The hypothalamus, anterior pituitary gland, and gonads play a critical role in the regulation of reproductive hormones in both males and females. Endocrine interactions in the hypothalamic-pituitary-ovarian axis result in the regulation of ovulation, or oocyte release from the ovary, while stimulation of the male gonads, the testes, results in the regulation of spermatogenesis. In both genders, the hypothalamus releases gonadotropin releasing hormone (GnRH) which triggers the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to stimulate the gonads. The testes or ovaries in turn can produce hormones that influence the development of primary and secondary gender characteristics as well as feedback and regulate the release of GnRH, FSH, and LH to modulate the axis and prevent overstimulation.
FSH stimulation of theca interna cells of the ovary promotes the growth of developing follicles while LH stimulates ovarian granulosa cells to produce the hormones estrogen and inhibin. Constant, moderate estrogen levels inhibit further hypothalamic release of GnRH while both estrogen and inhibin inhibit anterior pituitary release of FSH and LH, thus inhibiting follicular growth and overproduction of these hormones. In addition, estrogen causes proliferation of endometrial cells in the uterus and is also responsible for secondary female sex characteristics such as fat deposition and breast development. Meanwhile, an LH peak mid-cycle due to very high estrogen levels triggers ovulation, leaving behind remaining follicular cells to form the corpus luteum, which produces estrogen and the additional hormone progesterone to thicken the uterine endometrial lining in preparation for egg implantation.
