Capacitation

Sperm are being nourished all the time by secretions in the seminal fluid and cervical mucus. For example, one chemical from the epididymis (called carnatine) helps mature the sperm, while the zinc-rich prostatic secretions protect the sperm from infection. Sperm also benefits from certain chemicals in the cervical and uterine fluids, which make it possible for the sperm to penetrate the ovum before actual fertilisation. This process of simulation, known as capacitation, causes the sperm head’s outer layer (the acrosome tip) to leak some potent enzymes that help to dissolve the membrane surrounding the ovum (the zona pellucida). Capacitation also can be stimulated in the laboratory to make test-tube fertilisation more successful.

The point of contact

Once some capacitated sperm make contact with the ovum, their enzymes break down the zone pellucida until one sperm head (containing the 23 chromosomes from the father’s gene pool) enters the ovum, leaving its tail behind in the process. The penetrating sperm stimulates the ovum to finish the second meiotic division so that the 23 chromosomes from the mother’s gene pool appear. At this moment, all other sperm are excluded and the genentic material from the father’s sperm and mother’s ovum fuse together to form a new and unique combination. The newly fertilised ovum now contains 46 chromosomes with all the genetic information it needs to divide, grow and develop into a perfectly formed human body.

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