The Menstrual Cycle

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The Menstrual Cycle

• District 1 ACOG Medical Student Education Module
  2008

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From Up to Date The Normal Menstrual Cycle
2008
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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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Hormones in the Menstrual Cycle
From Up to Date The Normal Menstrual Cycle
2008
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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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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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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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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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LH Surge Effect on the Ovary

• In response to LH
• Oocyte in the dominant follicle completes 1st
  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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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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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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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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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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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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