Female Reproductive System
In the female reproductive system, the menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive-age females. Overt menstruation occurs primarily in humans and close evolutionary relatives such as chimpanzees. The females of other species of placental mammal have estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. The menstrual cycle is under the control of the hormone system and is necessary for reproduction. Menstrual cycles are counted from the first day of menstrual flow, because the onset of menstruation corresponds closely with the hormonal cycle. The menstrual cycle may be divided into several phases, and the length of each phase varies from woman to woman and cycle to cycle. The main phases are the follicular phase, ovulation, the luteal phase, and the menstrual phase. During the follicular phase the lining of the uterus thickens, stimulated by gradually increasing amounts of estrogen from the pituitary gland. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two follicles become dominant (non-dominant follicles atrophy and die). The dominant follicle releases an ovum or egg in an event called ovulation. An egg that is fertilized by a spermatozoon will become a zygote, taking one to two weeks to travel down the fallopian tubes to the uterus. If the egg is not fertilized within about a day of ovulation, it will die and be absorbed by the woman's body. After ovulation the remains of the dominant follicle in the ovary become a corpus luteum. This body has a primary function of producing large amounts of progesterone. Under the influence of progesterone, the endometrium (uterine lining) changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within approximately two weeks, the corpus