How do my menstrual cycles occur?

From your menarche, the first period, to your menopause (Greek: men- month; pausis cessation) the endometrium (lining of the womb) changes its structure cyclically under the influence of the sex steroids hormones, oestrogen and progesterone with menstruation marking just one of these cyclical changes. The menstrual cycle is interrupted by pregnancy when specific endometrial changes occur (decidualisation). The menstrual cycle is controlled by hypothalamic and pituitary hormones (the hypothalamus and pituitary gland are situated at the base of the brain) and by ovarian hormones. (Figure 2.6)

The hypothalamus provides the link between the brain and pituitary gland by producing releasing and inhibiting factors which control the pituitary gland's secretion of hormones into the blood. There are five releasing hormones each composed of a chain of peptides, which are the basic unit for proteins. Gonadotrophin releasing hormones, which controls the sex hormone stimulating hormone output of the pituitary gland, is a decapeptide - a chain of 10 peptides. Thyrotrophin releasing hormones, which controls the thyroid gland through thyroid stimulating hormone of the pituitary, consists of just three peptides. Reproductive dysfunction, an abnormality of the control of the menstrual cycle and fertility, can result from functional abnormality of any of these releasing hormones.

The pituitary gland functions as two separate areas the anterior pituitary, which is at the front of the gland and the posterior pituitary which is the part at the back. The anterior pituitary produces the two gonadotrophic hormones known as follicle stimulating hormone (FSH) and luteinising hormone (LH).

Ovarian control of pituitary function is principally through a negative feedback by oestradiol (Figure 2.6). A rise in LH level, the LH surge, triggers egg release (ovulation) 36 hours later. At the end of a menstrual cycle, oestradiol production falls with the demise of the corpus luteum (Figure 2.3) and FSH levels begin to rise. FSH release is inhibited by oestradiol. At the menopause, the ovaries run out of functional eggs and oestradiol levels fall. As a result, the pituitary releases high levels of FSH in an attempt to gain a response from the ovaries (Q 26.4). There are other regulators of pituitary gonadotrophin release. Current interest is focusing on one called inhibin.

A third important pituitary hormone involved in reproduction is prolactin. This hormone derives its name from the fact that it encourages milk production. Levels of prolactin normally rise after childbirth.

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