Oestrogens in the menstrual cycle

Oestrogens produced principally by the theca interna of the ovarian follicle act in a feedback loop on the anterior pituitary and the hypothalamus. The major oestrogen in the menstrual cycle is 17 beta-oestradiol.

In the hypothalamus oestrogens act to increase the frequency and amount of pulsed release of gonadotrophin releasing hormone - GnRH, which acts on the anterior pituitary to increase luteinizing hormone - LH - release.

In the anterior pituitary oestrogens have two types of effect. In constant, moderate levels they exert negative feedback on the release of LH. However, when levels are high, about 36 to 48 hours before ovulation, they increase the sensitivity of the anterior pituitary to GnRH, and thus there is a positive feedback effect which results in a peak in LH release.

As well as feedback regulation of the menstrual cycle, oestradiol-17beta has the following effects:

• oviducts:
  • increase in smooth muscle motility
  • increase in density of cilia
  • increased secretions
• endometrium: proliferation of functional layer from epithelial cells lining the remnants of endometrial glands; this effect is maximal just prior to ovulation
• myometrium:
  • increased contraction
  • increased excitability to other agents
• epithelium:
  • increased watery mucus that is less hostile to sperm
  • proliferates
  • becomes keratinized