Normal Menstrual cycle pdf - D. Enas

The Normal Menstrual cycle L1&2

Introduction :

The external manifestation of 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 within

the ovaries and fluctuation in ovarian hormone levels , that are

themselves controlled by the pituitary and hypothalamus , the

hypothalamo-pituitary-ovarian axis ( HPO ) .

Hypothalamus :

The hypothalamus in the forebrain secretes the peptide hormone

gonadotrophn-releasing hormone ( GnRH ) , which in turn controls

pituitary hormone secretion . GnRH must be released in a pulsatile

fashion to stimulate pituitary secretion of luteinizing hormone ( LH ) and

follicle stimulating hormone ( FSH ) . If GnRH is giving in a constant

high dose , it desensitizes the GnRH receptor and reduces LH and FSH

release .

Clinical view :

Drugs that are GnRH agonists ( e.g. buserelin and goserelin ) .

Although they mimic the GnRH hormone , when administered

continuously , they will downregulate the pituitary and consequently

decrease LH and FSH secretion . This has effects on ovarian function

such that oestrogen and progesterone levels also fall . These preparations

are used as treatments for endometriosis and to shrink fibroids prior

surgery .

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

oestrogen and progesterone ( see Figure 1 ) . Low levels of oestrogen

have an inhibitory effect on LH production ( negative feedback ) ,

whereas high levels of oestrogen will increase LH production ( positive

feedback ) . The mechanism of action for the positive feedback effect of

oestrogen involves an increase in GnRH receptor concentrations , while

the mechanism of the negative feedback effect is uncertain .

The high levels of circulating oestrogen in the late follicular phase of

the ovary act via the positive feedback mechanism to generate a

periovulatory LH surge from the pituitary .

The clinical relevance of these mechanisms is seen in the use of the

combined oral contraceptive pill , which artificially creates a constant

serum oestrogen level in the negative feedback range , inducing a

correspondingly low level of gonadotriphin hormone release .

Figure (1) Hyothalamo-pituitary-ovarian axis .

Unlike oestrogen , low levels of progresterone have a positive

feedback effect on pituitary LH and FSH secretion ( as seen immediately

prior to ovulation ) and contribute to the FSH surge . High levels of

progesterone , as seen in the luteal phase , inhibit pituitary LH and FSH

production . Positive feedback effects of preogesterone occur via

increasing sensitivity to GnRH in the pituitary . Negative feedback

effects are generated through both decreased GnRH production from the

hypothalamus and decreased sensitivity to GnRH in the pituitary . It is

known that progesterone can only have these effects on gonadotropic

hormone release after priming by oestrogen ( Figure 2 ) .

There are other hormones which are involved in pituitary

gonadotrophin secretion . Inhibin inhibits pituitary FSH secretion ,

whereas activin stimulates it .

Ovary

Ovaries with developing oocytes are present in the female fetus from

an early stage of development . By the end of the second trimester in

utero , the number of occytes has reached a maximum and they arrest at

the first prophase step in meiotic division . No new occytes are formed

during the female lifetime . With the onset of menarche , the primordial

follicles containing oocytes will activate and grow in a cyclical fashion ,

causing ovulation and subsequent menstruation in the event of non-

fertilization .

In the course of a normal menstrual cycle , the ovary will go through

three phases :

1. Follicular phase .

2. Ovulation .

3. Luteal phase .

Follicular phase :

The initial stages of follicular development are independent of

hormone stimulation . However , follicular development will fail at the

preantral stage and follicular atresia will ensue if pituitary hormones LH

and FSH are absent .

FSH levels rise in the first days of the menstrual cycle , when

oestrogen , progesterone and inhibin levels are low . This stimulates a

cohort of small antral follicles on the ovaries to grow .

Within the follicles , there are two cell types which are involved in

the processing of steroids , These are the theca and the granulosa cells ,

which respond to LH and FSH stimulation , respectively . LH

stimulation production of androgens from cholesterol within theca cells .

These androgens are converted into oestrogens by the process of

aromatization in granulose cells , under the influence of FSH . The roles

of FSH and LH in follicular development are demonstrated by studies on

women undergoing ovulation induction in whom endogenous

gonadotrophin production has been suppressed . If pure FSH alone is

used for ovulation induction , as ovulatory follicle can be produced , but

oestrogen production is markedly reduced . Both FSH and LH are

required to generate a normal cycle with adequate amounts of

oestrogen .

As the follicles grow and oestrogen secretion increases , there is

negative feedback on the pituitary to decrease FSH secretion . This

assists in the selection of one follicle to continue in its development

towards ovulation – the dominant follicle . In the ovary , the follicle

which has the most efficient aromatase activity and highest

concentration of FSH – induced LH receptors will be the most likely to

survive as FSH levels drop , while smaller follicles will undergo atresia .

The dominant follicle will go on producing oestrogen and also inhibin ,

which enhances androgen synthesis under LH control .

Figure (2) Changes in hormone levels , endometrium and follicle development

during the menstrual cycle .

