Chapter 17 Reproductive Systems.

Transcription

1 Chapter 17 Reproductive Systems. I. Gonads. a. Produce cells called gametes (sperm and oocytes = egg cells). b. Testes (in males). i. Produce sperm and the hormone testosterone. c. Ovaries (in females). i. Produce oocytes and the hormones estrogen and progesterone. II. Male and female reproductive roles. a. Most cells in the body have 46 chromosomes located in the nucleus. This is called the diploid number (2n). These 46 chromosomes come in a set of 23 homologous pairs. i. Exceptions: red blood cells do not have a nucleus at maturity; skeletal muscle cells have multiple nuclei b. Most cellular reproduction within the body is via a process called mitosis. i. In this form of cellular reproduction, a parent cell copies its chromosomes, and the parent cell splits into two new daughter cells, each having one complete set of chromosomes. 1. The daughter cells are identical to each other and to the parent cell. c. In the testes and ovaries, another kind of cell division, called meiosis, is responsible for producing the sperm and oocytes. i. These cells wind up having half as many chromosomes (23) as other body cells. This is called the haploid number (n). 1. A cell entering meiosis copies its chromosomes, splits into two daughter cells, and then the daughter cells divide again, but without coping the chromosomes again. This results in four cells that are haploid. 2. Meiosis also shuffles the genes (alleles) among the homologous pairs of chromosomes; in addition to this, other mechanisms ensure that each gamete produced is genetically unique. d. During fertilization, when a sperm cell (n) unites with an oocyte (n), a new cell called the zygote is formed, and the diploid number (2n) is restored. i. This cell then divides repeatedly via mitosis to form the embryo, then the fetus, and finally the baby. e. Although the male and female make an equal genetic contribution to the zygote, their overall reproductive strategies are very different; these strategies have been shaped by natural selection and evolution. i. The male reproductive strategy is to produce millions of sperm, and hope that one is able to fertilize an oocyte. After making his genetic contribution, no further investment is required on his part. ii. The female reproductive strategy is to produce one oocyte (normally) about once a month. After fertilization, the female must devote considerable time and energy for the zygote to develop into a human being. III. Form and function of the male reproductive system. Fig a. Table 17.1 provides an overview of the male reproductive structures. b. Testes. i. Pair of organs housed within the scrotum. ii. Beginning at puberty, sperm production begins, and more than 100 million sperm cells are produced each day. 1. Sperm production occurs within the seminiferous tubules of the testes. Fig Between the seminiferous tubules are the interstitial cells (of Leydig). These produce male sex hormones, the androgens, one of which is testosterone. iii. Production of sperm requires a slightly lower temperature than normal body temperature (about 2 degrees F less than normal body temp.). Fig

2 1. Musculature of the scrotum (dartos muscle) and an extension of the abdominal muscles (cremaster muscle) alter the proximity of the testes with respect to the body in order to regulate their temperature. c. Duct system. i. Epididymis. 1. Forming sperm cells arrive here from the seminiferous tubules for storage and further maturation. ii. Vas (ductus) deferens. 1. During sexual climax, sperm cells from the epididymis are propelled by waves of muscular contraction through the vas deferens to the urethra and out of the body via the penis. In addition to sperm, secretions of the accessory glands also contribute to the formation of the semen. d. Accessory glands. i. Prostate gland. [The material in your textbook is incorrect concerning this organ]. 1. Surrounds the upper portion of the urethra and secretes a slightly acidic fluid that contains a number of compounds with uncertain function, but may be involved in the activation of sperm cells. It also contains seminalplasmin, an antibiotic that may help prevent urinary tract infections in males. ii. Seminal vesicles. 1. Pair of glands that secrete an alkaline fluid rich in fructose, which the sperm cells use as an energy source. a. The alkalinity helps to neutralize the acidity of the prostatic secretions and the interior of the urethra. 2. Proteins within the fluid initially thicken the semen so it is better able to remain inside the female reproductive tract. 3. The fluid also contains prostaglandins, which reduce the thickness of cervical mucus and stimulates uterine contractions that help move the sperm. 4. Finally, this fluid begins the activation process of the spermatozoa (capacitation) which begin beating their flagella, and become highly motile. iii. Bulbourethral glands. 1. Pair of glands that produce a mucous secretion that helps to neutralize the acidity of the male urethra. e. Penis. i. [Discussion of anatomy; glans, prepuce (foreskin), vasculature, nerves, urethral orifice, etc.]. ii. Contains the urethra, which serves dual function in the male: passage for urine and for semen; only one fluid may pass at a time. iii. Contains erectile tissues that engorge with blood during sexual arousal, causing it to become enlarged and erect, to facilitate the deposit of semen within the female reproductive tract. f. Sperm development (spermatogenesis). Fig i. Occurs within the seminiferous tubules. ii. Diploid (2n) spermatogonia are found on the along the outer connective tissue capsule of the tubule. iii. These cells divide by mitosis, to make more spermatogonia (2n). iv. One of each pair of these daughter cells pushes toward the lumen of the tubule, while the other remains next to the connective tissue capsule, where it will continue to divide by mitosis. 1. In this way, there is a constant supply of spermatogonia, even though some spermatogonia enter into meiosis to form sperm v. The cell that gets pushed toward the lumen enlarges, becoming a primary spermatocyte (2n). vi. The primary spermatocyte then replicates its DNA and undergoes the first meiotic division to form a pair of secondary spermatocytes.

