Chapter 19 Summary

Section A Summary
The gonads have a dual function—gametogenesis and the secretion of sex hormones.

General Principles of Gametogenesis
1. The first stage of gametogenesis is mitosis of primordial germ cells.
2. This is followed by meiosis, a sequence of two cell divisions resulting in each gamete receiving 23 chromosomes.
3. Crossing-over and random distribution of maternal and paternal chromatids to the daughter cells during meiosis cause genetic variability in the gametes.

Spermatogenesis
1. The male gonads, the testes, produce sperm in the seminiferous tubules and secrete testosterone from the Leydig cells.
2. The meiotic divisions of spermatogenesis result in each sperm containing 23 chromosomes, compared to the original 46 of the spermatogonia.
3. The developing germ cells are intimately associated with the Sertoli cells, which perform many functions, as summarized in Table 19–2.

Transport of Sperm
1. From the seminiferous tubules, the sperm pass through the epididymis, where they are concentrated.
2. The epididymis and vas deferens store the sperm, and the seminal vesicles and prostate secrete the bulk of the semen.
3. Erection of the penis occurs because of vascular engorgement accomplished by relaxation of the small arteries and passive occlusion of the veins.
4. Ejaculation includes emission—emptying of semen into the urethra—followed by expulsion of the semen from the urethra.

Hormonal Control of Male Reproductive Functions
1. Hypothalamic GnRH stimulates the anterior pituitary to secrete FSH and LH, which then act on the testes: FSH on the Sertoli cells to stimulate spermatogenesis and inhibin secretion, and LH on the Leydig cells to stimulate testosterone secretion.
2. Testosterone, acting locally on the Sertoli cells, is essential for maintaining spermatogenesis.
3. Testosterone exerts a negative-feedback inhibition on both the hypothalamus and the anterior pituitary to reduce mainly LH secretion. Inhibin exerts a negative-feedback inhibition on FSH secretion.
4. Testosterone maintains the accessory reproductive organs and male secondary sex characteristics and stimulates growth of muscle and bone. In many of its target cells, it must first undergo transformation to dihydrotestosterone or to estrogen.

Ovarian Function
1. The female gonads, the ovaries, produce eggs and secrete estrogen, progesterone, and inhibin.
2. The two meiotic divisions of oogenesis result in each ovum having 23 chromosomes, in contrast to the 46 of the original oogonia.
3. The follicle consists of the egg, inner layers of granulosa cells surrounding the egg, and outer layers of theca cells.
4. At the beginning of each menstrual cycle, a group of preantral and early antral follicles begins to develop further, but soon only the dominant follicle continues its development to full maturity and ovulation.
5. Following ovulation, the remaining cells of that follicle differentiate into the corpus luteum, which lasts about 10 to 14 days if pregnancy does not occur.
6. The menstrual cycle can be divided, according to ovarian events, into a follicular phase and a luteal phase, which last approximately 14 days each and are separated by ovulation.

Control of Ovarian Function
1. The menstrual cycle results from a finely tuned interplay of hormones secreted by the ovaries, the anterior pituitary, and the hypothalamus.
2. During the early and middle follicular phases, FSH stimulates the granulosa cells to proliferate and secrete estrogen, and LH stimulates the theca cells to proliferate and produce the androgens that the granulosa cells use to make estrogen.
   1. During this period, estrogen exerts a negative feedback on the anterior pituitary to inhibit the secretion of the gonadotropins. It probably also inhibits secretion of GnRH by the hypothalamus.
   2. Inhibin preferentially inhibits FSH secretion.
3. During the late follicular phase, plasma estrogen becomes high enough to elicit a surge of LH, which then causes, via the granulosa cells, completion of the egg’s first meiotic division and cytoplasmic maturation, ovulation, and formation of the corpus luteum.
4. During the luteal phase, under the influence of small amounts of LH, the corpus luteum secretes progesterone and estrogen. Regression of the corpus luteum results in a cessation of the secretion of these hormones.
5. Secretion of GnRH and the gonadotropins is inhibited during the luteal phase by the combination of progesterone, estrogen, and inhibin.

