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Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube. It is also the initiation of prenatal development.
Fertilization constitutes the penetration of the oocyte which the sperm performs, fusion of the sperm and oocyte, succeeded by fusion of their genetic material.
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Penetration
A single sperm penetrates the cell membrane of the oocyte. To reach the oocyte, the sperm must pass through the corona radiata and the zona pellucida; two layers covering and protecting the oocyte from fertilization by more than one sperm.
Corona radiata
The sperm passes through the corona radiata, a layer of follicle cells on the outside of the secondary oocyte.
Acrosome reaction
The acrosome reaction must occur to mobilise enzymes within the head of the spermatozoon to degrade the zona pellucida.
Zona pellucida
The sperm then reaches the zona pellucida, which is an extra-cellular matrix of glyco-proteins. A special complementary molecule on the surface of the sperm head then binds to a ZP3 glyco-protein in the zona pellucida. This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida.
Cortical reaction
When the sperm penetrates the zona pellucida, the cortical reaction occurs: cortical granules inside the secondary oocyte fuses 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, making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm.
Fusion
The sperm fuses with the oocyte, enabling fusion of their genetic material, in turn.
Cell membranes
The cell membranes of the secondary oocyte and sperm fuse together.
Transformations
Both the oocyte and the sperm goes through transformations, as a reaction to the fusion of cell membranes, preparing for the fusion of their genetic material.
The oocyte now 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 new human for its first mitotic division.
Mitosis
The male and female pronuclei don't fuse, although their genetic material do so. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms around them to catch the chromosomes before they get lost in the egg cytoplasm. By subsequently performing a mitosis (which includes pulling of chromatids towards centrosomes 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 have one replica of each chromatid that was replicated in the previous stage. Thus, they are genetically identical.
In other words, the sperm and oocyte don't fuse into one cell, but into two identical cells.
Diseases
Various disorders can arise from defects in the fertilization process.
- Polyspermy results from multiple sperm fertilizing an egg.
