Ct

Testosterone-► Sex Characteristics

Figure 25.10 • Regulation of spermatogenesis.

tide from unmodified decapeptides. Analogs of GnRH that are used therapeutically are covered in Chapter 20.

Pituitary Gonadotropins: Luteinizing Hormone and Follicle-Stimulating Hormone

The pituitary gonadotropins LH and FSH, their structures, genes, receptors, biological roles, and their regulation (including by negative feedback actions of steroid hormones) have been studied intensively.33,34FSH, LH, and CG are all glycopeptide dimers with the same a subunit but different jS-subunits.

In females, LH and FSH regulate the menstrual cycle (see Figs. 25.9 and 25.11). At the start of the cycle, plasma concentrations of estradiol and other estrogens (Fig. 25.11) and progesterone are low. FSH and LH stimulate several ovarian follicles to enlarge and begin developing more rapidly than others. After a few days, only one follicle continues developing to the release of a mature ovum. The granulosa cells of the maturing follicles begin secreting estrogens, which then cause the uterine endometrium to thicken. Vaginal and cervical secretions increase. Gonadotropins and estrogen reach their

Secretions

Figure 25.9 • Regulation of ovulation by GnRH.

Secretions

Figure 25.9 • Regulation of ovulation by GnRH.

80 r 60

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