Chapter 28 Outline

MALE REPRODUCTIVE SYSTEM

Testes

1. The testes, or testicles, are paired oval-shaped glands (gonads) in the scrotum (Figure 28.1).

a. The testes develop high on the embryo’s posterior abdominal wall and usually begin their descent into the scrotum through the inguinal canals during the latter half of the seventh month of fetal development.

b. The testes contain seminiferous tubules (in which sperm cells are made) (figure 28.3).

c. Embedded among the spermatogenic cells in the tubules are large Sertoli cells or sustentacular cells (Figure 28.4).

1) The tight junctions of these cells form the blood-testis barrier that prevents an immune response against the surface antigens on the spermatogenic cells.

2) The Sertoli cells also nourish spermatocytes, spermatids, and spermatozoa; mediate the effects of testosterone and follicle stimulating hormone on spermatogenesis; phagocytose excess spermatid cytoplasm as development proceeds; control movements of spermatogenic cells and the release of spermatozoa into the lumen of the seminiferous tubule; and secrete fluid for sperm transport and the hormone inhibin.

d. The Leydig cells or interstitial cells found in the spaces between adjacent seminiferous tubules secrete testosterone (Figure 28.4).

2. Spermatogenesis is the process by which the seminiferous tubules of the testes produce haploid sperm.

a. It begins in the diploid spermatogia (stem cells). They undergo mitosis to reserve future stem cells and to develop cells (2n primary spermatocytes) for sperm production (Figure 28.5).

b. The diploid primary spermatocytes undergo meiosis I forming haploid secondary spermatocytes.

c. Meiosis II results in the formation of the haploid spermatids. The spermatids are connected by cytoplasmic bridges.

d. The final stage of spermatogenesis is spermiogenesis which is the maturation of the spermatids into spermatozoa (sperm cells).

3. Mature sperm cells consist of a head, midpiece, and tail (Figure 28.8). They are produced at the rate of about 300 million per day and, once ejaculated, have a life expectancy of 48 hours within the female reproductive tract. Their function is to fertilize a secondary oocyte.

4. Hormonal Control of spermatogenesis

a. At puberty, gonadotropin releasing hormone stimulates anterior pituitary secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH initiates spermatogenesis, and LH assists spermatogenesis and stimulates secretion of testosterone by the Leydig cells. Figure 28.7 summarizes the hormonal relationships of the hypothalamus, pituitary gland, and testes.

b. Testosterone controls the growth, development, functioning, and maintenance of sex organs; stimulates bone growth, protein anabolism, and sperm maturation; and stimulates development of male secondary sex characteristics. Negative feedback systems regulate testosterone production (Figure 28.8).

c. Inhibin is produced by Sertoli cells. Inhibition of FSH by inhibin helps to regulate the rate of spermatogenesis.

You should also know the functions and locations of the accessory sex glands (p.1066 gray box) and the general anatomy of the structures through which spermatozoa pass on their way to the external urethral orifice (Fig. 28.1, 28.3, 28.10).

FEMALE REPRODUCTION SYSTEM

Ovaries

1. The ovaries are paired glands that are homologous to the testes.

2. The ovaries are located in the upper pelvic cavity, on either side of the uterus. They are maintained in position by a series of ligaments.

3. The histology of the ovary is illustrated in Figure 28.13.

a. Ovarian follicles lie in the cortex and contain oocytes in various stages of development.

b. A mature (Graafian) follicle expels a secondary oocyte by a process called ovulation.

c. A corpus luteum contains the remnants of an ovulated follicle and produces progesterone, estrogens, relaxin, and inhibin until it degenerates into a corpus albicans.

4. Oogenesis and follicular development

a. Oogenesis occurs in the ovaries. It results in the formation of a single haploid secondary oocyte.

b. The oogenesis sequence includes reduction division (meiosis I), equatorial division (meiosis II), and maturation (Figure 28.15).

c. While oogenesis is occurring, the follicle cells surrounding the oocyte are also undergoing developmental changes. The sequence of follicular cell changes is: primordial, primary, secondary, and mature (Graffian) follicles, and corpus luteum and corpus albicans.

d. Table 28.1 summarizes the events of oogenesis and follicular development.

FEMALE REPRODUCTIVE CYCLE

A. The general term female reproductive cycle encompasses the ovarian and uterine cycles, the hormonal changes that regulate them, and cyclical changes in the breasts and the cervix.

