The opening at the lower end of the uterus is a constricted area called the cervix. The tube leading from the cervix to the exterior is a muscular organ called the vagina. During periods of sexual arousal, the vagina receives the penis and the semen. The sperm cells in the semen pass through the cervix and uterus into the Fallopian tubes, where fertilization takes place.
In the human female, egg cell production begins before birth, when about 2 million primitive cells known as oogonia accumulate in the ovaries. These oogonia are formed in the early stages of meiosis. After the age of puberty, the oogonia develop into primary oocytes and then into egg cells at a rate of one per month. Egg cell production occurs by the process of meiosis.
The egg cells develop within the ovary in a cluster of cells called the Graafian follicle, which secretes female hormones called estrogens that regulate the development of secondary female characteristics. Egg cell development within the follicle requires approximately 14 days. The development is controlled by two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both hormones are secreted by the anterior lobe of the pituitary gland.
During the 14 days of egg cell development, the endometrium increases its supply of blood and nutrients in anticipation of a fertilized egg cell. On about the 14th day, the release of the egg cell from the follicle takes place. This process is called ovulation. The egg cell is swept into the Fallopian tube and begins to move toward the uterus. Meanwhile, the follicle is changed into a mass of cells known as the corpus luteum. The LH stimulates the conversion. The corpus luteum then secretes the hormone progesterone, which together with estrogens continues to regulate the buildup of tissue in the endometrium and inhibit contractions of the uterus.
The egg cell remains alive in the Fallopian tubes for 24 to 72 hours. If fertilization by a sperm cell fails to occur, the egg cell moves toward the uterus, and the corpus luteum begins to degenerate. This degeneration causes the level of progesterone and estrogen to drop off. Within two weeks, the hormone level declines to a point where it cannot inhibit contractions of the uterus. Uterine contractions then occur, and the endometrium is released in the process of menstruation. Follicle development begins again, but in the opposite ovary.