Uterine Changes in the Menstrual Cycle
1. The ovarian follicular phase is equivalent to the uterine menstrual and proliferative phases, the first day of menstruation being the first day of the cycle. The ovarian luteal phase is equivalent to the uterine secretory phase.
   1. Menstruation occurs when the plasma estrogen and progesterone levels fall as a result of regression of the corpus luteum.
   2. During the proliferative phase, estrogen stimulates growth of the endometrium and myometrium and causes the cervical mucus to be readily penetrable by sperm.
   3. During the secretory phase, progesterone converts the estrogen-primed endometrium to a secretory tissue and makes the cervical mucus relatively impenetrable to sperm. It also inhibits uterine contractions.

Other Effects of Estrogen and Progesterone
1. The many effects of estrogen and progesterone are summarized in Table 19–8.

Pregnancy
1. After ovulation, the egg is swept into the uterine tube, where a sperm, having undergone capacitation and the acrosome reaction, fertilizes it.
2. Following fertilization the egg undergoes its second meiotic division, and the nuclei of the egg and sperm fuse. Reactions in the ovum block penetration by other sperm and trigger cell division and embryogenesis.
3. The conceptus undergoes cleavage, eventually becoming a blastocyst, which implants in the endometrium on approximately day 7 after ovulation.
   1. The trophoblast gives rise to the fetal part of the placenta, whereas the inner cell mass develops into the embryo proper.
   2. Although they do not mix, fetal blood and maternal blood both flow through the placenta, exchanging gases, nutrients, hormones, waste products, and other substances.
   3. The fetus is surrounded by amniotic fluid in the amniotic sac.
4. The progesterone and estrogen required to maintain the uterus during pregnancy come from the corpus luteum for the first 2 months of pregnancy, their secretion stimulated by chorionic gonadotropin produced by the trophoblast.
5. During the last 7 months of pregnancy, the corpus luteum regresses, and the placenta itself produces large amounts of progesterone and estrogen.
6. The high levels of progesterone, in the presence of estrogen, inhibit the secretion of GnRH and thereby that of the gonadotropins, so that menstrual cycles are eliminated.
7. Delivery occurs by rhythmical contractions of the uterus, which first dilate the cervix and then move the infant, followed by the placenta, through the vagina. The contractions are stimulated in part by oxytocin, released from the posterior pituitary in a reflex triggered by uterine mechanoreceptors, and by uterine prostaglandins.
8. The breasts develop markedly during pregnancy as a result of the combined influences of estrogen, progesterone, prolactin, and placental lactogen.
   1. Prolactin secretion is stimulated during pregnancy by estrogen acting on the anterior pituitary, but milk is not synthesized because high concentrations of estrogen and progesterone inhibit the milk-producing action of prolactin on the breasts.
   2. As a result of the suckling reflex, large bursts of prolactin and oxytocin occur during nursing; the prolactin stimulates milk production and the oxytocin causes milk ejection.

Sex Determination and Sex Differentiation
1. Sex is determined by the two sex chromosomes: Males are XY, and females are XX.
2. A gene on the Y chromosome is responsible for the development of testes. In the absence of a Y chromosome, testes do not develop and ovaries do instead.
3. When a functioning male gonad is present to secrete testosterone and MIS, a male reproductive tract and external genitalia develop. In the absence of testes, the female system develops.

Puberty
1. At puberty, the hypothalamic-anterior-pituitary-gonadal chain of hormones becomes active as a result of a change in brain function that permits increased secretion of GnRH.
2. The first sign of puberty is the appearance of pubic or axillary hair.

Menopause
1. Around the age of 50, a woman’s menstrual periods become less regular and ultimately disappear—menopause.
   1. The cause of menopause is a decrease in the number of ovarian follicles and their hyporesponsiveness to the gonadotropins.
   2. The symptoms of menopause are largely due to the marked decrease in plasma estrogen concentration.
2. Men show a steady decrease in testosterone secretion after age 40 but generally no complete cessation of reproductive function.