1. The ovarian cycle is a series of events associated with the maturation of an ovum.

2. The uterine (menstrual) cycle involves changes in the endometrium to prepare for the reception of a fertilized ovum.

B. Hormonal Regulation of the Female Reproductive Cycle

1. The menstrual and ovarian cycles are controlled by GnRH from the hypothalamus, which stimulates the release of FSH and LH by the anterior pituitary gland (Figure 28.23).

a. FSH stimulates the initial development of ovarian follicles and secretion of estrogens by the ovaries.

b. LH stimulates further development of ovarian follicles, ovulation, and the secretion of estrogens and progesterone by the ovaries.

2. At least six different estrogens have been isolated from the plasma of human females, with three in significant quantities: beta-estradiol, estrone, and estriol.

a. Estrogens have several important functions:

1) Promotion of the development and maintenance of female reproductive structures, secondary sex characteristics, and the breasts.

2) Increase protein anabolism and build strong bones.

3) Lower blood cholesterol.

b. Moderate levels of estrogens in the blood inhibit the release of GnRH by the hypothalamus and secretion of LH and FSH by the anterior pituitary gland.

3. Progesterone works with estrogens to prepare the endometrium for implantation and the mammary glands for milk synthesis.

4. A small quantity of relaxin is produced monthly to relax the uterus by inhibiting contractions (making it easier for a fertilized ovum to implant in the uterus). During pregnancy, relaxin relaxes the pubic symphysis and helps dilate the uterine cervix to facilitate delivery.

5. Inhibin inhibits secretion of FHS and GnRH and, to a lesser extent, LH. It might be important in decreasing secretion of FSH and LH toward the end of the uterine cycle.

C. Phases of the Female Reproductive Cycle

1. The female reproductive cycle may be divided into four phases (Figure 28.24).

a. The menstrual cycle (menstruation) lasts for approximately the first 5 days of the cycle.

1) During this phase, small secondary follicles in each ovary begin to develop.

2) Also during this phase, the stratum functionalis layer of the endometrium is shed, discharging blood, tissue fluid, mucus, and epithelial cells.

b. The preovulatory phase, or proliferative phase, is the time between menstruation and ovulation. This phase is more variable in length that the other phases, lasting from days 6-13 in a 28-day cycle.

1) During this phase, primary follicles develop into secondary follicles and a single secondary follicle (occasionally more than one) develops into a vesicular ovarian (Graafian) follicle, or mature follicle. This follicle produces a bulge on the surface of the ovary.

2) The dominant follicle continues to increase its estrogen production under the influence of an increasing level of LH.

3) During this phase, endometrial repair occurs.

c. Ovulation is the rupture of the vesicular ovarian (Graafian) follicle with release of the secondary oocyte into the pelvic cavity, usually occurring on day 14 in a 28-day cycle.

1) The high levels of estrogen during the last part of the preovulatory phase exert a positive feedback on both LH and GnRH to cause ovulation (Figure 28.25). (Remember, low to moderate levels of estrogen exert negative feedback on LH and GnRH, but high levels of estrogen exert a positive feedback on these hormones)

a) GnRH promotes release of FSH and more LH by the anterior pituitary gland.

b) The LH surge brings about the ovulation. This surge can be detected with ovulation test kits.

2) Following ovulation, the vesicular ovarian follicle collapses (and blood within it forms a clot) to become the corpus hemorrhagicum The clot is eventually absorbed by the remaining follicle cells. In time, the follicular cells enlarge, change character, and form the corpus luteum, or yellow body, under the influence of LH. Stimulated by LH, the corpus luteum secretes estrogens and progesterone.

d. The postovulatory phase is the most constant in duration and lasts from days 15-28 in a 28-day cycle, the time between ovulation and onset of the next menstrual period.

1) With reference to the ovaries, this phase of the cycle is also called the luteal phase, during which both estrogen and progesterone are secreted in large quantities by the corpus luteum.

A) If fertilization and implantation do not occur, the corpus luteum degenerates and becomes the corpus albicans, or white body. The decreased secretion of progesterone and estrogens then initiates another menstrual phase (uterine and ovarian cycle).

b) If fertilization and implantation do occur, the corpus luteum is maintained until the placenta takes over its hormone-producing function. During this time, the corpus luteum, maintained by human chorionic gonadotropin (hCG) from the developing placenta, secretes estrogens and progesterone to support pregnancy and breast development for lactation. Once the placenta begins its secretion, the role of the corpus luteum becomes minor. hCG can be detected in the urine of a pregnant woman as soon as two weeks after fertilization and is what is measured in the home pregnancy test kits.

2) With reference to the uterus, this phase is also called the secretory phase because of the secretory activity of the endometrial glands as the endometrium thickens in anticipation of implantation.

2. Figure 28.26 summarizes the hormonal interactions during the ovarian and uterine cycles.

You should know the generalstructure of the female reproductive organs as illustrated in Fig. 28.11, 28.16, 28.20 and 28.22.