The Normal Menstrual Cycle:

Introduction: The most obvious manifestation of the normal menstrual cycle is the presence of regular menstrual periods. These occur as the endometrium is shed following failure of implantation or fertilization of the oocyte. Menstruation is initiated in response to changes in steroids produced by the ovaries which themselves are controlled by the pituitary & hypothalamus.
Ovary:
Within the ovary the menstrual cycle can be divided into 3 phases:
1. Follicular phase
2. Ovulation
3. Luteal phase
The key event in the follicular phase is the development of the oocytes. The ovary contains thousands of primordial follicles that are in a continuous state of development from birth to the menopause. The initial stages of follicular development ( up to the pre-antral stage) are independent of hormonal stimulation, however, development beyond the pre-antral stage is stimulated by the pituitary hormones LH & FSH which are the key regulators of oocyte development.
At the start of the menstrual cycle the FSH level increase as the pituitary is released from the negative feedback effect of progesterone, estrogen & inhibin.

Steroidogenesis: The basis of hormonal activity in the pre-antral to the pre-ovulatory follicles is described as the two cell, two gonadotrophin hyothesis. Steroidogenesis occur in two cell types within the follicle, the theca & granulosa cells. The hypothesis states that these cells are responsive to gonadotrophins LH & FSH respectively. Within the theca cells, LH stimulate the release of androgen from cholesterol. Within the granulosa cells, FSH stimulate the conversion of thecally derived androgens to estrogens (aromatization). Both FSH & LH are required for normal follicular development, FSH alone produce ovulatory follicles but with low estrogen production. Androgen production within the follicle also regulate the development of pre-antral follicle. Low level of androgen enhance aromatization & increase oestrogen production where as high androgen levels inhibit aromatization & produce follicular atresia. The ideal situation for the initial stages of follicular development is low LH & high FSH.