Menstrual cycle

1. The Menstrual Cycle
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3. The Menstrual Cycle: Two Phases
•Ideally 14 days each
•Follicular and Luteal
–Describe changes in the ovary
•Proliferative and Secretory
–Describe changes in the endometrium
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4. Hormones in the Menstrual Cycle
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5. Follicular phase
•Begins with the onset of menses and ends on the day of
the luteinizing hormone (LH) surge
•During late luteal phase/ early follicular phase:
–withdrawal of estrogen and progesterone in late luteal phase
–In early follicular phase, ovary is least hormonally active  low
serum estradiol and progesterone
–release from negative feedback effects of estrogen +
progesterone  increase in GnRH pulse frequency in early
follicular phase
–Increased GnRH pulse  30% increase in serum FSH
concentrations
•FSH release from the pituitary stimulates growth of 5-15
primordial follicles
–By late follicular phase, a single dominant follicle has been
selected.
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6. Follicular Phase
•The developing dominant follicle produces
estrogen in a 2-cell process
–Theca interna cells produce androstenedione in
response to LH stimulation
–Granulosa cells convert androstenedione  estradiol
when stimulated by FSH
•Estrogen causes the uterine lining to thicken /
proliferate
•GnRH pulse frequency increase rise in LH
•Rise in LH also stimulates androgen synthesis
 androgens are converted to estrogens
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7. Follicular Phase
•Serum estradiol concentrations peak
approximately 1 day before ovulation
•Midcycle (~day 14) there is an LH spike in
response to this estrogen surge
•Ovulation occurs as increase in LH level
causes the follicle to rupture and release
mature ovum
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8. LH Surge
•Involves a poorly-understood
neuroendocrine phenomenon in which
there is a switch from negative feedback
control of LH secretion by estradiol and
progesterone to positive feedback
•Rising estradiol levels at the end of the
follicular phase result in a 10-fold increase
in serum LH concentration
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9. LH Surge: Effect on the Ovary
•In response to LH:
–Oocyte in the dominant follicle completes 1
st
meiotic
division
–Increase in local secretion of plasminogen activator
and cytokines required for ovulation
–Oocyte released from follicle at ovarian surface ~ 36
hrs after LH surge.
–Even before oocyte is released, granulosa cells
around it begin to “luteinize” and produce
progesterone. Progesterone slows LH pulses/
decreases LH pulse frequency
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10. Ovulation
•LH spike stimulates ovulation, the release of the ovum from the follicle
•After ovulation, the luteal phase begins, and remnants of the follicle left
behind in the ovary develop into the corpus luteum
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11. Ovulation
•Ovum usually passes into adjoining
fallopian tube and is swept down to the
uterus by the cilia lining the tube
•Takes 3-4 days for ovum to travel down
tube to the uterus
•Fertilization must occur within 24 hrs of
ovulation or ovum degenerates
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12. Luteal Phase
•After ovulation, granulosa and theca
interna cells lining the wall of the follicle
form the corpus luteum cyst (stimulated by
LH)
•The corpus luteum synthesizes estrogen
and large amounts of progesterone
 Progesterone stimulates the endometrium to
become more glandular/secretory in
preparation for implantation of fertilized ovum
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13. Luteal Phase
•If fertilization occurs:
–developing trophoblast synthesizes human
chorionic gonadotropin (hcg)
–hcg maintains the corpus luteum so it may
continue producing estrogen and
progesterone to support the endometrium
•By ~ 8-10 weeks gestation, the placenta is
developed, and takes over production of
estrogen and progesterone
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14. Luteal Phase
•If fertilization does not occur:
–corpus luteum is not maintained by hcg
–Corpus luteum degenerates after ~ 14 days
–Estrogen and progesterone levels fall
–Withdrawal of progesterone causes secretory
endometrium to slough
–FSH levels slowly rise again in absence of negative
feedback
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