Fertilization
During breeding the several million sperm are deposited into the vagina near the cervix. The cervix is protected by mucus secretions through which the sperm must pass. Sperm must then cross the uterus, enter a uterine tube and travel to the end nearest the ovary in order to fertilize the ovum (if one is present) while it is still viable. Only a small concentration of sperm will reach the ovum, this is to ensure only the most vigorous sperm play a part in reproduction. Sperm travel fastest immediately after ejaculation and slow down considerably after 2 hours. Whip like motions of the flagella help them to move and their travel maybe assisted by contractile movements of the uterus. A sperm may reach an ovum within an hour after deposition. Sperm can remain alive within the fallopian tubes for 48-72 hours while waiting for an ovum. Sperm cannot fertilize an ovum until it has been exposed to uterine secretions for at least 7 hours. This allows the sperm to undergo biochemical changes. The sperm lose the acrosome cap on their heads which allows digestive enzymes to escape. These enzymes help to break down the layers of the ovum; corona radiate and zona pellucida so that the sperm can penetrate it. Once one sperm breaks through the zona pellucida, a chemical change occurs in the zona that makes it impenetrable to all other sperm. This ensures that each ovum is only fertilized by one sperm. Within 12 hours of the sperm entering the ovum, the nuclear membranes of the ovum and the sperm fuse and fertilization is complete. The rest of the sperm then atrophies. A zygote forms and cell division begins.
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