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Endocrine Physiology |
259 |
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Chapter 7 |
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Cholesterol |
LH (theca cells) |
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Progesterone |
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Pregnenolone |
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17-Hydroxypregnenolone |
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Dehydroepiandrosterone |
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Androstenedione |
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Testosterone |
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FSH |
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aromatase |
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(granulosa cells) |
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fIGUre 7.18 Synthesis of estrogen and progesterone. FSH = |
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17β-Estradiol |
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follicle-stimulating hormone; LH = luteinizing hormone. |
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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.
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t a b l e |
7.13 |
Negative and Positive Feedback Control of the |
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Menstrual Cycle |
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Phase of menstrual Cycle |
Hormone |
Type of feedback and site |
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Follicular |
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Estrogen |
Negative; anterior pituitary |
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Midcycle |
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Estrogen |
Positive; anterior pituitary |
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Luteal |
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Estrogen |
Negative; anterior pituitary |
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Progesterone |
Negative; anterior pituitary |
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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.
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Ovulation |
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Follicular phase |
Luteal phase |
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Basal body |
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Progesterone |
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temperature |
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17β-Estradiol |
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LH |
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menses |
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FSH |
menses |
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24 26 |
0 |
2 |
4 |
6 |
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10 12 14 16 18 20 22 24 26 |
0 |
2 |
4 |
Figure 7.19 The menstrual cycle. FSH |
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Day of cycle |
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= follicle-stimulating hormone; LH = |
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luteinizing hormone. |
262 |
BRS Physiology |
2. First trimester
■The corpus luteum (stimulated by HCG) is responsible for the production of estradiol and progesterone.
■Peak levels of HCG occur at gestational week 9 and then decline.
3. Second and third trimesters
■Progesterone is produced by the placenta.
■Estrogens are produced by the interplay of the fetal adrenal gland and the placenta. The fetal adrenal gland synthesizes dehydroepiandrosterone-sulfate (DHEA-S), which is
then hydroxylated in the fetal liver. These intermediates are transferred to the placenta, where enzymes remove sulfate and aromatize to estrogens. The major placental estrogen is estriol.
■Human placental lactogen is produced throughout pregnancy. Its actions are similar to those of growth hormone and prolactin.
4. Parturition
■Throughout pregnancy, progesterone increases the threshold for uterine contraction.
■Near term, the estrogen/progesterone ratio increases, which makes the uterus more sensitive to contractile stimuli.
■The initiating event in parturition is unknown. (Although oxytocin is a powerful stimulant of uterine contractions, blood levels of oxytocin do not change before labor.)
5. Lactation
■Estrogens and progesterone stimulate the growth and development of the breasts throughout pregnancy.
■Prolactin levels increase steadily during pregnancy because estrogen stimulates prolactin secretion from the anterior pituitary.
■Lactation does not occur during pregnancy because estrogen and progesterone block the action of prolactin on the breast.
■After parturition, estrogen and progesterone levels decrease abruptly and lactation occurs.
■Lactation is maintained by suckling, which stimulates both oxytocin and prolactin secretion.
■Ovulation is suppressed as long as lactation continues because prolactin has the following effects:
a. Inhibits hypothalamic GnRH secretion.
b. Inhibits the action of GnRH on the anterior pituitary and consequently inhibits LH and FSH secretion.
c. Antagonizes the actions of LH and FSH on the ovaries.
