Describe the process of fertilization: include the major steps, location of event and specific str

Fertilization occurs when sexual intercourse allows sperm to travel through the female's vagina through the cervix and the uterus to meet the mature egg. Although males can release 200-300 million sperm, it only takes one to penetrate the egg.

After fertilization in the female's fallopian tube, the nuclei of the sperm and egg fuse to form a new cell (the zygote). This cell contains 46 chromosomes, 23 from each parent cell. As the zygote travels to the uterus, it divides, forming a cluster of cells (the morula) by about 3 days after fertilization.

The morula develops a cavity and is now known as a blastocyst, which will become the embryo. This blastocyst floats freely within the uterine cavity for about 48 hours before attaching itself to a site in the endometrium (uterine lining).

About 10 days following fertilization, the blastocyst is completely imbedded into the endometrium, and forms the placenta. Within the cell cover of the blastocyst's cavity, it then develops into a fluid- filled sac covering the embryo, and the yolk sac.
RU486 is a synthetic steroid which works by blocking the effects of progesterone, the natural hormone which is required to maintain the lining of the uterus during pregnancy. RU486 starves the womb of progesterone, the lining of the womb breaks down, and it is lost along with the developing embryo or foetus.

RU486 (mifepristone) has proved to be a remarkably active antiprogesterone and antiglucocorticosteroid agent in human beings. The mechanism of action involves the intracellular receptors of the antagonized hormones (progesterone and glucocorticosteroids). At the molecular level, the most important features are high binding affinity to the receptor, interaction of the phenylaminodimethyl group in the 11 beta-position with a specific region of the receptor binding pocket, and RU486-induced transconformation differences in the ligand-binding domain. These particularities have consequences at different steps of the receptor function as compared with agonists. However, the reasoning cannot be limited to the RU486-receptor interaction, and, for instance, there is the possibility of a switch from antagonistic property to agonist activity, depending on the intervention of other signaling pathways. It would be desirable to have derivatives with only one of the two antagonistic properties (antiprogestin, antiglucocorticosteroid) in spite of similarities between steroid structures, receptors involved, and responsive machineries in target cells. Clinically, the RU486-plus-prostaglandin method is ready to be used on a large scale and is close to being as convenient and safe as any medical method of abortion may be. The early use of RU486 as a contragestive as soon as a woman fears a pregnancy she does not want will help to defuse the abortion issue. Research should now be conducted to define an efficient and convenient contraceptive method with RU486 or other antiprogestins. The usefulness of RU486 for obstetric indications, including facilitation of difficult delivery, has to be assessed rapidly. Gynecologic trials, particularly in leiomyomata, should be systemically continued. The very preliminary results obtained with tumors, including breast cancers, indicate that further studies are necessary.