3 vii. The 2 secondary spermatocytes undergo the second meiotic division to produce 4 haploid (n) spermatids. viii. The spermatids undergo a process called spermiogenesis in which several structural changes occur that transform them into spermatozoa. ix. Spermatozoa are then released into the lumen of the seminiferous tubule, which transports them to the epididymis for further development and storage. x. It takes about 2 months for a spermatogonium to become a spermatozoon. xi. When spermatozoa mix with the secretions of the seminal vesicles, capacitation begins, which causes them to become motile sperm cells. xii. The structure of a mature sperm cell is shown in Fig g. Hormones. Fig and Table i. The hormones described below regulate sperm cell production through self-regulating negative feedback loops. ii. Cells of the hypothalamus produce gonadotropin-releasing hormone (GnRH) that stimulates the anterior pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). iii. LH stimulates the interstitial cells (of Leydig) secrete the hormone testosterone, which has the following effects: 1. Combined with growth hormone, testosterone is responsible for the male growth spurt during adolescence. 2. Causes the development of the secondary sex characteristics. 3. Responsible for male sex drive. 4. Stimulates sperm cell production; ~100 million are produced daily. 5. Inhibits the production of GnRH and LH. iv. FSH (and testosterone) stimulates sperm cell production. v. Large numbers of spermatozoa stimulate sustentacular (Sertoli) cells in the seminiferous tubules to secrete the hormone inhibin, which inhibits the production of GnRH and FSH, slowing the production of more spermatozoa. IV. Form and function of the female reproductive system. Fig a. Ovaries. i. Paired organs, each about the size of your thumb, that are superior and lateral to the uterus. ii. Produce oocytes (egg cells) and the hormones estrogen and progesterone. b. Oviducts (fallopian tubes/uterine tubes). i. Pair of muscular, open-ended tubes that each face an ovary on one end, and connect to the uterus on the other end. ii. The end facing the ovary does not touch the ovary, but is very close to it. It has a number of finger-like projections called fimbriae. iii. The fimbriae and lumen of the oviduct are covered with a ciliated columnar epithelium. 1. The cilia draw the oocyte into the oviduct when it is released from the ovary during ovulation, and along with muscular contraction of the tube, help transport the oocyte to the uterus. 2. Fertilization typically occurs while the oocyte is within the oviduct. c. Uterus. i. Muscular organ (smooth muscle of uterus = myometrium) that looks like an inverted pear. 1. The narrow end contains the cervix, which has a small opening where sperm can enter at certain times. ii. If fertilization occurs, the zygote implants within the endometrium (inner layer of the uterus) and embryonic development ensues. 1. As the embryo (up to 8 weeks of age) and fetus (after week 8) grows within the uterus, it expands about 60 times its original size. a. Initially, the endometrium directly nourishes the fetus, but as development progresses, the placenta forms; the placenta remains in direct contact with the

