Sperm are produced in the seminiferous tubules of the testes and mature in the nearby epididymis. During the ejaculatory process, mature sperm are propelled out of the epididymis and through the vas deferens by peristalsis. Once they reach the ampulla, sperm combine with seminal vesicle secretions to become seminal fluid. Seminal fluid is further mixed with secretions from the prostate and bulbourethral glands before it exits the urethra and enters the anterior vagina. Here, seminal fluid combines with cervical mucus in order to survive the acidic environment of the vagina during ovulation (ovulation lowers the pH of vaginal secretions and thins the otherwise tacky cervical mucus). Uterine contractions help propel the most motile sperm cells into the cervix and, beyond it, a fallopian tube.
Oocytes are produced in the ovaries and transported by the fallopian tubes. During ovulation, an oocyte enters a fallopian tube traveling in the opposite direction as the sperm cells. Fallopian tubes are hostile environments for sperm cells, which are destroyed en masse by the woman’s immune system. Of the approximately 250 million sperm cells present in the ejaculatory fluid, only between a few hundred to a few thousand make it to a fallopian tube, the usual site of fertilization.
The oocyte is surrounded by two membranes, the outer corona radiata, and the inner zona pellucida. In order to reach the ovum, surviving sperm must undergo capacitation, a maturation process that stimulates their whip-like tails, called flagella, and releases digestive enzymes to enable the sperm to break through the zona pellucida. Once a single sperm cell succeeds, the oocyte depolarizes its plasma membrane and destroys its sperm receptors to prevent multiple sperm cells from fertilizing a single oocyte (polyspermy).
The ovum begins to divide over the next 3-4 days as it continues downward toward the uterus. Once there, surface molecules on the ovum bind to the uterine wall, a process called implantation. The uterine lining anticipates this, as it does every month, because it has been prepared for a potential pregnancy by estrogen produced early in the menstrual cycle and progesterone produced during ovulation. If an oocyte is not fertilized, these hormone levels will decrease, and the excess uterine lining will be shed via menstruation. If an oocyte is fertilized, estrogen and progesterone will maintain the uterine environment, stimulate breast tissue growth, and contribute to fetal development.