The Menstrual Cycle And Its Hormone Control

While the length of the normal menstrual cycle varies from woman to woman, the average cycle lasts 28 days.With significant variations, cycles can be as short as 21 days or as long as 36 days. Longer cycles are more common in women at the beginning or the end of their reproductive lives. This reflects the development or the deterioration, of ovarian function (Silverton, 1993).

The menstrual cycle is under the control of the higher centres of the brain (the cerebrum). It has been said that, as these areas are also concerned with the emotions, psychological upset can therefore alter menstrual regularity.

The major hormones involved in the control of reproductive function (Lewis and Chamberlain, 1990):

• Gonadotrophin-releasing hormone (GnRH) is a small polypeptide produced and released by the hypothalamus in pulses. It then flows through the pituitary portal system to stimulate the secretion of luteinising hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland.

• The pulsatile secretion of GnRH every 90 minutes is vital for the reproductive cycle to continue. Continuous secretion of the hormone leads to decreased stimulation of the pituitary gland. The secretion of GnRH is controlled by neurotransmitters from the higher cerebral centres, and by feedback of oestrogen and progesterone from the ovary.

• Luteinising hormone (LH) and Follicle stimulating hormone (FSH) are two glycoproteins secreted by the pituitary in response to pulsatile GnRH, and therefore have pulsatile patterns of secretion themselves. FSH stimulates the development of Graafian follicles within the ovary.

• Oestrogen is secreted by the granulose and theca cells of the ovarian follicles and, after luteinisation, by the same cells in the corpus luteum. Its physiological actions include:

Control of the monthly proliferative phase of the endometrium, with menstruation occurring following its withdrawal. An increase in circulating oestrogen mid-cycle that stimulates cervical glands to produce clear alkaline mucus that protects spermatozoa from the vagina's acid environment. Inhibition of pituitary activity through positive feedback. A rise in oestrogen concentration mid-cycle increases the sensitivity of the pituitary gonadotrophs to GnRH, increasing the frequency and height of LH pulses, and thereby increasing the LH and accompanying FSH surges that cause ovulation.

• Progesterone is produced by the corpus luteum. Its actions include: Responsibility for the luteal phase of the menstrual cycle, which will, however, only be taken up if the endometrium has already been primed by the action of oestrogens.

Anterior pituitary

PJtH

hormones Fön

Anterior pituitary

PJtH

hormones Fön

Ovarian hormones

Progesterone

Ovary

Uterine endometrium

Progesterone

Ovarian hormones

Ovary

Uterine endometrium

Corpus albicans

Follicular phase Luteal phase Menstruation

Corpus albicans

Follicular phase Luteal phase Menstruation

The menstrual cycle. (Reproduced by permission of the University of Wisconsin Digital Collections Center.)

The ovarian cycle describes the events that occur in the ovary during a menstrual cycle before, during and after ovulation. The ovarian cycle is divided into the follicular phase (pre-ovulatory) and the luteal phase (post-ovulatory).

THE FOLLICULAR PHASE

FSH starts to rise two days prior to menstruation, stimulating the growth and maturation of ovarian follicles. Each of these follicles consists of a maturing ovum, with surrounding granulosa and theca interna cells that are derived from the ovarian stroma. These granulosa and theca cells produce oestrogens (mainly oestradiol).

The level of LH is fairly constant during the follicular phase of the cycle. However, the rising oestrogen levels (along with increasing levels of inhibin, a polypeptide) inhibit the output of FSH by negative feedback, as they alter the sensitivity of the pituitary to GnRH. This results in a fall in FSH level on about day 5 of the menstrual cycle.

Oestrogen levels rise with continuing maturation and growth of the dominant follicle. During the final maturation phase, a rapid increase in the levels of oestrogen (around day 12) increases the sensitivity of the pituitary gonadotrophs to GnRH, in turn increasing the frequency and height of LH pulses, hence also producing the mid-cycle LH and accompanying FSH surges that cause ovulation at around day 14 (positive feedback to oestrogen). While there are up to 50 follicles beginning to mature in the early part of each cycle, generally only one matures fully and ovulates.

THE LUTEAL PHASE

When the ovum has been released from the follicle there is a temporary fall in the oestrogen level, and the FSH and LH levels are reduced. The granulosa and the theca cell of the now empty follicle become swollen and take up fat. It becomes yellow, and is thus named corpus luteum 'yellow body'. The corpus luteum secretes increasing amounts of progesterone and oestradiol, so that levels begin to rise once more.

However, progesterone dominates this phase. It prepares the endometrium to receive the fertilized egg if fertilisation occurs. Progesterone also alters the pituitary sensitivity to GnRH, resulting in less frequent pulses of LH, but increasing the height of each LH pulse.

