In order to appreciate the niceties of ovulation induction, a basic understanding of the mechanism of ovulation is essential. In the normal course of events, ovulation occurs once a month between the time of menarche and menopause.The release of a mature, fertilizable oocyte from the dominant follicle is the culmination of a wonderfully integrated and synchronized succession of hormonal actions and morphological changes involving principally the anterior hypothalamus, the anterior pituitary and the ovaries. The major players in this system are gonadotropin releasing hormone (GnRH), follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen and progesterone, but essential fine-tuning is provided by a large number of other factors including inhibin, activin and growth factors. An appreciation of the steps involved in the process of ovulation, necessitating the exact sequence of so many events, leaves one in awe of the ingenuity of the system and a little surprised that its breakdown, i.e. anovulation, does not occur much more frequently than is actually seen.
The normal functioning of this axis is dependent on the correct synchronization of the timing of release and the quantity of the hormones involved. These change dramatically throughout the cycle as a result of the various feedback mechanisms involved. First, we will consider the individual hormones involved, and their target organs and actions, before piecing together the mosaic of the feedback mechanisms to complete the hormonal profile of the normal ovulatory cycle. Figure 2.1 provides a simple representation of the origin, target organ and feedback mechanisms of the principal hormones involved in this axis.