Reproductive endocrinology is the study of hormones and neuroendocrine factors that are produced by and/or affect reproductive tissues. These tissues include the hypothalamus, anterior pituitary gland, ovary, endometrium, and placenta. A hormone is classically described as a cell product that is secreted into the peripheral circulation and that exerts its effects in distant target tissues (Fig. 15-1). This is termed endocrine secretion. Additional forms of cell-to-cell communication exist in reproductive physiology. Paracrine communication, common within the ovary, refers to chemical signaling between neighboring cells. Autocrine communication occurs when a cell releases substances that influence its own function. Production of a substance within a cell that affects that cell before secretion is termed an intracrine effect.
Different types of hormone communication. Endocrine: hormones travel through the circulation to reach their target cells. Paracrine: hormones diffuse through the extracellular space to reach their target cells, which are neighboring cells. Autocrine: hormones feed back on the cell of origin, without entering the circulation.
A neurotransmitter, in classic neural pathways, crosses a small extracellular space called a synaptic junction and binds to dendrites of a second neuron (Fig. 15-2). Alternatively, these factors are secreted into the vascular system and are transported to other tissues where they exert their effects in a process termed neuroendocrine secretion or neuroendocrine signaling. One example is gonadotropin-releasing hormone (GnRH) secretion into the portal vasculature with effects on the gonadotropes within the anterior pituitary gland.
Types of neurotransmitter secretion. A. Classic neurotransmitter release and binding. Transmission of an action potential down a neural axon leads to release of neurotransmitters, which travel across a synaptic cleft to reach their target cell. B. Neurohormonal secretion. An action potential leads to release of neurotransmitters. In this instance, neurotransmitters enter into and travel through the circulation to reach their target organ.
Normal reproductive function requires precise quantitative and temporal regulation of the hypothalamic-pituitary-ovarian axis (Fig. 15-3). Within the hypothalamus, specific centers or nuclei release GnRH in pulses. This decapeptide binds to surface receptors on the gonadotrope subpopulation of the anterior pituitary gland. In response, gonadotropes secrete glycoprotein gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), into the peripheral circulation. Within the ovary, LH and FSH bind to the theca and granulosa cells to stimulate folliculogenesis and ovarian production of steroid hormones (estrogens, progesterone, and androgens), gonadal peptides (activin, inhibin, and follistatin), and growth factors. Among other functions, these ovarian-derived factors feed back to the hypothalamus and pituitary gland to inhibit or, at the midcycle surge, to augment GnRH and gonadotropin secretion. The ovarian steroids are also critical for preparing the endometrium for placental implantation if pregnancy ensues.
Positive and negative feedback loops seen with ...