Normally, in the ovary the corpus luteum secrets steroid hormones (estrogen and progesterone), which are responsible for endometrium thickening as well as its development and maintenance. It is one of the most important roles that corpus luteum actually plays.
If the implantation does not take place the secretion of progesterone stops (normally it happens in 12 days) and corpus luteum decays. Corpus ablicans is the result of that degradation and it is in fact a fibrous scar tissue. Afterwards it is expelled through the vagina. The entire process in big mammals is referred too as menstruation.
However, if fertilized, the embryo starts secreting the HCG (human chorionic gonadotropin). Corpus luteum is sensitive to the hormone and it becomes a signal for corpus luteum to continue with progesterone secretion. In this way the thick lining of the uterus is maintained and the area rich in blood vessels preserved for the development of the zygote. It should be noted that this process actually determines the significance of corpus luteum in the early stages of fertilization and explains its biological and biochemical role. At this stage, placenta takes over the production of progesterone. As the result, corpus luteum goes through the degradation into a corpus albicans without the loss of embryo.
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As one cam see from mechanism described above, it is critical that corpus luteum would remain functional during the introduction since it plays the essential role in hormone secretion that allows zygote to develop. For this reason the life circle of corpus luteum is extended till the moment the placenta can take over the progesterone production and corpus luteum can be expelled. (http://www.drdaiter.com/14.html)
Fertilization
Fertilization is the complicated process that involves other stages except for the actual gene sharing. Sperm-egg recognition and gamete interaction are prior to the actual development of the embryo. Those processes are necessary to be examined since they are of the same importance as the fertilization itself.
Sperm-egg recognition is the process based on the presence of certain chemical elements in both egg and sperm. Different in different species they play the same role. ZP-3 actually functions as the sperm receptor that is mediated by oligosaccharide that is critical for sperm-binding activity.
When the sperm binds to the zona the acrosome reaction takes place. This reaction enables the sperm to enter zona. Acrosome reaction release enzymes are able to digest and in combination with for create a slit.
After the sperm penetrates the area, it starts sperm-egg adhesion with the equatorial segment on its head. Fertilin (the sperm protein) is believed to be involved in the process. The molecular domain of fertilin in thought to interact with membrane proteins (integrins). The gamete interaction is actually the result of binding between integrines in the egg plasma membrane and disintegrines in sperm.
Fusion is the last step in the fertilization. It is facilitated by the acrosome reaction, however, the actual mechanism of the fusion is nuclear. Sperm-egg fusion is facilitated by the alterations to the equatorial segment.
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