Chapter Summary

Sexual Reproduction

1. Sperm that bear X chromosomes produce XX zygotes when they fertilize an ovum; sperm that bear Y chromosomes produce XY zygotes.
1. Embryos that have the XY genotype develop testes; those without a Y chromosome produce ovaries.
2. The testes of a male embryo secrete testosterone and müllerian inhibitory factor. MIF causes degeneration of female accessory sex organs and testosterone promotes the formation of male accessory sex organs.
2. The male accessory sex organs are the epididymis, ductus (vas) deferens, seminal vesicles, prostate, and ejaculatory duct.
1. The female accessory sex organs are the uterus and uterine (fallopian) tubes. They develop when testosterone and müllerian inhibitory factor are absent.
2. Testosterone indirectly (acting via conversion to dihydrotestosterone) promotes the formation of male external genitalia; female genitalia are formed when testosterone is absent.
3. Numerous disorders of embryonic sexual development can be understood in terms of the normal physiology of the developmental processes.

Endocrine Regulation of Reproduction

1. The gonads are stimulated by two anterior pituitary hormones: FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
1. The secretion of FSH and LH is stimulated by gonadotropin-releasing hormone (GnRH), which is secreted by the hypothalamus.
2. The secretion of FSH and LH is also under the control of the gonads by means of negative feedback exerted by gonadal steroid hormones and by a peptide called inhibin.
2. The rise in FSH and LH secretion that occurs at puberty may be due to maturational changes in the brain and to decreased sensitivity of the hypothalamus and pituitary gland to the negative feedback effects of sex steroid hormones.
3. The pineal gland secretes the hormones melatonin. This hormone has an inhibitory effect on gonadal function in some species of mammals, but its role in human physiology is presently controversial.

Male Reproductive System

1. In the male, the pituitary secretion of LH is controlled by negative feedback from testosterone, whereas the secretion of FSH is controlled by the secretion of inhibin from the testes.
1. The negative feedback effect of testosterone is actually produced by the conversion of testosterone to 5-reduced androgens and to estradiol.
2. The secretion of testosterone is relatively constant rather than cyclic, and it does not decline sharply at a particular age.
2. Testosterone promotes the growth of soft tissue and bones before the epiphyseal discs have sealed; thus, testosterone and related androgens are anabolic steroids.
1. Testosterone is secreted by the interstitial cells of Leydig under stimulation of LH.
2. LH receptor proteins are located in the interstitial tissue. FSH receptors are located in the Sertoli cells within the seminiferous tubules.
3. The Leydig cells of the interstitial compartment and the Sertoli cells of the tubular compartment of the testes secrete autocrine regulatory molecules which allow these compartments to interact.
3. Diploid spermatogonia in the seminiferous tubules undergo meiotic cell division to produce haploid sperm.
1. At the end of meiosis, four spermatids are formed. They develop into spermatozoa by a maturational process called spermiogenesis.
2. Sertoli cells in the seminiferous tubules are required for spermatogenesis.
3. At puberty, testosterone is required for the completion of meiosis, and FSH is required for spermiogenesis.
4. Spermatozoa in the seminiferous tubules are conducted to the epididymis and drained from the epididymis into the ductus deferens. The prostate and seminal vesicles add fluid to the semen.
5. Penile erection is produced by parasympathetic-induced vasodilation. Ejaculation is produced by sympathetic nerve stimulation of peristaltic contraction of the male sex accessory organs.

Female Reproductive System

1. The human sexual response is divided into four phases: excitation, orgasm, plateau, and resolution. Both sexes follow a similar pattern.
2. Primordial follicles in the ovary contain primary oocytes that have become arrested at prophase of the first meiotic division. Their number is maximal at birth and declines thereafter.
1. A small number of oocytes in each cycle are stimulated to complete their first meiotic division and become secondary oocytes.
2. At the completion of the first meiotic division, the secondary oocyte is the only complete cell formed. The other product of this division is a tiny polar body, which disintegrates.
3. One of the secondary follicles grows very large, becomes a graafian follicle, and is ovulated.
1. Upon ovulation, the secondary oocyte is extruded from the ovary. It does not complete the second meiotic division unless it becomes fertilized.
2. After ovulation, the empty follicle becomes a new endocrine gland called a corpus luteum.
3. The ovarian follicles secrete only estradiol, whereas the corpus luteum secretes both estradiol and progesterone.
4. The hypothalamus secretes GnRH in a pulsatile fashion, causing pulsatile secretion of gonadotropins. This is needed to prevent desensitization and down regulation of the target glands.