If the ovum is not fertilized, there is atresia of the courpus luteum in the last week of the cycle. This results in a hyaline body known as a corpus albi-cans 'white body'. The levels of oestrogen and progesterone decrease, and the ovarian cycle ends as the support given to the endometrium by these hormones ceases, and menstruation occurs.

ENDOMETRIAL CYCLE

The endometrium undergoes cyclical regeneration and disintegration under the influence of oestrogen and progesterone. The endometrium is composed of glands supported on a bed of stroma that contains fibroblasts and white blood cells. The cycle is divided into three phases.

The menstrual phase lasts about 4-6 days, and occurs as a result of the degeneration of the luteum following a fall in oestrogen and progesterone production. The spiral arterioles that supply the endometrium undergo vasospasm, which leads to necrosis and desquamation of the endometrium. This leaves the basal layer from which the next cycle's endometrium will form.

The proliferative/follicular phase starts with low levels of oestrogen, which stimulate the hypothalamus to produce FSH-releasing hormone, resulting in the release of FSH. This stimulates the developing Graafian follicle to produce oestrogen, which causes growth of the endometrium. Ovulation occurs at the end of this phase at about day 14.

In the secretory/luteal phase, the corpus luteum produces oestrogen and progesterone, causing the endometrium to thicken further, so becoming even more vascular. This produces an environment into which the fertilized ovum can implant and obtain nourishment.

If fertilization does not take place, the corpus luteum will start to degenerate as levels of progesterone drop. In response to this, the spiral arterioles supplying the endometrium undergo vasospasm, which leads to necrosis and desquamation of the endometrium. This results in menstruation.

CERVICAL MUCUS

In the early pre-ovulatory phase of the menstrual cycle the cervical mucus is thick, sticky and opaque. It forms a plug that blocks the cervical canal. Increasing concentrations of oestrogen alter the mucus, which becomes copious, thin elastic and clear. This facilitates the ascent of spermatozoa to the endometrial cavity and the upper genital tract. Following ovulation progesterone alters its state again. The secretions become viscid, scarce, sticky and opaque. This acts as a barrier to spermatozoa. Along with alterations in cervical mucus, there is a change in pH from 4 pre-ovulation, to a pH of 7-8 in the peri- and post-ovulatory part of the cycle.

VAGINAL DISCHARGE

Vaginal discharge varies dramatically during a woman's lifetime and during each phase of the menstrual cycle. These changes in the vaginal epithelium and vaginal secretion influence the defence against vaginal infection.Vaginal secretions consist of a mixture of vulval secretion, composed of secretions from sebaceous glands, transudates from Bartholin's and Skene's glands, and cervical mucus (Emens (1983) cited by Silverton (1993)). It also consists of vaginal transudate containing desquamated vaginal epithelial cells, giving it a creamy colour. The viscosity of the secretion is influenced mainly by the cervical component (Lewis and Chamberlain, 1990).

Vaginal secretions are acid, with a pH of 3.8-4.2 (Emens 1983) during the reproductive years. This is a result of high levels of lactic acid (2-3 per cent), which is produced by the action of commensal organisms (Doderlein's bacilli) on glycogen contained in desquamated vaginal epithelial cells. This level of acidity prevents the multiplication of most pathogenic organisms (Lewis and Chamberlain, 1992). As has been mentioned, the normal production of vaginal secretion varies considerably between individuals. This is important to bear in mind when differentiating between physiological and pathological discharges.

Table 8 Common causes of vaginal discharge

Physiological

Local factors

Hormones

Prepubertal

Reproductive

Post-menopausal

Menstruation Post partum Malignancy Semen

Personal habits and hygiene

Hormonal contraception Cyclical hormonal changes Pregnancy

Pathological

Infective discharge

Other cause of discharge

Conditions:

Bacterial vaginosis

• Acute pelvic inflammatory disease

• Pelvic infection post operatively

• Puerperal sepsis

Organisms:

Candida albicans

• Trichomonas vaginalis

Chlamydia trachomatis

• Neisseria gonorrhoeae

Less common causes:

Human papillomavirus Primary syphilis Mycoplasma genitalium Ureaplasma urealyticum Escherichia coli

• Retained tampon or condom

• Chemical irritation

• Allergic responses

• Endocervical polyp

• Intrauterine device

• Atrophic changes

Less common causes

• Physical trauma

• Vault granulation tissue

• Vesicovaginal fistula

• Rectovaginal fistula

Source: Adaped from Soutter 1998 and Mitchell 2004.

Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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