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Details fertilization and the natural processes involved in achieving pregnancy.

Fertilization (Fertilisation)

The term fertilization describes the joining of the egg and sperm. It occurs in the outer third of the Fallopian tube near the ovary. It usually occurs within 36 hours of ovulation.

How do the sperm reach the Fallopian tube?

An average man ejaculates anything between 100 to 800 million sperm into the vagina. Some of these sperm trickle out of the vagina. Most just remain inside the vagina, where they rapidly die because of the vaginal acidity. A small proportion of sperm find their way into the cervix (the neck of the womb) which lies at the top of the vagina. Once sperm have entered the mucus, the majority will stay there; the cervical mucus acts as a reservoir. Over the next few days, a few sperm will swim up through the mucus and into the cavity of the womb. The other function of the cervical mucus is to increase the ability of the sperm to penetrate and fertilize the egg (this is known as sperm capacitation). The release of sperm from the mucus will continue until the mucus becomes thick. From the cavity of the womb the sperm will travel to the Fallopian tube. It is possible that the contraction of the womb acting like a pump may help in this transport mechanism. Sperm can be found in the Fallopian tube about 15 minutes after having been deposited into the vagina.Of the millions of sperm in the ejaculate, only a few hundred will make the trip to the egg successively.

How a human egg captures a sperm and how a sperm recognizes an egg?

A sperm recognises an egg when proteins on the head of the sperm meet and match a series of specific sugars in the egg's outer coating. The outer coat of the egg is sticky to help the egg and sperm bind together. As the sperm reaches the egg, the shell which covers the head of the sperm begins to dissolve; this will allow the sperm to penetrate the outer coat of the egg (known as the zona pellucida). As the sperm enters the egg its tail is left behind. As soon as one sperm enters the egg, the egg produces a chemical barrier that prevents any other sperm, of the thousands surrounding it, from penetrating the egg. The Zona Pellucida hardens after fertilization in order to prevent the occurance of other sperm entering the egg and protect the migrating embryo integrity from mechanical stress until its implantation.

Although one sperm is required for fertilization, the enzymes from many sperm are required to breakdown the zona pellucida to allow the fertilizing sperm to penetrate the egg.

Once the egg is fertilized, it is known as a zygote.

Zygote - two pronucleate embryo

The zygote moves down the Fallopian tube towards the uterus by a combination of rhythmic contractions of the muscular wall, and the action of the cilia. The tube also supplies foodstuffs to the developing early embryo and removes waste products. The fertilized egg divides into two cells, each of these divide into two more and so on. The division occurs at intervals of about 15-18 hours. By the time it enters the uterus it has about 64 cells.

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