Menstrual Cycle

1. During the follicular phase of the cycle, the ovarian follicles are stimulated by FSH from the anterior pituitary.
1. Under FSH stimulation, the follicles grow, mature, and secrete increasing amounts of estradiol.
2. At about day 13, the rapid rise in estradiol secretion stimulates a surge of LH from the anterior pituitary. This represents positive feedback.
3. The LH surge stimulates ovulation at about day 14.
4. After ovulation, the empty follicle is stimulated by LH to become a corpus luteum, at which point the ovary is in a luteal phase.
5. The secretion of progesterone and estradiol rises during the first part of the luteal phase and exerts negative feedback on FSH and LH secretion.
6. Without continued stimulation by LH, the corpus luteum regresses at the end of the luteal phase, and the secretion of estradiol and progesterone declines. This decline results in menstruation and the beginning of a new cycle.
2. The rising estradiol concentration during the follicular phase produces the proliferative phase of the endometrium. The secretion of progesterone during the luteal phase produces the secretory phase of the endometrium.
3. Oral contraceptive pills usually contain combinations of estrogen and progesterone, which exert negative feedback control of FSH and LH secretion.

Fertilization, Pregnancy, and Parturition

1. The sperm undergoes an acrosomal reaction, which allows it to penetrate the corona radiata and zona pellucida.
1. Upon fertilization, the secondary oocyte completes meiotic division and produces a second polar body, which degenerates.
2. The diploid zygote undergoes cleavage to form a morula and then a blastocyst. Implantation of the blastocyst in the endometrium begins between the fifth and seventh day.
2. The trophoblast cells of the blastocyst secrete human chorionic gonadotropin (hCG), which functions in the manner of LH and maintains the motherís corpus luteum for the first 10 weeks of pregnancy.
1. The trophoblast cells provide the fetal contribution to the placenta. The placenta is also formed from adjacent maternal tissue in the endometrium.
2. Oxygen, nutrients, and wastes are exchanged by diffusion between the fetal and maternal blood.
3. The placenta secretes chorionic somatomammotropin (hCS), chorionic gonadotropin (hCG), and steroid hormones.
1. The action of hCS is similar to that of prolactin and growth hormone. The action of hCG is similar to that of LH and TSH.
2. The major steroid hormones secreted by the placenta is estriol. The placenta and fetal glands cooperate in the production of steroid hormones.
4. Contraction of the uterus in labor is stimulated by oxytocin from the posterior pituitary and by prostaglandins, produced within the uterus.
1. Androgens, primarily DHEA, secreted by the fetal adrenal cortex are converted into estrogen by the placenta.
2. Estrogen secreted by the placenta induces oxytocin synthesis; enhances uterine sensitivity to oxytocin, and promotes prostaglandin synthesis in the uterus. These events culminate in labor and delivery.
5. The high levels of estrogen during pregnancy, acting synergistically with other hormones, stimulate growth and development of the mammary glands.
1. Prolactin (and the prolactin-like effects of hCS) can stimulate the production of milk proteins. Prolactin secretion and action, however, are blocked during pregnancy by the high levels of estrogen secreted by the placenta.
2. After delivery, when estrogen levels fall, prolactin stimulates milk production.
3. The milk-ejection reflex is a neuroendocrine reflex. The stimulus of suckling causes reflex secretion of oxytocin, which stimulates contractions of the lactiferous ducts and the ejection of milk from the nipple.

After studying this chapter, students should be able to . . .

1. describe how the chromosomal content determines the sex of an embryo and how this relates to the development of testes or ovaries.
2. explain how the development of accessory sex organs and external genitalia is affected by the presence or absence of testes in the embryo.
3. describe the hormonal changes that occur during puberty, the mechanisms that may control the onset of puberty, and the secondary sexual characteristics that develop during puberty.
4. explain how the secretions of FSH and LH are regulated in the male and describe the actions of these hormones on the testis.
5. describe the structure of the testis and the interaction between interstitial Leydig cells and seminiferous tubules.
6. describe the stages of spermatogenesis and the functions of Sertoli cells in this process.
7. explain the hormonal control of spermatogenesis and describe the effects of androgens on the male accessory sex organs.
8. describe the composition of semen, explain the physiology of erection and ejaculation, and discuss the various factors that affect male fertility.
9. describe the four phases of the human sexual response
10. describe oogenesis and the stages of follicle development through ovulation and the formation of a corpus luteum.
11. explain how the hormonal interactions involved in the control of ovulation.
12. describe the changes in the secretion of ovarian sex steroids during a nonfertile cycle and explain the function and fate of the corpus luteum.
13. explain how the secretion of FSH and LH is controlled through negative and positive feedback mechanisms during a menstrual cycle.
14. explain how contraceptive pills function to prevent ovulation.
15. describe the cyclic changes that occur in the endometrium and the hormonal mechanisms that cause these changes.
16. describe the acrosomal reaction and the events that occur at fertilization, blastocyst formation, and implantation.
17. explain how menstruation and further ovulation are normally prevented during pregnancy.
18. describe the structure and functions of the placenta.
19. list the hormones secreted by the placenta and describe their actions.
20. discuss the factors that stimulate uterine contractions during labor and parturition, and explain how the onset of labor may be regulated.
21. describe the hormonal requirements for development of the mammary glands during pregnancy and how lactation is prevented during pregnancy.
22. describe the milk-ejection reflex.