Long before the actual fertilization and immediately after, several events must succeed: making gametes, pathways, proper fertilization, segmentation and implantation.
Sperm and oocytes :
First, it is necessary that the gametes or reproductive cells called sperm and oocytes in humans, are present.
In humans, production of sperm in the testes begins in adolescence and continues normally throughout life. This "production" or spermatogenesis takes place in the seminiferous tubules of testes from stem cells or spermatogonia. It occurs in the lining of the tubes during 74 days before the sperm are released into the center of the seminiferous tubules. At this stage, they contain genetic material adequate or 23 chromosomes, but they do not yet have any mobility or all of their fertilizing capacity. Only during their journey through the epididymis, where they are stored between ejaculations, they will achieve full maturity. Sperm constitute only a small fraction of the volume of ejaculate (2.5 ml on average) the largest part consists of seminal plasma from the seminal vesicles and seminal fluid secreted by the prostate.
In women, the maturation of gametes or oocytes, occurs in the ovaries from a stockpile in the womb. The maximum number of about 7 million is reached in the 3rd trimester of pregnancy.From this stage, the population of oocytes gradually decreasing during puberty, there are only 400 000 and a few thousand just before menopause. In the ovary, oocytes are surrounded by follicular cells that are transformed as well. All form a follicle. From puberty to menopause, during each ovulatory cycle, a mature follicle ruptures and releases an egg which still contains 46 chromosomes. Half will be deported upon contact with a sperm, making fertilization possible.This dominant follicle began developing its final 85 days before ovulation, with the departure of many follicles, all forming a cohort. Only the dominant follicle reached maturity. Other halt their growth at different stages.
If intercourse takes place during a period of fertility, 200 to 300 million sperm are deposited in the vagina after ejaculation encounter a supportive environment. The most mobile of them will be back in the cervical canal of the uterus through the mucus that ensures their survival and migration capability. The few hundred sperm which travel in the tube were acquired after the passage of the uterine cavity, their fertilizing capacity: the capacitation. They are attracted to a dilated portion of the tube, the bulb, where the oocyte is surrounded by a cell mass: the cumulus.
The end of the tube is indeed provided with fringes to capture the egg from an ovary removed.During the last hours before ovulation itself, the dominant follicle, which measures 16 to 20 mm in diameter, deforms the surface of an ovary. He has received major hormonal stimuli making the oocyte capable of being fertilized. They allow the rupture of the follicular wall and expulsion of the oocyte which has a lifetime of only 8 to 10 hours.
In the tube, the sperm capacity through the cumulus cells come and join a second cell barrier surrounding the oocyte: zona pellucida. This causes a reaction at the head of some sperm which will release their enzymes to cross the zona pellucida. The first sperm that will cross merges with the cell membrane of the oocyte, resulting in a few seconds ofPregnancy.