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Hormonal Control of Female Reproductive

Functions

The hypothalamus, the anterior pituitary gland, and the ovaries secrete hormones that control development and maintenance of female secondary sex characteristics, maturation of female sex cells, and changes that occur during the monthly reproductive cycle.

Female Sex Hormones

A girl's body is reproductively immature until about ten years of age. Then, the hypothalamus begins to secrete increasing amounts of GnRH, which, in turn, stimulate the anterior pituitary gland to release the gonadotropins FSH and LH. These hormones play primary roles in controlling female sex cell maturation and in producing female sex hormones.

Several tissues, including the ovaries, the adrenal cortices, and the placenta (during pregnancy), secrete female sex hormones. These hormones include the group of estrogens (es'tro-jenz) and progesterone (pro-jes'ti-ron). Estradiol is the most abundant of the estrogens, which also include estrone and estriol.

The primary source of estrogens (in a nonpregnant female) is the ovaries, although some estrogens are also synthesized in adipose tissue from adrenal androgens. At puberty, under the influence of the anterior pituitary gland, the ovaries secrete increasing amounts of the hormone. Estrogens stimulate enlargement of accessory organs including the vagina, uterus, uterine tubes, and ovaries, as well as the external structures, and is also responsible for the development and maintenance of female secondary sex characteristics. These are listed in figure 22.32 and include the following:

1. Development of the breasts and the ductile system of the mammary glands within the breasts.

2. Increased deposition of adipose tissue in the subcutaneous layer generally and in the breasts, thighs, and buttocks particularly.

3. Increased vascularization of the skin.

The ovaries are also the primary source of progesterone (in a nonpregnant female). This hormone promotes changes that occur in the uterus during the female reproductive cycle, affects the mammary glands, and helps regulate secretion of gonadotropins from the anterior pituitary gland.

Certain other changes that occur in females at puberty are related to androgen (male sex hormone) concentrations. For example, increased growth of hair in the pubic and axillary regions is due to androgen secreted by the adrenal cortices. Conversely, development of the female skeletal configuration, which includes narrow shoulders and broad hips, is a response to a low concentration of androgen.

Female athletes who train for endurance events, such as the marathon, typically maintain about 6% body fat. Male endurance athletes usually have about 4% body fat. This difference of 50% in proportion of body fat reflects the actions of sex hormones in males and females. Testosterone, the male hormone, promotes deposition of protein throughout the body and especially in skeletal muscles. Estrogens, the female hormones, deposit adipose tissue in the breasts, thighs, buttocks, and the subcutaneous layer of the skin.

U What stimulates sexual maturation in a female?

^9 Name the major female sex hormones.

^9 What is the function of estrogens?

□ What is the function of androgen in a female?

Female Reproductive Cycle

The female reproductive cycle, or menstrual cycle (men'stroo-al si'kl), is characterized by regular, recurring changes in the endometrium, which culminate in menstrual bleeding (menses). Such cycles usually begin near the thirteenth year of life and continue into middle age, then cease.

Hypothalamus

Women athletes may have disturbed menstrual cycles, ranging from diminished menstrual flow (oligomenorrhea) to complete stoppage (amenorrhea). The more active an athlete, the more likely are menstrual problems. This effect results from a loss of adipose tissue and a consequent decline in estrogens, which adipose tissue synthesizes from adrenal androgens.

GnRH

FSH, LH (gonadotropins)

Hypothalamus

GnRH

FSH, LH (gonadotropins)

Bloodstream

Estrogens inhibit oversecretion of gonadotropins

Bloodstream

Gonadotropins

A female's first menstrual cycle (menarche) occurs after the ovaries and other organs of the female reproductive control system mature and respond to certain hormones. Then, the hypothala-mic secretion of gonadotropin-releasing hormone (GnRH) stimulates the anterior pituitary gland to release threshold levels of FSH (follicle-stimulating hormone) and LH (luteinizing hormone). As its name implies, FSH stimulates maturation of an ovarian follicle. The granulosa cells of the follicle produce increasing amounts of estrogens and some progesterone. LH also stimulates certain ovarian cells (theca interna) to secrete precursor molecules (testosterone) used to produce estrogens.

In a young female, estrogens stimulate development of various secondary sex characteristics. Estrogens secreted during subsequent menstrual cycles continue development of these traits and maintain them. Table 22.3 summarizes the hormonal control of female secondary sex characteristics.

Increasing concentration of estrogens during the first week or so of a menstrual cycle changes the uterine lining, thickening the glandular endometrium (prolifera-tive phase). Meanwhile, the developing follicle completes maturation, and by the fourteenth day of the cycle, the follicle appears on the surface of the ovary as a blisterlike bulge.

Within the follicle, the granulosa cells, which surround the oocyte and connect it to the inner wall, loosen. Follicular fluid accumulates rapidly.

While the follicle matures, estrogens that it secretes inhibit the release of LH from the anterior pituitary gland but allow LH to be stored in the gland. Estrogens also make the anterior pituitary cells more sensitive to the action of GnRH, which is released from the hypothalamus in rhythmic pulses about ninety minutes apart.

Near the fourteenth day of follicular development, the anterior pituitary cells finally respond to the pulses

Gonadotropins

Estrogens

Ovaries

Estrogens inhibit oversecretion of gonadotropins

Breasts develop

Increased deposition of adipose tissue in breasts, thighs, and buttocks

Breasts develop

Increased vascularization of the skin

Accessory reproductive organs enlarge

Estrogens

Increased deposition of adipose tissue in breasts, thighs, and buttocks

Ovaries

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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