Figure 42.12 illustrates the human female reproductive system. Eggs are produced in and released from the ovaries located on each side of the lower abdominal cavity. When an egg leaves the ovary (ovulation), it enters the abdominal cavity, but it does not go far. The ovaries are close to the openings of the oviducts (also known as the Fallopian tubes). The openings are surrounded by undulating, fringed tissues called fimbria that sweep the egg into the oviduct. Cilia lining the oviduct propel the egg toward the uterus, a muscular, thick-walled cavity shaped in humans like an upside-down pear. The uterus is where the embryo will develop if the egg is fertilized. At its bottom, the uterus narrows into a region called the cervix, which opens into the vagina.

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Externally, the vagina is enclosed by two sets of skin folds. The inner, more delicate folds are the labia minora; the outer, thicker folds are the labia majora. At the anterior tip of the labia minora is the clitoris, a small bulb of highly sensitive erectile tissue that has the same developmental origins as the male glans penis. Clitoral tissues, homologous to the erectile tissues of the penis extend deep under the labia minora and the walls of the vagina. The labia minora and the clitoral tissues become engorged with blood in response to sexual stimulation. Just as stimulation of the glans penis during sexual intercourse culminates in orgasm in the male, stimulation of the clitoral tissues results in orgasm in the female.

The external opening of an infant’s vagina is usually, but not always, partly covered by a thin membrane, the hymen. Eventually the hymen can be torn by vigorous physical activity or by first sexual intercourse; it can sometimes make first intercourse difficult or painful for the woman.

Sperm deposited in the vagina swim and are propelled by contractions of the female reproductive tract through the cervical opening, across the uterus, and most of the way up an oviduct, where fertilization takes place. Before meeting a sperm, the egg is still a secondary oocyte. Fertilization stimulates the egg to complete its second meiotic division. Following that division, the haploid nuclei of the sperm and the egg can fuse to produce a diploid zygote nucleus.

The *zygote undergoes its first few cell divisions to become a blastocyst while still in the oviduct. The blastocyst moves down the oviduct to the uterus, where it attaches itself to the epithelial lining of the uterus—the endometrium. Once attached, the blastocyst implants in the endometrium and interacts with it to form the placenta, as you will see in Chapter 42. The mother’s body nurtures the embryo through the placenta, which also produces hormones that sustain pregnancy.

The maturation of eggs and ovulation are cyclical events in the ovaries. Changes in the uterus are also critical processes. These cycles in humans average about 28 days, but they can vary considerably in an individual and among different individuals. In anticipation of receiving a blastocyst, the endometrium thickens and develops lots of blood vessels. The endometrium regresses if a blastocyst does not arrive within a certain window of time. Thus female reproductive functions consist of two linked cycles: an ovarian cycle that produces eggs and hormones; and a uterine, or menstrual, cycle that prepares the endometrium for the arrival of a blastocyst (Focus: Key Figure 42.13). The two cycles must be synchronized so that a blastocyst arrives in the uterus at the optimal time to implant in the endometrium and continue its development.

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