Menstrual Cycle

• Primordial germ cells migrate to developing ovary around 5-6 weeks gestation and differentiate into oogonia as the ovaries develop.
• Oogonia are diploid egg-precursors, and multiply by MITOSIS.
• Once numbers have been increased by mitosis, Primordial follicles form and the oocytes enter into MEIOSIS.
• At 16-24 weeks, Meiosis I is paused in prophase I.
• Primordial follicles only resume growth after puberty.
• From then on, a few begin to enter the menstrual cycle each month.
• Primordial follicles are laid down before birth.
• In the foetal ovary, the surrounding cells condense around the oocyte and differentiate into the granulosa cells.
• These then secrete an acellular basal lamina.
• The oocyte enlarges.
• Granulosa cells multiply and the oocyte secretes zona pellucida.
• A second layer of theca cells differentiates around the basal lamina.
• Granulosa cells divisions increase and follicle becomes larger forming an antrum filled with follicular fluid.
• Sustained high estrogen levels cause LH surge by reversing feedback on pituitary leading to ovulation.
• Endometrium disintegrates due to lack of progesterone.
• Corpus luteum disintegrates and progesterone falls.
• Inter-cycle rise in FSH recruits follicles.
• High progesterone inhibits gonadotropin production by pituitary leading to failure of corpus luteum.
• Corpus luteum secretes progesterone (some estrogen) stimulating differentiation and maintenance of endometrium.
• Menstrual cycle summary.
• Estrogen from follicle causes proliferation of endometrium.
• The oocyte matures and is released during ovulation.
• The corpus luteum forms from the empty follicle and is dominated by progesterone production.
• The follicular phase of the menstrual cycle is FSH/LH dependent for 14 days.
• The proliferative phase of the menstrual cycle is cycle independent for 65 days.
• The secretory phase of the menstrual cycle is FSH independent for 65 days.
• The ovaries and uterus are part of the female reproductive system.
• The hypothalamus, GnRH, pituitary, LH, FSH, ovary, estrogen, growing follicles, endometrial proliferation, rise in FSH and growth of antral follicles are all parts of the female reproductive system.
• Dominant follicle selection occurs at the small antral stage.
• Follicle initiation is the process where a cohort of early follicles leave the resting pool and grow continuously independent of gonadotrophins.
• Follicle recruitment is the process where a dozen or so follicles become responsive to FSH and are recruited into the current cycle.
• Dominant follicle selection is the process where from the group that are recruited only one will become the dominant follicle and be selected for ovulation.
• The proliferative phase of the menstrual cycle is driven by estrogen from the growing dominant follicle.
• The endometrial lining regrows and thickens during the proliferative phase of the menstrual cycle.
• Maximal cell division occurs by days 12-14 during the proliferative phase of the menstrual cycle.
• When endometrium >4mm progesterone receptors begin to be induced during the proliferative phase of the menstrual cycle.
• The secretory phase of the menstrual cycle is characterized by sustained estrogen and ovulation.