Cyclic Function of the Mammalian Ovary

Abstract

The maturation and release of eggs from the mammalian ovary occur at regular and predictable intervals that are species-specific. This cyclicity of the ovary has been the subject of intensive interdisciplinary investigation in the past several decades. At base, the ovarian cycle is a product of the interaction of a number of components of the neuroendocrine system. Tonic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary is stimulated by the tonic release of luteinizing-hormone-releas-ing hormone (LH-RH) from the ventromedial arcuate region of the hypothalamus (Figure 1). In addition to this tonic component, there is a complex of trophic hormones (LH, FSH, and prolactin) released in large quantities from the pituitary on a regular, periodic basis in most mammals; this rhythmic component is dependent on the integrity of the preoptic-suprachiasmatic area of the hypothalamus. The tonic release of gonadotropins stimulates maturation of ovarian follicles and secretion of ovarian steroid hormones, notably estradiol. The cyclic release of gonadotropins results in ovulation and the secretion of more ovarian steroid hormones, notably progesterone. These ovarian steroids act both alone and in synergy in complex positive and negative feedback relationships to ensure the periodic release of gonadotropins.