Gnrh Enters Blood Vessels In

Seminiferous tubules

Production of sperm cells

Interstitial cells

Production and secretion of male sex hormones

Epididymis

Storage and maturation of sperm cells; conveys sperm cells to vas deferens

Vas deferens

Conveys sperm cells to ejaculatory duct

Seminal vesicle

Secretes an alkaline fluid containing nutrients and prostaglandins; fluid helps neutralize acidic semen

Prostate gland

Secretes an alkaline fluid that helps neutralize the acidic components of semen and enhances motility of sperm cells

Bulbourethral gland

Secretes fluid that lubricates end of the penis

Scrotum

Encloses and protects testes

Penis

Conveys urine and semen to outside of body; inserted into the vagina during sexual intercourse; the glans penis is richly supplied with sensory nerve endings associated with feelings of pleasure during sexual stimulation

Hypothalamic and Pituitary Hormones

Prior to ten years of age, a boy's body is reproductively immature. During this period, the body is childlike, and the spermatogenic cells of the testes are undifferenti-ated. Then a series of changes leads to development of a reproductively functional adult. The hypothalamus controls many of these changes.

Recall from chapter 13 (p. 518) that the hypothalamus secretes gonadotropin-releasing hormone (GnRH), which enters the blood vessels leading to the anterior pituitary gland. In response, the anterior pituitary gland secretes the gonadotropins (go-nad"o-trop'inz) called luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH, which in males is called interstitial cell-stimulating hormone (ICSH), promotes development of the interstitial cells (cells of Leydig) of the testes, and they, in turn, secrete male sex hormones. FSH stimulates the sustentacular cells of the seminiferous tubules to proliferate, grow, mature, and respond to the effects of the male sex hormone testosterone. Then, in the presence of FSH and testosterone, these cells stimulate the spermato-genic cells to undergo spermatogenesis, giving rise to sperm cells (fig. 22.18). The sustentacular cells also

Hypothalamus

GnRH

PitL

FSH 9lai

Hypothalamus

PitL

FSH 9lai

Androgens prevent oversecretion of GnRH

Androgens prevent over-secretion of LH (ICSH)

Inhibin prevents oversecretion of FSH

Bloodstream

FSH stimulates meiosis in primary spermatocytes to form immature sperm cells; FSH stimulates secretion of inhibin by sustentacular cells

LH (ICSH) stimulates interstitial cells to secrete androgens

(primarily testosterone)

Bloodstream

FSH stimulates meiosis in primary spermatocytes to form immature sperm cells; FSH stimulates secretion of inhibin by sustentacular cells

Androgens

Testes

Androgens stimulate the development of male secondary sex characteristics and maturation of sperm cells

Androgens

Testes

Figure 22.18

The hypothalamus controls maturation of sperm cells and development of male secondary sex characteristics. A negative feedback mechanism operating among the hypothalamus, the anterior lobe of the pituitary gland, and the testes controls the concentration of testosterone.

Inhibin prevents oversecretion of FSH

secrete a hormone called inhibin, which inhibits the anterior pituitary gland by negative feedback, and thus prevents oversecretion of FSH.

Male Sex Hormones

Male sex hormones are termed androgens (an'dro-jenz). The interstitial cells of the testes produce most of them, but small amounts are synthesized in the adrenal cortex (see chapter 13, p. 529).

The hormone testosterone (tes-tos'te-ron) is the most abundant androgen. It is secreted and transported in the blood, loosely attached to plasma proteins. Like other steroid hormones, testosterone combines with receptor molecules usually in the nuclei of its target cells (see chapter 13, p. 507). However, in many target cells, such as those in the prostate gland, seminal vesicles, and male external accessory organs, testosterone is first converted to another androgen called dihydrotestosterone, which stimulates the cells of these organs. Androgen molecules that do not reach receptors in target cells are usually changed by the liver into forms that can be excreted in bile or urine.

Testosterone secretion begins during fetal development and continues for a few weeks following birth, then nearly ceases during childhood. Between the ages of thirteen and fifteen, a young man's androgen production usually accelerates. This phase in development, when an individual becomes reproductively functional, is puberty (pu'ber-te). After puberty, testosterone secretion continues throughout the life of a male.

In a group of disorders called male pseudohermaphroditism, testes are usually present, but a block in testosterone synthesis prevents the fetus from developing male structures, and as a result, later, the child appears to be a girl. But at puberty, the adrenal glands begin to produce testosterone, as they normally do in any male. This leads to masculinization: The voice deepens, and muscles build up into a masculine physique; breasts do not develop, nor does menstruation occur. The clitoris may enlarge so greatly under the adrenal testosterone surge that it looks like a penis. Individuals with a form of this condition that is prevalent in the Dominican Republic are called guevedoces, which means "penis at age 12."

Actions of Testosterone

Cells of the embryonic testes first produce testosterone after about eight weeks of development. This hormone stimulates the formation of the male reproductive organs, including the penis, scrotum, prostate gland, seminal vesicles, and ducts. Later in development, testosterone causes the testes to descend into the scrotum.

