Menstrual Cycle

Menstrual Cycle2110571 Sharimilee a/p Ratnam

1Definition: regular cyclical changes occurring in females of child bearing age characterized by periodic vaginal bleeding

Usual length; 28 days (may vary according to individual and external factors)If unusual?

Made up of 2 cyclesOvarian cycleUterine cycle

Ovarian Cycle

Fetal life; even before we ladies are born, weve already got meiosis up and running.

The primordial germ cells, a.k.a the Oogonia start to undergo meiosis, but unfortunately, they dont complete it. (meiotic arrest..) These half cooked oocytes remain this way up to puberty. We call them

Single layer of granulosa cells proliferate to form several layers that surround the oocyte

These granulosa cells themselves make the zona pellucida as a membrane that separate themselves from the oocyte.

The zona pellucida has gap junctions through which the granulosa cells supply ions and small molecules to the oocyte (akin to how the sertoli cell nurses the developing sperm)

Granulosa cells also release paracrines that form an outer layer known as the Theca cells.

Granulosa cells + Theca cells = FOLLICULAR CELLS (whose main function is estrogen secretion)

So now that the follicular cells have start secreting stuff, all of its secretion has to go somewhere, right?

A fluid filled cavity (antrum) forms in the middle of the granulosa cells. Fluid is made up of secretions of the follicular cells (eg. Estrogen).

The antral follicle undergoes rapid growth. (more estrogen produced more growth,)

During each cycle 15-20 primary follicles mature, but only one dominant follicle will fully mature.

Greatly expanded antrum pushes oocyte (which has developed in to 2ndry oocyte) to one side of the growing follicle. The greatly expanded mature follicle bulges on the ovarian surface, creating a thin surface that ruptures to release the oocyte at ovulation.

Release of ovum is triggered by LH SURGE

Rupture follicle develops into corpus luteum via process called luteinization whereby cells enlarge and are converted to active steroid hormone producing tissue Contains an abundance on cholesterol (steroid hormone precursor) which gives its yellow colour

Highly vascularised to enable secretion of high amouts of progesterone and smaller amounts of estrogen it blood.Human ovary with fully developed Corpus Luteum

Follicular PhaseFSH; antrum formationLH + FSH; proliferation of granulosa cellsLH; theca cells to produce androgensFSH; granulosa cells to produce enzyme aromatase to convert androgen to estrogenThus, rate of estrogen production depends primarily on circulating LH levels.

So what happens to the estrogen secreted?Partly enters blood (to supply sex-specific organs e.g. uterus), while the rest remains in antrum, further stimulating growth of granulosa cell.

Further growth of granulosa cells, more estrogen produced.

Estrogen also has negative feedback on hypothalamus in NEGATIVE FEEDBACK MECHANISM, resulting in reduced GnRH-prompted released

Secretion of inhibin by follicular cell also contributes to fall in FSH. Decline in FSH results in atresia of all but the single most mature follicle. (Sort of like survival of the fittest)

Estrogen alone cannot inhibit tonic secretion of LH because both estrogen and progesterone is needed. At this stage, progesterone secretion hasnt started yet. Thus, LH levels slowly increases.

If there is a negative feedback mechanism, why only FSH is reduced while LH continues to rise?

OvulationLH is secreted in two modesTonic secretion partially suppressed by inhibitory action of estrogen during follicular phase and completely suppressed by progesterone in luteal phase. (NEGATIVE FEEDBACK EFFECT)LH surge triggered by POSITIVE FEEDBACK where peak levels of estrogen, instead if inhibiting, now acts directly on hypothalamus to INCREASE GnRH, thus, both FSH and LH levels increase. However, FSH does not increase as much as LH because continued inhibin secretion keeps it low.

The amount of estrogen required to trigger an LH surge can only be produced by a mature follicle. This way, it can be made sure that follicle will only be ovulated when it has reached the proper degree of maturation

Process of ovulation in women 45 years old canada, taken by a doctor named Dr Donnez, from the Catholic University of Louvain, using a small camera-shaped tube

Result of LH surgeHalts estrogen synthesisReinitiates mitosis by blocking oocyte maturation-inhibiting factorTrigger production of local prostaglandins to promote vascular changes to cause rapid follicular swelling, resulting in swelling which would eventually lead to rupture of ovarian wall a.k.a ovulationCause differentiation of follicular cell into luteal cell

Luteal PhaseUnder influence of LH, corpus luteum secretes both estrogen and progesterone (more abundant).

Estrogen level climbs again, but not as much as it did in the follicular phase. Progesterone prevents estrogen from trigerring another LH surge by powerfully inhibiting LH itself, and FSH too.

No inhibin released, as progesterone is adequate to inhibit FSH so that preparation to receive fertilised ovum is carried out instead of preparation of other ova for release.

Corpus luteum does its job for about 2 weeks; if still no fertilisation, degenerates. But how?

LH influences corpus luteum to release progesterone which in turn, inhibits LH itself. Eventually, no LH, no stimulation of corpus luteum, no more progesterone, no more inhibition on LH and FSH

Cycle repeats all over again.

Uterine CycleInfluence of Estrogen and Progesterone on the two layers of the uterusMyometium: outer smooth muscle layerEndometrium: inner lining containing numerous blood vessels and gland

Estrogen stimulate growth of both myometrium and endometrium, induces synthesis of progesterone receptors in the endometrium

Progesterone makes endometrial tissue loose and edematous to facilitate implantation, secrete and store glycogen , growth of endometrial blood vessels

Menstrual PhaseCharacterised by discharge of blood and endometrial debris from the vagina

First day of menstuation is the day 1 of cycle

No fertilisation degeneration of corpus luteum drop in progesterone and estrogen levels deprive uterine lining of its hormonal support death of endometrium

Fall in ovarian hormone levels uterine prostaglandin release vasoconstriction of endometrial vessels reduction in O2 supply death of endometrium

Uterine prostaglandin release rhythmic contraction of uterine myometrium expulsion of blood and endometrial debris from uterine cavity

Excessive uterine contraction; dysmenorrhea (menstrual cramps)

Average blood loss in a single menstrual period is 50-150 ml

Lasts 5-7 days

After 5-7 days, FSH and LH from newly growing follicles secrete enough estrogen to induce repair and growth of endometrium

Proliferative phaseEndometrium starts to repair itself and proliferate under influence of estrogen from newly developing folliclesSecretory phaseCoincides with ovarian luteal phase

Large amounts of progesterone from corpus luteum converts endometrium to richly vascularised, glycogen filled tissue

Secretory; because endometrial glands are actively secreting glycogen into uterine lumen for nourishment of developing embryo.