OVARIAN AND MENSTRUAL CYCLES

What is the female reproductive cycle?

• non pregnant females show cyclical changes in overies and uterus

  • leads to ovarian + uterine (menstrual) cycles

• each cycle is ~28 days

• involves completion of oogenesis and preparation of uterus to receive fertilized ovum

What is the ovarian cycle?

• in ovary

• series of events associated with maturation of ovum

• stimulated by level changes of FSH and LH

• results in varying secretion of estrogen and progesterone

What is the menstrual / uterine cycle?

• in uterus

• development + shedding of uterine lining

• occurs in non pregnant females

• after puberty, before menopause

• stimulated by changing levels of estrogen + progesterone

What are some important notes regarding hormone release?

• GnRH stimulates gonadotropin (FSH and LH) release

• rising estrogen levels inhibit GnRH release (-ve feedback)

• large + sudden surge of estrogen stimulates large release of LH in middle of reproductive cycle (+ve feedback)

• high progesterone levels inhibit relaese of GnRH and gonadotropins (-ve feedback)

What are the phases of the ovarian cycle?

• follicular phase (day 1-14)

• ovulation (day 14)

• luteal phase (day 14-28)

What processes occur in the ovarian cycle?

• each ovary has ~200k primary follicles (total 400k at birth)

  • primary oocyte is found within primary follicle

• each month, 1 egg matures and is released; plus minus 28 day cycle

How is the ovarian cycle regulated?

• hypothalamus which releases gonadotropin releasing hormone

• anterior pituitary which releases FSH and LH

• ovaries which releases estrogen and progesterone

What occurs in day 1-5 of the follicular phase in the ovarian cycle?

• hypothalamus secretes gonadotropin releasing hormone

• anterior pituitary secretes FSH and LH

• 4 to 5 primary follicles begin development (follicle = ovum surrounded by cell layers)

• all primary follicles degenerate except one at the end

• remaining one develops into secondary follicle

What occurs in day 6-13 of the follicular phase in the ovarian phase?

• follicle grows + internal fluid increases

• ovum is pushed to side

• moderate estrogen levels from follicle act as negative feedback on hypothalamus to stop FSH and LH secretion

What occurs in ovulation / day 14 of the ovarian cycle?

• estrogen levels peak; positive feedback on hypothalamus to increase LH secretion

• ovulation occurs from LH level peak

• follicle bursts + ovum released into fallopian tubes

  • meiosis 1 completed, meosis 2 begins, oocyte arrested in metaphase 2 until fertilization

• LH transforms ruptured follicle into corpus luteum for progesterone and estrogen production

What occurs in day 15 to 28 of the luteal phase in the ovarian cycle?

• high levels of progesterone from corpus luteum inhibits GnRH secretion (also FSH / LH)

• if egg is not fertilized at the end, corpus luteum degrades to corpus albicans + progesterone is no longer produced

• FSH and LH are no longer blocked

• cycle repeats

What occurs in the uterine / menstrual cycle?

• series of cyclic changes that the uterine endometrium goes through each month in response to ovarian hormones in the blood

What occurs in day 1-5 of the menstrual phase of the menstrual cycle?

• uterus sheds all but deepest parts of the endometrium

• low levels of estrogen and progesterone

What occurs in day 6-14 of the proliferative phase of the menstrual cycle?

• endometrium rebuilds itself

• high levels of estrogen

What occurs in day 15-28 of the secretory phase of the menstrual cycle?

• endometrium prepares for implantation of the embryo where it becomes thicker and mroe vascularized

• high levels of progesterone

What happens if fertilization does not occur?

• progesterone levels fall which deprives endometrium of hormonal support

• spiral arteries kink and go into spasms + endometrial cells begin to die

• functional layer begins to digest itself and is sloughed off

• 25 to 65 mL of blood, tissue fluid, mucous, and epithelial cells in menstruation

What occurs in the corpus luteum if fertilization occurs?

• secretes estrogen and progesterone to maintain endometrium

• maintained by human chorionic gonadotropin (hCG) which is produced by implanted blastocyst and then by developing placenta

What occurs in the placenta if fertilization occurs?

• produce estrogen and progesterone once well developed in 4 months + when corpus luteum begins degeneration