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Sperm Capacitation

Freshly ejaculated sperm are unable or poorly able to fertilise. Rather, they must first undergo a series of changes known collectivelly as capacitation. This process takes several hours. It involves changes in the membrane covering the acrosome, such that small pores open. Capacitation is associated with removal of some seminal plasma proteins, reorganisation of plasma membrane lipids and proteins. Most importantly however, capacitation appears to destabilise the sperms membrane to prepare it for the acrosome reaction. Once capacitation has occurred, it is known that the flagellum of the sperm (tail of the sperm) moves faster to propel the sperm forward even quicker.

The acrosome reaction

The secondary oocyte which is discharged at ovulation is surrounded by a number of follicle cells which form the corona radiate. Within this a glycoprotein barrier called the zone pellucida. When a sperm reaches the oocyte, contact with the zone pellucida results in the acrosome membrane rupturing releasing hyaluronidase and protease enzymes. These enzymes soften the layers of the cells surrounding the oocyte and digest away the zone pellucida and corona radiate. Inversion of the acrosome results in a fine needle like filament developing at the tip of the spam and this pierces the already softened portion of the membrane. The whole process is called the acrosome reaction and it enables the sperm to penetrate the egg. This entry stimulates the reactions of the oocyte that brings about the formation of the fertilisation membrane (altered/ thickened zone pellucid) preventing the entry of further sperm. Entry of the sperm also stimulates the completion of the second meiotic division of the oocyte nucleus. Only one sperm enters the oocyte. The nucleus of the secondary oocyte undergoes meiosis 2 and forms an ovum and a secondary polar body. The nuclei of the ovum and sperm are drawn together and fuse to form a diploid zygote - fertilisation.

Biology


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