Following the deposition of sperm in the vagina at the cervix, it is subsequently transported to the uterus. Here it fertilises the ovum and implants in the uterine stroma, and this process is known as conception.
Transport of sperm
Immediately after ejaculation, the semen enters the vagina. Within a further 5 to 10 minutes, the sperm have travelled through the vagina and uterus to the ampulla in the fallopian tube. Approximately half a billion sperm are ejaculated into the vagina, but only a tiny percentage (usually a few thousand) successfully reach the site of fertilisation.
In addition to its propulsive activity, the movement of sperm is facilitated by uterine and uterine tube contractions, which is stimulated by prostaglandins present in the semen and oxytocin released during female orgasm.
Capacitation of sperm
For a sperm to successfully penetrate the oocyte, it must first undergo a process called capacitation. This is the final step in the maturation of the sperm that increases its mobility and prepares for the acrosome reaction.
Capacitation is a biochemical event that reorganises the sperm cell membrane. This results in the tail movement changing from a beat-like action to a thrashing whip-like action, which helps to propel the sperm forward. Capacitation is facilitated by the removal of the protein coat of sperm, which exposes the enzymes required for the acrosome reaction.
Fertilisation usually takes place in the ampulla of the uterine tubes. The first step in the process is the penetration of the granulosa cell layers surrounding the ovum) by the sperm (the corona radiata). Following this, the sperm then penetrates the glycoprotein membrane around the ovum (the zona pellucida).
Once the sperm contacts the corona radiata, the acrosome reaction is initiated. The acrosome is the cap-like structure that overlies the head of the sperm cell. It contains large quantities of hyaluronidase and proteolytic enzymes, which help the sperm break through the corona radiata and the zona pellucida.
Following the penetration of the zona pellucida by a sperm, the properties of the zona pellucida and plasma membrane change so that other sperm are no longer able to enter. The entry of a sperm into the ovum results in changes in the zona pellucida and plasma membrane that prevent further sperms from entering. This is referred to as the zona reaction.
Once the sperm is inside the ovum, the oocyte divides again. This expels a second polar body and results in the formation of the mature ovum. The nucleus of this mature ovum is referred to as the female pronucleus, which contains single copies of 23 chromosomes. The male pronucleus is formed by the swollen head of the sperm. The 23 unpaired chromosomes of these male and female pronuclei then align along the centre of the cell to form a full chromosomal complement in the fertilised ovum. At the same time, usually within 30 minutes of the sperm entering the ovum, the two cell membranes fuse to form a single cell that is referred to as the zygote. Following fertilisation, the zygote undergoes several changes, which will result in the implantation within the wall of the uterus in a successful pregnancy.
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