Menstrual Cycle Physiology

Menstrual Cycle Physiology

A normal menstrual cycle is the presence of regular vaginal bleeding. This occurs as a result of the shedding of the endometrial lining following failure of fertilization of the oocyte or failure of implantation. The cycle depends on changes occurring after puberty within the ovaries and fluctuation in ovarian hormone levels, which are themselves controlled by the pituitary and hypothalamus within the hypothalamo–pituitary–ovarian axis (HPO).

Hormones of menstutral cycle:

The hypothalamus :

The hypothalamus in the forebrain secretes the peptide hormone gonadotrophin-releasing hormone (GnRH), which in turn controls pituitary hormone secretion. GnRH

It must be released in a pulsatile fashion to stimulate pituitary secretion of;

1. Luteinizing hormone (LH)

2. Follicle-stimulating hormone (FSH).

 The pituitary gland:

GnRH stimulation of the basophil cells in the anterior pituitary gland causes synthesis and release of the gonadotrophic hormones FSH and LH. This process is modulated by the ovarian sex steroid hormones;

1. Oestrogen

 2. Progesterone.

 These hormones are released from ovaries by response of LH and FSH.

 Low levels of oestrogen have an inhibitory effect on LH production (negative feedback), whereas high levels of oestrogen will increase LH production (positive feedback).

 

Phases of menstutral cycle:

There are four phases of menstural cycle.

1. Mensturation

2. Follicular phase

3. Ovulation

4. Luteal phase

Mensturation:

Menstruation (day 1) is the shedding of the ‘dead’ endometrium and ceases as the endometriumregenerates (which normally happens by day 5–6 of the cycle). Immediately prior to menstruation, three distinct layers of endometrium can be seen. The basalis is the lower 25% of the endometrium, which will remain throughout menstruation and shows few changes during the menstrual cycle. The midportion is the stratum spongiosum with oedematous stroma and exhausted glands. The superficial portion (upper 25%) is the stratum compactum with prominent decidualized stromal cells. A fall in circulating levels of oestrogen and progesterone approximately 14 days after ovulation leads to loss of tissue fluid, vasoconstriction of spiral arterioles and distal ischaemia. This results in tissue breakdown and loss of the upper layers, along with bleeding from fragments of the remaining arterioles, seen as menstrual bleeding. Enhanced fibrinolysis reduces clotting. The effects of oestrogen and progesterone on the endometrium can be reproduced artificially, for example in patients taking the combined oral contraceptive pill or hormone replacement therapy (HRT), who experience a withdrawal bleed during their pill-free week each month. Vaginal bleeding will cease after 5–10 days as arterioles vasoconstrict and the endometrium begins to regenerate. Haemostasis in the uterine endometrium is different from haemostasis elsewhere in the body as it does not involve the processes of clot formation and fibrosis.

Menstural cycle video: