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258Brs Physiology

3.negative feedback control—testosterone and inhibin

■Testosterone inhibits the secretion of lH by inhibiting the release of GnRH from the hypothalamus and by directly inhibiting the release of LH from the anterior pituitary.

■Inhibin (produced by the Sertoli cells) inhibits the secretion of fsH from the anterior pituitary.

C.Actions of testosterone or dihydrotestosterone

1.Actions of testosterone

■Differentiation of epididymis, vas deferens, and seminal vesicles

■Pubertal growth spurt

■Cessation of pubertal growth spurt (epiphyseal closure)

■Libido

■Spermatogenesis in Sertoli cells (paracrine effect)

■Deepening of voice

■Increased muscle mass

■Growth of penis and seminal vesicles

■Negative feedback on anterior pituitary

2.Actions of dihydrotestosterone

■Differentiation of penis, scrotum, and prostate

■Male hair pattern

■Male pattern baldness

■Sebaceous gland activity

■Growth of prostate

3.Androgen insensitivity disorder (testicular feminizing syndrome)

■is caused by deficiency of androgen receptors in target tissues of males.

■Testosterone and dihydrotestosterone actions in target tissues are absent.

■There are female external genitalia (“default”), and there is no internal genital tract.

■Testosterone levels are elevated due to lack of testosterone receptors in the anterior pituitary (lack of feedback inhibition).

D.Puberty (male and female)

■is initiated by the onset of pulsatile GnrH release from the hypothalamus.

■FSH and LH are, in turn, secreted in pulsatile fashion.

■GnRH up-regulates its own receptor in the anterior pituitary.

e.variation in fsH and lH levels over the life span (male and female)

1.In childhood, hormone levels are lowest and FSH > LH.

2.At puberty and during the reproductive years, hormone levels increase and LH > FSH.

3.In senescence, hormone levels are highest and FSH > LH.

x. femAle rePrODUCTIOn

A.synthesis of estrogen and progesterone (figure 7.18)

■Theca cells produce testosterone (stimulated at the first step by LH). Androstenedione diffuses to the nearby granulosa cells, which contain 17β-hydroxysteroid dehydrogenase,

which converts androstenedione to testosterone, and aromatase, which converts testosterone to 17β-estradiol (stimulated by FSH).

B.regulation of the ovary

1. Hypothalamic control—GnrH

■ As in the male, pulsatile GnRH stimulates the anterior pituitary to secrete FSH and LH.

2.Anterior lobe of the pituitary—fsH and lH

■ FSH and LH stimulate the following in the ovaries:

a.Steroidogenesis in the ovarian follicle and corpus luteum

b.Follicular development beyond the antral stage

c.Ovulation

d.Luteinization

3.negative and positive feedback control—estrogen and progesterone (Table 7.13)

C. Actions of estrogen

1.Has both negative and positive feedback effects on FSH and LH secretion.

2.Causes maturation and maintenance of the fallopian tubes, uterus, cervix, and vagina.

3.Causes the development of female secondary sex characteristics at puberty.

4.Causes the development of the breasts.

5.Up-regulates estrogen, LH, and progesterone receptors.

6.Causes proliferation and development of ovarian granulosa cells.

7.Maintains pregnancy.

260 BRS Physiology

8.  Lowers the uterine threshold to contractile stimuli during pregnancy. 9.  Stimulates prolactin secretion (but then blocks its action on the breast).

D.Actions of progesterone

1.  Has negative feedback effects on FSH and LH secretion during luteal phase. 2.  Maintains secretory activity of the uterus during the luteal phase.

3.  Maintains pregnancy.

4.  Raises the uterine threshold to contractile stimuli during pregnancy. 5.  Participates in development of the breasts.

E.Menstrual cycle (Figure 7.19)

1.  Follicular phase (days 0 to 14)

■A primordial follicle develops to the graafian stage, with atresia of neighboring follicles.

■LH and FSH receptors are up-regulated in theca and granulosa cells.

■Estradiol levels increase and cause proliferation of the uterus.

■FSH and LH levels are suppressed by the negative feedback effect of estradiol on the anterior pituitary.

■Progesterone levels are low.