Reproductive system of female mammals

The function of the mammalian female reproductive system, in cooperation with the male reproductive system, is to produce offspring. The role of the female is very complex: She must produce gametes (sex cells) called ova (singular, ovum) or eggs, provide the site for the combination of ova with sperm from the male (fertilization), and nourish and protect a developing fetus during pregnancy. She also must provide for the delivery of offspring from her body to the outside. These functions are carried out by a group of organs or structures and a number of chemicals called hormones. The major organs of the system include ovaries, which produce ova and hormones; uterine (Fallopian) tubes that transport the ovum and provide the site of fertilization; the uterus, which houses the developing offspring; the vagina, which receives the male penis and sperm during sexual intercourse and also functions as a birth canal; and the external genitals. Hormones important to the function of the female reproductive system include estrogen and progesterone from the ovaries, and follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. Release of FSH and LH is under control of chemicals called releasing factors, which are produced by a small region of the brain called the hypothalamus.

The Ovaries

The paired ovaries are the primary sex organ, or gonad, of the female. They are analogous to the testes in the male reproductive system and actually develop from the same tissue. The size and shape of the ovary depend on the age and size of the female and whether the female usually has a single offspring or several at one time. Before birth, small groups of cells called follicles are formed in each ovary. In the center of each follicle is a single large cell called an oocyte, which is able to mature into an ovum. In other words, all the oocytes a female will ever produce were already in place before she was born. The ova are special cells; they are formed by a process (meiosis) that results in a cell with only half of the chromosomes found in other body cells. The only other cells that divide by this process are those that form sperm in males. When an ovum and sperm unite, each cell contributes half the necessary chromosomes to make a new complete cell. This new cell, the first cell of an offspring, will have characteristics of each parent. The follicles develop in response to the hormone FSH from the anterior pituitary gland. A mature follicle releases its mature ovum through the wall of the ovary into the pelvic cavity. This process is called ovulation and is controlled by LH and FSH.

The ovary also produces the female sex hormones—estrogen and progesterone. Estrogen is produced by maturing follicle cells. In addition to causing growth of the sex organs at puberty and stimulating growth of the uterine lining each month, estrogen is responsible for the appearance of female secondary sex characteristics. The follicle cells that are left behind following ovulation form a structure called the corpus luteum, which produces both estrogen and progesterone. The most important function of progesterone is to stimulate the lining of the uterus to complete its preparation for pregnancy.

The Accessory Organs

The rest of the internal structures of the reproductive system are called accessory organs. The first of these is a pair of uterine (Fallopian) tubes, or oviducts, which extend from each ovary into the uterus. They are frequently shaped like funnels, with fingerlike ends, called fimbria, that partially surround each ovary. Movements of the fimbria sweep the ovum and some attached cells into the uterine tube following ovulation. If fertilization is to take place, it will be in the uterine tube.

The uterus in most mammals consists of two horns and a body, although much variation occurs. Marsupials, mammals that have pouches, such as the opossum, have two separate uteri, each opening to the outside through a separate vagina. Rats, mice, and rabbits have uteri with two horns. Primates have simple uteri with no horns. The uterus has an amazing ability to expand during pregnancy. In all cases, the wall of the uterus is thick and muscular. This muscle layer, the myometrium, is able to contract rhythmically and powerfully to move the young down the birth canal and out of the mother’s body during the birth process. The lining of the uterus is called the endometrium.

The uterus narrows down into a muscular, necklike region called the cervix. This structure acts like a valve to keep the opening into the uterus closed most of the time. This prevents bacteria and other harmful objects from entering. The final internal accessory structure is the vagina, a thin-walled muscular tube. The vagina surrounds the cervix of the uterus at its anterior end and extends to its opening to the outside of the body. It allows for childbirth, sexual intercourse, and, in primates, menstrual flow. The walls of the vagina normally touch one another and have deep folds that allow for stretching without damage. A thin fold of tissue called the hymen partially covers the external opening of the vagina. This structure has no function and varies considerably in different mammals.

The external structures of the female reproductive system are called the external genitals. These include the labia majora, labia minora, and clitoris. Two thick, hair-covered folds of skin, the labia majora, protect and enclose other structures. In some mammals, two smaller hair-free folds of skin are located within the labia majora. These folds, the labia minora, are very prominent in primates but small in most other mammals and completely lacking in some. They enclose a region called the vestibule. Within the vestibule are located the clitoris, the external opening from the urinary system (the urethra), and the external opening of the vagina. The clitoris is a small structure almost covered by the anterior ends of the labia minora. It is very sensitive, being richly supplied with nerve endings and blood vessels.

Sexual Maturity

Reproduction can occur only after females reach sexual maturity. In mammals, this requires the full development of the reproductive structures. The point at which maturity is attained is ultimately under the control of the hypothalamus, as it controls the release of FSH and LH. Many factors, such as attainment of a particular body weight, other physiological and psychological factors, temperature, day length, and climate may influence the release of hormones.

After the female reaches maturity, reproductive activities are cyclic. In mammals, there are two different kinds of reproductive cycles. Most mammals have an estrus cycle in which females will mate with a male only if they are “in heat,” which happens at certain restricted times. An estrus cycle is divided into stages: an inactive phase, called anestrus, which may last for days, weeks, months, or years; proestrus, during which the follicles are developing; estrus, when ovulation occurs; and metestrus, when the ova are moving into the oviduct. Females mate, and may become very aggressive about finding a mate, during estrus only. Usually, ovulation is triggered by LH from the pituitary gland. In some mammals, including cats and rabbits, ovulation does not occur until the animal mates. Many females signal that they are in estrus. The signals may be chemical—a special scent which carries for a long distance, for example—or visual. Chimpanzees, for example, develop pink swollen skin on the external genitals during estrus.

