Chapter 22

After studying this chapter, you should be able to:

• Name the key hormones secreted by Leydig cells and Sertoli cells of the testes and by Graafian follicles and corpora lutea of the ovaries.
• Outline the role of chromosomes, hormones, and related factors in sex determination and development.
• Summarize the hormonal changes that occur at puberty in males and females.
• Outline the hormonal changes and their physiologic effects during perimenopause and menopause.
• Describe the physiologic changes that occur in the female reproductive organs during the menstrual cycle.
• Know the general structures of 17β-estradiol and progesterone, and describe their biosynthesis, transport, metabolism, and actions.
• Describe the roles of the pituitary and the hypothalamus in the regulation of ovarian function, and the role of feedback loops in this process.
• Describe the hormonal changes that accompany pregnancy and parturition.
• Outline the processes involved in lactation.

Modern genetics and experimental embryology make it clear that, in most species of mammals, the multiple differences between the male and the female depend primarily on a single chromosome (the Y chromosome) and a single pair of endocrine structures, namely the testes in the male and the ovaries in the female. The differentiation of the primitive gonads into testes or ovaries in utero is genetically determined in humans, but the formation of male genitalia depends on the presence of a functional, secreting testis; in the absence of testicular tissue, development is female. Evidence indicates that male sexual behavior and, in some species, the male pattern of gonadotropin secretion are due to the action of male hormones on the brain in early development. After birth, the gonads remain quiescent until adolescence, when they are activated by gonadotropins from the anterior pituitary. Hormones secreted by the gonads at this time cause the appearance of features typical of the adult male or female and the onset of the sexual cycle in the female. In human females, ovarian function regresses after a number of years and sexual cycles cease (the menopause). In males, gonadal function slowly declines with advancing age, but the ability to produce viable gametes persists.

In both sexes, the gonads have a dual function: the production of germ cells (gametogenesis) and the secretion of sex hormones. The androgens are steroid sex hormones that are masculinizing in their action; the estrogens are those that are feminizing. Both types of hormones are normally secreted in both sexes. The ovaries secrete large amounts of estrogens and small amounts of androgens, a pattern that is reversed in males. Androgens are secreted from the adrenal cortex in both sexes, and some of the androgens are converted to estrogens in fat and other extragonadal and extra-adrenal tissues. The ovaries also secrete progesterone, a steroid that has special functions in preparing the uterus for pregnancy.

Particularly during pregnancy, the ovaries secrete the polypeptide hormone relaxin, which loosens the ligaments of the pubic symphysis and softens the cervix, facilitating delivery of the fetus. ...

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