Introduction

Abstract

Reproductive function in vertebrate organisms is rejgulated by the hypothalamic-pituitary-gonadal (hpg) axis. Each level of the hpg axis synthesizes and releases a hormone. The hypothalamus produces a ten amino acid peptide called gonadotropin-releasing hormone (GnRH; also called luteinizing hormone-releasing hormone, or LHRH). The anterior pituitary secretes the gonadotropins, which consist of the glycoprotein molecules luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The gonads release the sex steroid hormones, comprising the estrogens, progestins, and androgens (Figure 1). The hpg axis is hierarchically organized, with the GnRH molecule being directly responsible for synthesis and release of the pituitary gonadotropins, and these molecules in turn directly regulating biosynthesis and secretion of sex steroid hormones. More specifically, the hypothalamic GnRH peptide is released ip discrete pulses from neuroterminals in the median eminence. There, it binds to its receptors in the anterior pituitary gland, just below the brain, to stimulate the pulsatile release of the gonadotropins into the systemic circulation. These hormones in turn bind to LH or FSH receptors on ovarian or testicular cells to regulate folliculogenesis in females, spermatogenesis in males, and steroid biosynthesis in both sexes.