What is Capacitation?
Posted on September 26, 2007
Filed Under Pregnancy | Leave a Comment
Before fertilization can be accomplished, the sperm must undergo the process of capacitation – the process by which sperm become transformed and thus able to enter the egg cell. Freshly ejaculated spermatozoa are incapable of causing fertilization. Capacitation is accomplished by exposure of the sperm to secretions of the uterus, the fallopian tube, or the ovary’s grafian follicle. Capacitation requires as little as 2 hours in the hamster and as long as 11 hours in the rabbit. It requires about 7 hours in the human being.Use of an electron microscope, which permits magnification from ten thousand to more than one hundred thousand times, has shown that each sperm head is surrounded by two membranes, a plasma membrane closely applied to it and a loose, veil-like outer membrane, called acrosomal membrane. As far as can be observed, a capacitated sperm appears the same as a sperm before capacitation. However, several hours exposure to fluids of the female reproductive tract enables it to undergo the acrosomal reaction, which ruptures the outer membrane surrounding the sperm head and releases enzymes beneath the membrane. These enzymes dissolve cumulus cells, cutting a path through the corona radiata-halo surrounding the egg’s surface. Next, the enzymes must carve out a pathway through the zona pellucida-the egg’s covering.
The egg capsule, the zona pellucida, is relatively firm and rigid, its thickness is approximately one-tenth the diameter of the egg. Precisely how a spermatozoon gets through the capsule is not completely known. One effect of capacitation is to increase the speed of the sperm. This may be important in allowing it to move through the zona. Once a sperm passes through the zona pellucida, the zona undergoes a reaction called the zona reaction. This makes it impervious to other sperm. Only a single sperm-one that has undergone the acrosomal reaction-makes its way through the perivitelline membrane, just beneath the zona pellucida, to pair the twenty-three chromosomes of its nucleus with the twenty-three chromosomes of the nucleus of the egg.
Tags:acrosomal reaction, capacitation, corona radiata, egg capsule, egg cell, enzymes, female reproductive tract, fertilization, plasma membrane, Pregnancy, sperm head, spermatozoon zona pellucida
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