Clinical view :

Administration of exogenous gonadotrophins is likely to stimulate

growth of multiple follicles which continue to develop and are released

at ovulation ( and can lead to multiple gestations at a rate of around 30

per cent ) .

This situation is used to advantage in patients requiring in vitro

fertilization ( IVF ) , as many occytes can be harvested from ovaries

which have been stimulated as described above .

There are other autocrine and paracrine mediators playing a role in the

follicular phase of the menstrual cycle .

These include : Inhibin and activin . Inhibin participates in feedback to

the pituitary to downregulate FSH release , and also appears to enhance

ongoing androgen synthesis . Activin is structurally similar to inhibin ,

but has an opposite action is structurally similar to inhibin , but has an

opposite action . It is produced in granulosa cells and in the pituitary ,

and acts to increase FSH binding on the follicles .

Insulin-like growth factors ( IGF – I , IGT – II ) act as paracrine

regulators .

1. In the follicular phase , IGF-I is produced by theca cells under the

action of LH. IGF-I receptors are present on both theca granulosa

cells . Within theca , IGF-I augments LH-induced steroidogenesis .

In granulosa cells , IGF-I augments the stimulatory effects of FSH

on mitosis , aromatase activity and inhibin production .

2. In the preovulatory follicle , IGF-I enhances LH-induced

progesterone production from granulosa cells .

3. Following ovulation , IGF-II is produced from luteinized

granulosa cells , and acts in an autocrine manner to augment LH-

induced proliferation of granulosa cells .

Kisspeptins are proteins which have more recently been found to play a

role in regulation of the HPO axis , via the mediation of the metabolic

hormone leptin's effect on the hypothalamus , Leptin is thought to be key

in the relationship between energy production , weight and reproductive

health . Mutations in the kisspeptin receptor , gpr-54 , are associated

with delayed or absent puberty , probably due to a reduction in leptin-

liked triggers for gonadotrophin release .

Ovulation

By the end of the follicular phase , which lasts an average of 14 days , the

dominant follicle has grown to approximately 20 mm in diameter . As the

follicle matures :

1. FSH induces LH receptors on the granulosa cells to compensate for

lower FSH levels and prepare for the signal ovulation .

2. Production of oestrogen increases until they reach the necessary

threshold to exert a positive feedback effort on the hypothalamus and

pituitary to cause the LH surge .

3. This occurs over 24 – 36 hours , during which time the LH-induced

luteinization of granulosa cells in the dominant follicle causes

progesterone to be produced , adding further to the positive feedback

for LH secretion and causing a small periovulatory rise in FSH .

4. Androgens , synthesized in the theca cells , also rise around the time of

ovulation and this is thought to have an important role in stimulating

libido , ensuring that sexual activity is likely to occur at the time of

greatest fertility .

Clinical view :

1. The LH surge is one of the best predictors of imminent ovulation , and

this is the hormone detected in urine by most over-the-counter

'ovulation predictor' tests .

2. The LH surge has another function in stimulating the resumption of

meiosis in the occyte just prior to its release . The physical ovulation of

the oocyte occurs after breakdown of the follicular was occurs under

the influence of LH , FSH and progesterone-controlled proteolytic

enzymes , such as plasminogen activators and protaglandins . There

appears to be an inflammatory-type response within the follicle wall

which may assist in extrusion of the oocyte by stimulating smooth

muscle activity .

Thus , women wishing to become pregnant should be advised to avoid

taking prostaglandin synthetase inhibitors .

Luteal phase :

After the release of the oocyte , the remaining granulosa and theca

cells on the ovary form the corpus luteum . The granulosa cells have a

vacuolated appearance with accumulated yellow pigment , hence the

name corpus luteum ( ' yellow body ' )

Ongoing pituitary LH secretion and granulosa cell activity ensures a

supply of progesterone which stabilizes the endometrium in preparation

for pregnancy . Progesterone levels are at their highest in the cycle

during the luteal phase . This also has the effect of suppressing FSH and

LH secretion to a level that will not produce further follicular growth in

the ovary during that cycle .

The luteal phase lasts 14 days in most women , without great

variation . In the absence of beta human chorionic ganadotrophin

( BHCG ) being produced from an implanting embryo , the corpus

luteum will regress in a process known as luteolysis .

The withdrawal of progesterone has the effect on the uterus of causing

shedding of the endometrium and thus menstruation . Reduction in

levels of progesterone , oestrogen and inhibin feeding back to the

pituitary cause increased secretion of gonadotrophic hormone ,

particularly FSH . New preantral follicles begin to be stimulated and the

cycle begins anew .

Endometrium :

The specific secondary changes in the uterine endometrium give the

most obvious external sign of regular cycles .

Menstruation :

The endometrium is under the influence of sex steroids that circulate

in females of reproductive age .

During the ovarian follicular phase , the endometrium undergoes

profileration ( the ' proliferative phase ' ) ; during the ovarian luteal

phase , it has its ' secretory phase ' , Decidualization , the formation of a

specialized glandular endometrim , is an irreversible process and

apoptosis occurs if there is no embryo implantation . Menstruation

(day 1) is the shedding of the 'dead' endometrium and ceases as the

endometrium regenerates ( which normally happens by day 5 – 6 of the

cycle ) .

