida to cross-link with each other—that is, the enzymes cause the ZP2 to hydrolyse into ZP2f—making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm.
Fusion
After the sperm enters the cytoplasm of the oocyte, the cortical reaction takes place, preventing other sperm from fertilizing the same egg. The oocyte now undergoes its second meiotic division producing the haploid ovum and releasing a polar body. The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material.
Cell membranes
The cell membranes of the secondary oocyte and sperm fuse.
Transformations
In preparation for the fusion of their genetic material both the oocyte and the sperm undergo transformations as a reaction to the fusion of cell membranes.
The oocyte completes its second meiotic division. This results in a mature ovum. The nucleus of the oocyte is called a pronucleus in this process, to distinguish it from the nuclei that are the result of fertilization.
The sperm's tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.
Replication
The pronuclei migrate toward the center of the oocyte, rapidly replicating their DNA as they do so to prepare the embryo for its first mitotic division.
Mitosis
The male and female pronuclei don't fuse, although their genetic material do. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms between them. The spindle captures the chromosomes before they disperse in the egg cytoplasm. Upon subsequently undergoing mitosis (which includes pulling of chromatids towards centrioles in anaphase) the cell gathers genetic material from the male and female together. Thus, the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes.
Each of the two daughter cells resulting from that mitosis has one replica of each chromatid that was replicated in the previous stage. Thus, they are genetically identical.
Diseases
Various disorders can arise from defects in the fertilization process.
Polyspermy results from multiple sperm fertilizing an egg.
However, some researchers have found that in rare pairs of fraternal twins, their origin might have been from the fertilization of one egg cell from the mother and two sperm cells from the father. This possibility has been investigated by computer simulations of the fertilization process.
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