Fertilization - Pre-Embryonic Development
Ovulation generally takes place between 9th to 14th day after the end of last menstrual period. Fertilization of oocyte initiates development and should take place within about 24 hours after ovulation; or else the oocyte degenerates. The female reproductive tract plays a very active role in the fertilization process. Newly ejaculated mammalian sperms are not able to undergo acrosomal reaction unless they reside in the female reproductive tract for some time. This need is known as capacitation. During this process the sperm membrane cholesterol is thought to be reduced like some of it is taken up by albumin molecules present in the female reproductive tract. With no these changes in the membrane the acrosomal reaction cannot occur. Except for this, there are coating factors or inhibitors that are to be removed that would otherwise prevent fertilization.
To reach the ovum the sperm has to move upwards by the uterus aided by its tail as well as the secretions of the cervix which contains a protein mucin. This forms threadlike highways all along which the sperm travels. While the sperm reaches the ovum (secondary oocyte) it invades the follicular cells. An enzyme hyaluronidase is released from the acrosome of the sperm head. This enzyme take out the extracellular matrix and disperses the corona radiata cells. Another enzyme acrosin digests a path for the sperm through the zona pellucida. In the acrosome of the sperm this trypsin like enzyme is inactive and requires to be activated by a glycoprotein in the female reproductive tract. Once a sperm passes by the zona pellucida, this covering becomes impenetrable by other sperms. This is termed as the zona reaction. The structure of the membrane changes and lysosomal enzymes are released from the secondary oocyte which prevents other sperms from attaching to the membrane. Usually only one sperm enters the ovum. Two sperms may take part in fertilization under an abnormal process known as dispermy. The resulting embryo consists of 69 chromosomes and may appear normal, but it is always aborted. Occasionally a triploid infant may be born but it dies shortly.
Thus polyspermy does not generate viable embryos. As the sperm head enters the egg it loses its tail. The secondary oocyte completes its second meiotic division making a mature ovum and a second polar body. The nuclei of the sperm and ovum fuse to form the zygote completing the process of fertilization. The offspring's sex is ascertained at fertilization by the type of sperm that enters the ovum. You would recall that all female gametes bear X chromosome while the male gametes may bear either the X or the Y chromosome. If the ovum is fertilized by a sperm bearing X chromosome it will develop into a female; and into a male child if it is fertilized by a chromosome bearing the Y chromosome. Which type of sperm may fertilize the ovum de ends entirely on chance. Therefore, whether the offspring will be a boy or a girl is the genetic responsibility of the father rather than of the mother.