4 endometrium throughout the rest of pregnancy, and is the interface between the fetus and mother where nutrients, gasses, wastes, etc., are exchanged. 2. During labor, muscular contraction of the myometrium expels the child and placenta at birth through the opening of the uterus (cervix) and out the vagina (birth canal). a. The vagina is a muscular tube that serves as the receptacle for the penis and semen during sexual intercourse. d. External genitalia (vulva). i. [Discussion of anatomy: labia majora, labia minora, clitoris, hymen, urethral orifice, vasculature, nerves, etc.]. e. Breasts (mammary glands). Fig i. In females, composed of adipose and groups of milk-secreting glands, each of which drain via their own duct through the nipple. f. Ovarian cycle. Fig i. [Much of the material in your textbook is incorrect concerning this process]. ii. Table 17.4 summarizes the hormones and their effects that bring about the events discussed in this section, and in the section describing the uterine cycle. iii. The production of oocytes in the female begins before she is born, accelerates at puberty, and ends at menopause. iv. Throughout oocyte development within the ovary, cellular structures called follicles surround each developing oocyte. v. During fetal development in females, oogonia (2n) complete all their mitotic divisions. The resulting cells are called primary oocytes (2n), which are contained in primordial follicles. 1. The number of primary oocytes at this time is about 2 million. vi. The primary oocytes then enter the first stage of meiosis, which is halted before the first meiotic division occurs. 1. Only about 700,000 primary oocytes remain at birth; the rest have degenerated. 2. They remaining primary oocytes stay in a state of suspended animation until puberty, when hormonal changes will cause a few of them to continue the process of meiosis each month. a. At the onset of puberty, the number of viable primary oocytes has decreased to about 400,000. b. Only will ever mature in the female s lifetime. vii. When puberty begins, FSH from the anterior pituitary gland stimulates a few primordial follicles to become primary follicles, which then begin to enlarge as the follicular cells proliferate. The primary oocyte within also begins to enlarge. 1. The follicular cells begin secreting hormones known as estrogens. viii. Only a few of these primary follicles will grow to become secondary follicles, which are much larger than the primary follicles. The primary oocyte also continues to enlarge within the follicle, but not as quickly as the follicle itself. 1. At this time, a fluid filled space called the antrum cavity develops between the layers of the follicular cells. ix. By the 10 th to the 14 th day of the ovarian cycle and after further growth, typically only a single follicle remains. This is the tertiary = Graffian = mature follicle, with the primary oocyte inside. 1. The mature follicle is about 15 mm in diameter and forms a visible bulge on the surface of the ovary. 2. Much of the large size of the mature follicle is due to the large antrum cavity that has developed. x. As the anterior pituitary gland releases a surge of LH around day 13, the primary oocyte is stimulated to continue meiosis.

5 1. The first meiotic division does not produce 2 equivalent cells; each gets a complete compliment of DNA, but one cell gets most of the cytoplasm and its contents. The other is essentially a garbage sac containing DNA. a. This small sac of DNA is called the first polar body, and will not continue further development. b. The remaining cell is called a secondary oocyte. c. At this point, meiosis is halted again, until fertilization occurs. xi. About 12 hours after the secondary oocyte has formed, ovulation occurs. The mature follicle ruptures, releasing the secondary oocyte and the fluid from the antrum cavity into the abdominopelvic cavity, where the oocyte will be swept into the uterine tube by its ciliated epithelium. 1. If fertilization does not occur, meiosis will not finish and the secondary oocyte will not survive. 2. If fertilization does occur, the second meiotic division occurs, and a second polar body is cast off. The remaining cell is called the ovum (n), which fuses with a sperm cell (n), resulting in the formation of the diploid zygote (2n). (The second polar body may undergo the 2 nd meiotic division ). 3. It is important to note here that in meiosis in males, a single spermatogonium (2n) eventually gave rise to 4 functional sperm cells (n). In contrast, in females, a single oogonium (2n) eventually gave rise to a single ovum (n), and 3 non-functional polar bodies. xii. After the release of the secondary oocyte at ovulation, LH causes the follicular cells to proliferate and fill the hollow wound cavity left on the ovary due to the rupture of the mature follicle. 1. The resulting structure is called the corpus luteum, which actively secrets the hormones estrogen and progesterone. a. If pregnancy does not occur, it will degenerate at the end of the ovarian cycle. b. If pregnancy does occur, the developing embryo will secrete a hormone called human chorionic gonadotropin, which will maintain the corpus luteum so it will continue secreting estrogen and progesterone. i. When the placenta forms, the corpus luteum will finally degenerate. g. Coordination of the ovarian and uterine (menstrual) cycles. Fig i. The events of the ovarian cycle take about 28 days to produce a mature ovum. ii. At the same time, the uterus goes through a coordinated cycle to prepare for the possible arrival of a zygote and pregnancy. iii. Day 1 of the cycle is determined by the onset of the menses (menstrual bleeding), which typically lasts about 5 days. 1. During this time, the amounts of circulating estrogen and progesterone are at their lowest levels. These hormones have a negative feedback effect on GnRH production in the hypothalamus; therefore GnRH secretion increases at this time. iv. GnRH stimulates the release of FSH and LH from the anterior pituitary gland. v. FSH and LH cause the following to occur: 1. New follicles to begin maturing (as described in the ovarian cycle above). 2. Follicle cells start producing estrogen and progesterone. a. Estrogen and progesterone causes the uterine endometrial cells proliferate in preparation for possible pregnancy. vi. As the levels of estrogen and progesterone rise, they exert a negative feedback (up to a point) on the anterior pituitary, causing FSH and LH production to decline. vii. However, when the increasing amount of estrogen and progesterone (produced by the follicles and other ovarian cells) crosses a critical threshold, it causes a surge of LH (and to a much lesser extent, FSH) to be released, which triggers ovulation. The LH also stimulates the development of the corpus luteum.

