D 0 12 3

\ TIME (hrs)

neuroendocrine relationships controlling LH/FSH release

FIGURE 57-2 Neuroendocrine control of gonadotropin secretion in females. The hypothalamic pulse generator functions as a neuronal "clock" that fires at regular hourly intervals (A), resulting in the periodic release of gonadotropin-releasing hormone (GnRH) from GnRH neurons into the hypothalamic-pituitary portal vasculature (B). GnRH neurons (B) receive inhibitory input from opioid, dopamine, and GABA neurons and stimulatory input from noradrenergic neurons (NE, norepinephrine). The pulses of GnRH trigger the intermittent release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from pituitary gonadotropes (C), resulting in the pulsatile plasma profile (D). FSH and LH regulate ovarian production of estrogen and progesterone, which exert feedback controls (E). (See text and Figure 57-3 for additional details.)

estrogen progesterone estrogen progesterone

FIGURE 57-2 Neuroendocrine control of gonadotropin secretion in females. The hypothalamic pulse generator functions as a neuronal "clock" that fires at regular hourly intervals (A), resulting in the periodic release of gonadotropin-releasing hormone (GnRH) from GnRH neurons into the hypothalamic-pituitary portal vasculature (B). GnRH neurons (B) receive inhibitory input from opioid, dopamine, and GABA neurons and stimulatory input from noradrenergic neurons (NE, norepinephrine). The pulses of GnRH trigger the intermittent release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from pituitary gonadotropes (C), resulting in the pulsatile plasma profile (D). FSH and LH regulate ovarian production of estrogen and progesterone, which exert feedback controls (E). (See text and Figure 57-3 for additional details.)

Menstruation marks the start of the menstrual cycle. During the follicular (or proliferative) phase, estrogen stimulates proliferation and differentiation. One important effect of estrogen in the endometrium and other tissues is induction of the progesterone receptor (PR), which enables cells to respond to this hormone during the second half of the cycle.

In the luteal (or secretory) phase of the cycle, elevated progesterone limits the proliferative effect of estrogens on the endometrium by stimulating differentiation. Progesterone is thus important in preparation for implantation and for the decidual reaction that takes place in the uterus at the implantation site. There is a narrow "window of implantation," spanning days 19—24 of the cycle, when the epithelial cells of the endometrium are receptive to blastocyst implantation.

If implantation occurs, human chorionic gonadotropin, produced initially by the trophoblast and later by the placenta, maintains steroid hormone synthesis by the corpus luteum during the early stages of pregnancy. Thereafter, the placenta itself becomes the major site of estrogen and progesterone synthesis.

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