During puberty, testosterone stimulates enlargement of the testes (the primary male sex characteristic) and accessory organs of the reproductive system, as well as development of male secondary sex characteristics, which are special features associated with the adult male body. Secondary sex characteristics in the male include

1. Increased growth of body hair, particularly on the face, chest, axillary region, and pubic region. Sometimes growth of hair on the scalp decreases.

2. Enlargement of the larynx and thickening of the vocal folds, with lowering of the pitch of the voice.

3. Thickening of the skin.

4. Increased muscular growth, broadening shoulders, and narrowing of the waist.

5. Thickening and strengthening of the bones.

Testosterone also increases the rate of cellular metabolism and production of red blood cells by stimulating release of erythropoietin. For this reason, the average number of red blood cells in a cubic millimeter of blood is usually greater in males than in females. Testosterone stimulates sexual activity by affecting certain portions of the brain.

OO Reconnect to chapter 14, Red Blood Cell Production and Its Control, page 552.

Regulation of Male Sex Hormones

The extent to which male secondary sex characteristics develop is directly related to the amount of testosterone that the interstitial cells secrete. A negative feedback system involving the hypothalamus regulates testosterone output (fig. 22.18).

As the concentration of testosterone in the blood increases, the hypothalamus becomes inhibited, decreasing its stimulation of the anterior pituitary gland by GnRH. As the pituitary's secretion of LH (ICSH) falls in response, the amount of testosterone the interstitial cells release decreases.

As the blood concentration of testosterone drops, the hypothalamus becomes less inhibited, and it once again stimulates the anterior pituitary gland to release LH. The increasing secretion of LH causes the interstitial cells to release more testosterone, and its blood concentration increases. Testosterone level decreases somewhat during and after the male climacteric, a decline in sexual function that occurs with aging. At any given age, the testosterone concentration in the male is regulated to remain relatively constant.

H Which hormone initiates the changes associated with male sexual maturity?

Describe several male secondary sex characteristics.

List the functions of testosterone.

Explain the regulation of secretion of male sex hormones.

Organs of the Female Reproductive System

The organs of the female reproductive system are specialized to produce and maintain the female sex cells, or egg cells; to transport these cells to the site of fertilization; to provide a favorable environment for a developing offspring; to move the offspring to the outside; and to produce female sex hormones.

The primary sex organs (gonads) of this system are the ovaries, which produce the female sex cells and sex hormones. The other parts of the system comprise the internal and external accessory organs.

Ovaries

The ovaries are solid, ovoid structures measuring about 3.5 centimeters in length, 2 centimeters in width, and 1 centimeter in thickness. An individual ovary is located in a shallow depression (ovarian fossa) on each side in the lateral wall of the pelvic cavity (fig. 22.19).

Ovary Attachments

Several ligaments help hold each ovary in position. The largest of these, formed by a fold of peritoneum, is called the broad ligament. It is also attached to the uterine tubes and the uterus.

A small fold of peritoneum, called the suspensory ligament, holds the ovary at its upper end. This ligament also contains the ovarian blood vessels and nerves. At its lower end, the ovary is attached to the uterus by a rounded, cordlike thickening of the broad ligament called the ovarian ligament (fig. 22.20).

Ovary Descent

Like the testes in a male fetus, the ovaries in a female fetus originate from masses of tissue posterior to the parietal peritoneum, near the developing kidneys. During development, these structures descend to locations just inferior to the pelvic brim, where they remain attached to the lateral pelvic wall.

Ovary Structure

The tissues of an ovary can be subdivided into two rather indistinct regions, an inner medulla and an outer cortex. The ovarian medulla is mostly composed of loose connective tissue and contains many blood vessels, lymphatic vessels, and nerve fibers. The ovarian cortex consists of

Uterine tube Ovary Uterus

Urinary bladder-

Labium Minus

Symphysis pubis Urethra Clitoris Labium minus

Labium majus Vaginal orifice

Fimbriae

Rectouterine pouch

Fornix Cervix

Rectum Vagina Anus

Level of section

Coccyx

Inferior gluteal vein and

Sciatic nerve

Femur Ureter

Ischium

Sciatic nerve

Femur Ureter

Ischium

Round And Ovarian Ligaments

Femoral nerve, artery, and vein

Anterior

Femoral nerve, artery, and vein

Anterior u

Gluteus maximus m. Rectum Levator ani m. Uterus

Urinary bladder Symphysis pubis

Figure 22.19

(a) Sagittal view of the female reproductive organs. (b) Transverse section of the female pelvic cavity.

Suspensory ligament of ovary

Fimbriae of uterine tube

Suspensory ligament of ovary

Fimbriae of uterine tube

Round Ligament Uterus

Round ligament of uterus

Broad ligament

Uterus

Figure

Round ligament of uterus

Broad ligament

Uterus

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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