6 viii. As the corpus luteum forms, it secretes estrogen, and an increasing amount of progesterone (both of which maintain endometrial proliferation, even as estrogen levels fluctuate). 1. The high levels of estrogen and progesterone inhibit secretion of FSH and LH. ix. If fertilization occurs, the zygote will implant in the endometrial wall, which will serve to nourish and protect the embryo until the placenta forms. 1. As described above, the embryo will secrete human chorionic gonadotropin, which will maintain the corpus luteum so it will continue secreting estrogen and progesterone, which are necessary to maintain the endometrium. a. Human chorionic gonadotropin is detectable in the female 7-9 days after implantation; home pregnancy kits test for this. b. When the placenta forms, the corpus luteum will finally degenerate. x. If fertilization does not occur, the corpus luteum degenerates, levels of estrogen and progesterone fall, and the endometrium degenerates, sloughing off as menses. 1. The decline in estrogen and progesterone cause GnRH production to increase, and the cycle starts again. h. Menopause. i. A female s fertility declines beginning in her late 20s. The older she gets, the less responsive the ovaries become to LH and FSH; the number of oocytes also declines. Eventually ovulation and menstruation become irregular and eventually stop in her late 40s or early 50s. ii. The associated decline in estrogen levels can have a number of negative effects: 1. Hot flashes, reduction in body fat and associated wrinkling/sagging, vaginal dryness, growth of facial hair, increased risk of cardiovascular disease and osteoporosis. a. Hormone replacement therapy with estrogen and progesterone can alleviate these problems, but has been implicated in increased risk of stroke. V. Disorders of the female reproductive system. You will not be tested on this, but it is interesting to read. VI. Stages of the human sexual response. a. Both males and females exhibit the same series of events, centered on vasocongestion of certain tissues, and rhythmic muscular contractions associated with orgasm. b. The first stage is excitement, in which heart rate and ventilation rate increase. In addition, blood is diverted to the erectile tissues of the penis in the male and to the breasts, labia, vagina, and clitoris in the female. In the vagina, increased blood flow causes lubrication to occur. c. In the plateau stage, sexual arousal is maintained, and the events described above become more pronounced. d. Orgasm is the height of sexual response, and is similar in both sexes in that involuntary contraction of numerous structures occurs. i. In males, these contractions result in ejaculation of semen. ii. In females, these contractions occur in the uterus and vagina. e. During resolution, the body returns to normal, but usually takes longer in females. i. Many females are capable of becoming aroused again during this stage, and are able to have additional orgasms. ii. Men undergo a refractory period in which another orgasm and ejaculation is not possible. 1. The refractory period increases with age; it may last only a few minutes, or could last for many hours. VII. Birth control. You will not be tested on this material, but it is very relevant to your health and future. Study suggestions for this chapter: In the textbook at the end of the chapter, the sections entitled 1) Highlighting the Concepts, 2) Recognizing Key Terms, and 3) Reviewing the Concepts are all good for you to gauge your comprehension and focus your study efforts.