Physiology 5th Ed.

Chapter 10. Reproductive Physiology

 

The gonads are endocrine glands whose functions are to support development and maturation of the male and female germ cells. The male gonads, the testes, are responsible for development and maturation of sperm and synthesis and secretion of the male sex steroid hormone, testosterone. The female gonads, the ovaries, are responsible for development and maturation of ova and synthesis and secretion of the female sex steroid hormones, estrogen and progesterone.

SEXUAL DIFFERENTIATION

PUBERTY

MALE REPRODUCTIVE PHYSIOLOGY

FEMALE REPRODUCTIVE PHYSIOLOGY

Appendix I. Common Abbreviations and Symbols

Appendix II. Normal Values and Constants

Challenge Yourself Answers

Frequently Asked Questions

SUMMARY

ent Genetic sex is determined by the sex chromosomes, either XX or XY. Gonadal sex is defined by the presence of testes or ovaries. Phenotypic sex is determined by the hormonal output of the gonads.

ent Puberty in males and females is initiated by the pulsatile secretion of GnRH, which drives the pulsatile secretion of FSH and LH. Pulsatile secretion of FSH and LH drives the testes and ovaries to secrete their respective sex steroid hormones (testosterone, and progesterone and estrogen).

ent In males, the testes are responsible for spermatogenesis and secretion of testosterone. Testosterone is synthesized from cholesterol by the Leydig cells. In some target tissues, testosterone must be converted to dihydrotestosterone by the action of 5α-reductase. Testosterone acts locally to support spermatogenesis, as well as on extratesticular target tissues such as skeletal muscle.

ent Regulation of the testes is via negative feedback effects of testosterone and inhibin on the hypothalamus and anterior pituitary.

ent In females, the ovaries are responsible for oogenesis and secretion of progesterone and estrogen. Progesterone and 17β-estradiol are synthesized from cholesterol by theca and granulosa cells, respectively. Theca cells synthesize progesterone and testosterone, and granulosa cells convert testosterone to 17β-estradiol by the action of aromatase.

ent The menstrual cycle has a follicular (proliferative) phase and a luteal (secretory) phase. The follicular phase is dominated by estrogen, and the luteal phase is dominated by progesterone. Ovulation occurs on day 15 of a 28-day menstrual cycle. If fertilization occurs, the corpus luteum synthesizes steroid hormones to support the developing zygote. If fertilization does not occur, the corpus luteum regresses and menses occurs.

ent Early pregnancy is supported by steroid hormones produced by the corpus luteum, as directed by HCG from the trophoblast. The second and third trimesters of pregnancy are supported by steroid hormones from the placenta. Progesterone, estriol, and prolactin levels increase steadily during pregnancy.

ent Menopause is the cessation of menstrual cycles. During this period, the number of functioning ovarian follicles decreases, estrogen secretion declines, and the circulating levels and pulsatility of FSH and LH secretion increase.


Challenge Yourself

Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. Correct answers are supplied at the end of the book.

1 In which of the following causes of delayed puberty would pulsatile administration of gonadotropin-releasing hormone (GnRH) be effective: hypothalamic dysfunction; Leydig cell dysfunction; androgen insensitivity syndrome?

2 Which step in testosterone synthesis is activated by luteinizing hormone (LH): androstenedione to testosterone; cholesterol to pregnenolone; testosterone to dihydrotestosterone?

3 Which steroidogenic enzyme is not present in the gonads: 17α-hydroxylase; 21β-hydroxylase; cholesterol desmolase?

4 Which hormone maintains the corpus luteum of pregnancy: LH; human chorionic gonadotropin (HCG); estradiol; progesterone?

5 Which of the following organs are needed to synthesize estrogen during the third trimester of pregnancy: corpus luteum; maternal ovaries; placenta; fetal liver; maternal adrenal cortex; maternal liver; fetal adrenal cortex?

6 During which period of the menstrual cycle does the dominant follicle produce most of its estrogen: days 1–4; days 5–14; days 15–20; days 21–25; days 26–28?

7 During which period of the menstrual cycle does the corpus luteum regress (if fertilization does not occur): days 1–4; days 5–14; days 15–20; days 21–25; days 26–28?

8 In a genetic male with deficiency of 5α-reductase, which of the following masculine features is/are present: testes; muscle mass; male hair distribution; epididymis; deepening of the voice?

9 Which of the following is present in androgen insensitivity disorder? Male phenotype, testes, increased levels of androgen receptors, vagina.

10 Which step in ovarian estradiol synthesis is stimulated by FSH? Cholesterol → pregnenolone, androstenedione → testosterone, testosterone → 17β-estradiol.


SELECTED READINGS

Pohl CR, Knobil E: The role of the central nervous system in the control of ovarian function in higher primates. Ann Rev Physiol 44:583, 1982.

Veldhuis JD: The hypothalamic-pituitary axis. In Yen SSC, Jaffe RB (eds): Reproductive Endocrinology. Philadelphia, WB Saunders, 1991.

White PC, New MI, Dupont B: Congenital adrenal hyperplasia. N Engl J Med 316:1519, 1987.

Yen SSC: The human menstrual cycle: Neuroendocrine regulation. In Yen SSC, Jaffe RB (eds): Reproductive Endocrinology. Philadelphia, WB Saunders, 1991.