The endometrium is composed of two layers , the uppermost of which

is shed during menstruation . 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 layer along with

bleeding from fragments of the remaining arterioles is seen as menstrual

bleeding . Enhanced fibrinolysis reduces clotting .

Clinical view :

1. 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

who experience a withdrawal bleed during their pill-free week

each month . Vaginal bleeding will cease after 5 – 10 days as

arterioles vasconstrict and the endometrium begins to regenerate .

2. In rare cases , the tissue breakdown and vasoconstriction does not occur

correctly and the endometrium may develop scarring which goes on to

inhibit its function . This is known as ' Asherman's syndrome ' . The

endocrine influences in menstruation are clear . However there is also

paracrine mediators influence in menstruation ,include : prostaglandin

F2 a , endothelin-1 and platelet activating factor ( PAF ) are

vasoconstrictors which are produced within the endometrium . They

may be balanced by the effect of vasodilator agents , such as

prostaglandin E

, prostacyclin ( PGI ) and nitric oxide (NO) , which

are also produced by the endometrium .

3. Recent research has shown that progesterone withdrawal increases

endometrial prostaglandin ( PG ) synthesis and decreases PG

metabolism . The COX-2 enzyme and chemokines are involved in PG

synthesis and this is likely to be the target of non-steroidal anti-

inflammatory agents used for the treatment of heavy and painful

periods .

The proliferative phase :

Once endometrial repair is complete . After this time , the endometrium

enters the proliferative phase , when glandular and stromal growth occur . The

epithelium lining the endometrial glands changes from a single layer of

columnar cells to a pseudostratified epithelium with frequent mitoses .

Endometrial thickness increases rapidly , from 0.5 mm at menstruation to

3.5 – 5 mm at the end of the proliferative phase .

Figure (3) Tissue sections of normal endometrium during proilferative and

secretory phases of the menstrual cycle .

The secretory phase

After ovulation ( generally around day 14 ) , there is a period of

endometrial glandular secretory activity . following the progesterone

surge , the oestrogen-induced cellular proliferation is inhibited and the

endometrial thickness does not increase any further . However , the

endometrial glands will become more tortuous , spiral arteries will

grow , and fluid is secreted into glandular cells and into the uterine

lumen . Later in the secretory phase , progesterone induces the formation

of a temporary layer , known as the decidua . Stromal cells show

increased mitotic activity , nuclear enlargement and generation of a

basement membrane .

Recent research into infertility has identified apical membrane

projections of the endometrial epithelial cells known as pinopodes ,

which appear after day 21-22 and appear to be a progesterone-dependent

stage in making the endmetrium receptive for embryo implantation

( Figure 4 ) .

Figure (4) photomicrograph of endometrial pinopodes from the implantation

window .

Immediately prior to menstruation , three distinct layers of

endometrium can be seen .

1. The basalis is the lower 25 per cent of the endomtrium , which will

remain throughout menstruation and shows few changes during the

menstrual cycle .

2. The mid-portion is the stratum spongiosum with oedematous

stroma and exhausted glands .

3. The superficial portion ( upper 25 per cent ) is the stratum

compactum with prominent decidualized stromal cells . On the

withdrawal of both oestrogen and progesterone , the decidua will

collapse , with vasoconstriction and relaxation of spiral arteries

and shedding of the outer layers of the endometrium .

New developments :

Measurement of ovarian reserve :

Female reproductive potential is directly proportionate to the

remaining number of oocytes in the ovaries . This number decreases

from birth onwards .

It is desirable to be able to quantify the residual ovarian capacity of

women of older age or after undergoing treatment in order to give

prognostic information and management advice to patients , and also to

compare different forms of treatment . Research using :

1. Altrasound markers has looked at measurements of ovarian

volume , mean ovarian diameter and antral follicle count to

calculate ovarian reserve .

2. Biochemical markers include FSH . oestrodiol , inhibin B , anti-

Mullerian hormone ( AMH ) . AMH is produced in the granulosa

cells of ovarian follicles and does not change in response to

gonadotrophins during the menstrual cycle . As a result , it can be

measured and compared from any point in the cycle .

Harvesting ovarian tissue :

Harvesting and cryopreservation of ovarian tissue is an emerging

technique in reproductive biology . At present , its use is experimental and

offered to nulliparious women or young females undergoing gonadotrophic

therapy , for example to treat cancer . The theory is that strips of ovarian

cortex can be removed at laparoscopy or laparotomy and preserved by

freezing , in the hope that future technology will allow them to be thawed and

used to generate occytes for IVF treatment .

Objective

It is important to have an understanding of the physiology of the

normal menstrual cycle to understand the causes of any abnormalities ,

and also to tackle problems , such as infertility and the prevention of

unwanted pregnancy . This lecture aims to describe the mechanisms

involved in the normal menstrual cycle , with emphasis on the clinical

relevance of each phase .