Chapter 38 Reproductive hormones

There are several major classes of steroid hormone:

These are all derived from cholesterol. There is an interconversion between male and female hormones, as the structures are very similar (see Figure 38.3, p. 305). This occurs naturally in both males and females, although in females there is a predominance of female hormones and in males a predominance of male hormones.

Figure 38.3 Structure of various reproductive hormones and related plant compounds.

The Menstrual Cycle

Gonadotrophin-releasing hormone (GnRH) stimulates the pituitary gland, which secretes follicle-stimulating hormone (FSH) and is responsible for the synthesis and secretion of oestrogen in the ovaries. When oestrogen levels rise to a critical point, production is switched off by shutting off the GnRH release from the hypothalamus (Figure 38.1).

Figure 38.1 Hormonal feedback in the female reproductive system.

Progesterone production is stimulated by luteinizing hormone (LH), which is produced by the pituitary as a result of stimulation by GnRH. Progesterone levels cannot rise too high, as they are controlled in the same way as oestrogen. A new follicle begins to form in one of the ovaries at this stage and, when menstruation ceases, the follicle continues to develop while at the same time secreting oestrogen (Figure 38.2). The rising oestrogen levels cause the endometrium to thicken and become richly supplied with blood vessels and glands.

Figure 38.2 The menstrual cycle, showing the effects on the ovary and hormonal fluctuations.

Rising levels of LH create a change in the follicle until a sudden surge in LH levels at about 2 weeks triggers ovulation (mid-cycle). The unfertilized egg is then released into the fallopian tube. The presence of LH encourages the empty follicle to develop into a corpus luteum, which then starts to secrete progesterone. This:

• continues preparation of the endometrium for pregnancy

• inhibits the contraction of the uterus

• inhibits the development of a new follicle.

If fertilization does not occur then the elevated levels of progesterone inhibit the release of GnRH, which inhibits further production of progesterone. When the progesterone levels drops, the:

• corpus luteum and endometrium break down

• uterus is able to contract and menstruation begins.

Menstruation occurs every 28 days, when the lining of the uterus breaks down and is discharged.

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