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Fertilization is the process whereby genetic material from a male and female gamete fuse to form a single diploid nucleus. It normally takes place in the ampullary region of the uterine tube.
Shortly after ejaculation, spermatozoa swim from the vagina to the uterine cavity via the cervix, propelled by whip-like motions of their flagella.
Of the 200 million spermatozoa found in each ejaculation, approximately 3-5 hundred will reach the secondary oocyte.
The secondary oocyte cannot be fertilized until capacitation and the acrosomal reaction have taken place, which can take several hours.
During capacitation, two changes occur to the spermatozoa. First, the flagella beat more quickly triggered by the accompanying seminal secretions.
Second, vaginal secretions promote the removal of glycoproteins and seminal plasma proteins from the heads of the spermatozoa, resulting in the membrane becoming more fluid, in preparation for the events of fertilization.
A spermatozoon must penetrate two layers before it can fertilize the secondary oocyte. These layers are the corona radiata, composed of granulosa cells and the deeper zona pellucida, made up of glycoproteins.
Spermatozoa penetrate down to the zona pellucida where they bind to a sperm-receptor molecule called ZP3 glycoprotein, triggering the acrosomal reaction.
During this process, the acrosome releases acrosomal enzymes, which digest the zona pellucida and allow the spermatozoa to cross the perivitelline space, the space between the zona pellucida and the oocyte membrane.
The head of the first spermatozoon to reach the plasma membrane of the secondary oocyte fuses with it. Fusion of the spermatozoon and oocyte membranes activates two mechanisms that help prevent polyspermy, or fertilization of the secondary oocyte by more than one spermatozoon. These are a fast and a slow block to polyspermy.
During fast block to polyspermy, the oocyte membrane depolarizes, preventing other spermatozoa from fusing with the membrane. This activity stimulates slow block to polyspermy.
During slow block to polyspermy, a wave of intracellular calcium is released, causing the secondary oocyte to release cortical granules by exocytosis, rendering ZP3 inactive, and making the zona pellucida impermeable.
Immediately after the fusion of the spermatozoon and oocyte membranes, the nucleus of the spermatozoon containing haploid genetic material and associated centrioles is released into the cytoplasm of the secondary oocyte.
Upon the spermatozoon entering, the oocyte undergoes meiosis II, and develops into the female pronucleus. During this time, the tail of the sperm detaches and degenerates, and the spermatic nucleus develops into the male pronucleus. The two pronuclei then fuse to form a single cell with a diploid nucleus. This cell is known as a zygote.