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Menstrual Cycle (Ovarian and Uterine Cycles)

Menstrual Cycle (Ovarian and Uterine Cycles)

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Hypothalamic-pituitary-ovarian axis
  • Regulates oocyte development and release.
  • Ovarian follicle produces hormones that prepare the endometrium for implantation in the case of oocyte fertilization.
Follicular/Proliferative Phase:
GnRH
In response, both LH and FSH are released.
LH
  • Stimulates theca interna cells of the ovarian follicle to produce androgens, some of which move into the nearby granulosa cells.
FSH
  • Stimulates granulosa cells to increase aromatase activity
Aromatase converts androgens to estrogens, which exit the granulosa cell.
Estrogen
  • Stimulates growth of the endometrium; this characterizes the proliferative phase of the uterine cycle.
  • Stimulates growth and activity of granulosa cells, which, in turn, produce more estrogen;
  • And, at levels below a set threshold, estrogen inhibits release of FSH and LH from the anterior lobe of the pituitary gland; thus, during the follicular phase of the ovarian cycle, estrogen has negative feedback effects.
Ovulation
  • Growing population of granulosa cells raises the plasma estrogen concentration, it eventually surpasses a set threshold and has a positive effect on the anterior lobe of the pituitary gland.
  • In response, there is a surge in LH and FSH release (the "pre-ovulatory surge").
  • LH and FSH triggers ovulation, which is the ejection of the secondary oocyte and corona radiate into the peritoneum. Recall that, from here, the oocyte (typically) moves into the uterine tubes.
  • Ruptured follicle remains within the ovary.
Luteal/Secretory Phase
  • Follicular cells are morphologically and physiologically transformed into the corpus luteum, which is now the primary hormone-producing structure
Progesterone
  • Stimulates growth and differentiation of the endometrium, thus ushering it to the secretory phase of the uterine cycle.
  • Inhibits endocrine activities of the hypothalamus and anterior lobe of the pituitary gland.
Estrogen
  • Promotes additional endometrial growth.
Corpus luteum
  • The fate of the corpus luteum and endometrium are contingent upon successful fertilization and implantation of the ovulated oocyte.
If fertilization and implantation occur, the corpus luteum persists and continues to produce the hormones necessary to sustain the endometrium and developing blastocyst until the placenta can take over its endocrine duties. If fertilization and/or implantation do not occur, the corpus luteum involutes, aka, regresses, and becomes the corpus albicans ("white body") which does not produce hormones.
Menses
  • In the absence of progesterone and estrogen, the endometrial tissues begin to desquamate, aka, shed, and are discharged as menstrual blood.
  • Menses marks the end of the luteal phase of the ovarian cycle and overlaps with the early events of follicular phase.
Timing:
  • Inter and intra-individual variation in cycle length is typical; moreover, some cycles, especially in young women, are anovulatory (an oocyte is not released from the ovary).
Clinical correlation: Dysmenorrhea
  • Clinical term for painful abdominopelvic cramping with menstruation
  • When over-the-counter drugs aren't sufficient, dysmenorrhea is often treatable with hormonal contraceptives, which inhibit the cyclic events that produce cramping.

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