The Menstrual Cycle

Primates have a menstrual cycle instead of an estrus cycle. The menstrual cycle is coordinated by estrogen and progesterone from the ovary. These hormones, in turn, are controlled by FSH and LH from the anterior pituitary gland, so all the functions of the reproductive structure are coordinated and synchronized. The three stages of the menstrual cycle are the menses, proliferation, and secretion stages.

In menses, the thick endometrium is sloughed off and flows out of the uterus and out of the body through the vagina. This is also called the menstrual flow or menstrual period. The menstrual fluid consists of roughly equal parts of blood and other accumulated bodily fluids. In the proliferation phase, the endometrium again grows thick. Ovulation occurs in the ovary at the end of this stage, following a sudden increase in the release of LH from the anterior pituitary gland. In the secretion stage, the endometrium becomes very thick and cushiony and prepared to nourish a developing embryo if fertilization has occurred. If fertilization does not occur, the endometrial cells die, and the cycle begins again. These stages are controlled by estrogen and progesterone from the ovary. Female primates will mate throughout the entire menstrual cycle.

Studying the Female Reproductive System

Detailed examination of individual reproductive tissues is performed using a variety of very thin tissue slices, various dyes and stains, and microscopes. Frequently, preserved tissue is used. Electron microscopes have made it possible to magnify single cells, or parts of cells, several thousand times to observe minute details of structure. Fresh tissue is also examined. It is possible to freeze a small tissue sample quickly, slice it very thin, and then expose the tissue to chemicals, which can add to researchers’ understanding of the function of particular cells.

The study of reproductive hormones and the understanding of their function demand the use of many different methods. Again, much information comes from nonhuman studies. The procedures vary widely, but a typical laboratory experiment may involve removing the ovaries from a female rat and then injecting small amounts of estrogen or progesterone to observe the response of the endometrium. It is also possible to use chemicals that block, or inhibit, one or more specific hormones. By creating an abnormal, controlled situation and observing the results, an understanding of the role of individual hormones within a complex interrelated system can be obtained.

It is also frequently necessary to measure how much hormone is present in some bodily fluid, either for research to gain understanding of normal function or for medical diagnosis. This is very difficult, as most hormones occur in very minute concentrations. Procedures called radioimmunoassay (RIA) techniques, introduced during the late 1950s and early 1960s, represent a very important advance in the study of hormone concentrations. These procedures, which use special recognition molecules for each hormone, plus certain hormones that have been purified and made radioactive, make it possible to measure levels of hormones as low as one trillionth of a gram (a picogram).

In a system as complex in its function as the female reproductive system, it is not surprising that information is obtained from a variety of sources. Each technique has contributed to an understanding of the whole system.

Understanding the Female Reproductive System

The reproductive system is unique among all body systems. It is the only system not called upon to function continuously for the well-being of the individual. It is nonfunctional during the early part of the female’s life, then is activated by chemical messages from the anterior pituitary gland. Its primary function is not, after all, the well-being of one individual but rather the continued existence of the species. It is also unique in that it must interact with another individual, a male, to fulfill this function. Throughout the reproductive years, all the functions of the female reproductive systems are directed toward pregnancy.

Controlling Reproduction

While most female mammals can only reproduce during certain times and cycles, like the estrus and menstrual cycles, there are ways to further control and dictate reproduction, which can serve a variety of purposes. In humans, the use of contraceptives, informally known as birth control, can block or prevent pregnancy. Hormonal contraceptives use hormones to prevent the ovaries from releasing an egg during a menstrual cycle. Depending on the method of ingestion and the dosage, bleeding may still occur. Some methods, though, such as injections and intrauterine device (IUD) implantation, may greatly lessen or eventually stop monthly bleeding. Some types of birth control, like copper IUDs, do not contain hormones and instead release copper, which creates a toxic environment for the sperm and prevents them from reaching the egg.

Certain animal species that can overpopulate, especially in residential areas, are often subject to spaying, or ovariohysterectomy, the removal of the reproductive organs. Domesticated female dogs and cats are typically spayed in order to avoid unwanted or offspring or an abundance of litters. Additionally, spaying pets can help prevent certain diseases and cancers. A potential negative side effect of spaying is weight gain due to increased metabolism and maturation. However, such changes can be monitored and food intake can be adjusted to keep the pet a healthy weight.

Principal Terms

Anterior Pituitary Gland: The front portion of the pituitary gland, which is attached to the base of the brain; the source of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)

Estrus Cycle: Hormonally controlled changes that make up the female reproductive cycle in most mammals; ovulation occurs during the estrus (heat) period

External Genitals: The external reproductive parts of the female

Gonad: The primary reproductive organ (the ovary in females and the testes in males), which produces sex cells (gametes) and sex hormones

Menstrual Cycle: A series of regularly occurring changes in the uterine lining of a nonpregnant primate female that prepares the lining for pregnancy

Ovary: The female gonad, which produces ova and the hormones estrogen and progesterone

Ovum (pl. ova): The female reproductive cell (gamete); a mature egg cell

Uterus: The hollow, thick-walled organ in the pelvic region of females that is the site of menstruation, implantation, development of the fetus, and labor

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