The Anterior Pituitary and Hypothalamus

The hypothalamic-pituitary complex represents the core of the neuroendocrine system. The hypothalamus is composed of a diversity of neurosecretory cells arranged in groups, which secrete their products either into the portal blood system that connects the hypothalamus to the adenohypophysis (see later) or directly into the general circulation after storage in the neurohypophysis (see Chapter 6). Because of the nature of their actions, the hypothalamic hormones are classified as releasing or inhibiting hormones. The hypothalamic hormones delivered to the portal blood system are transported to the adenohypophysis, where they stimulate or inhibit the synthesis and secretion of different trophic hormones. In turn, these hormones regulate gonadal, thyroid, and adrenal function, in addition to lactation, bodily growth, and somatic development. No attempt will be made in this chapter to cover the actions of the different pituitary trophic hormones on their target glands, because they are discussed in detail in other chapters. An exception to this is growth hormone (GH). Although Chapter 11 considers several aspects of the control and actions of GH, a broader discussion of its physiological actions will be presented here because GH is the only anterior pituitary hormone that does not have a clear-cut target gland. The pituitary gland has two parts: the neurohypophysis, of neural origin (see Chapter 6), and the adenohypophysis, of ectodermal origin. In embryonic development, an evagination from the roof of the pharynx pushes dorsally to reach a ventrally directed evagination from the base of the diencephalon. The dorsally projecting evagination, known as Rathke’s pouch , forms the adenohypophysis, whereas the ventrally directed evagination of neural tissue forms the neurohypophysis. The neurohypophysis has three parts: the median eminence, the infundibular stem, and the neural lobe itself. The median eminence represents the intrahypothalamic portion and lies just ventral to the floor of the third ventricle protruding slightly in the midline. The main part of the neurohypophysis, the neural lobe, is connected to the median eminence by the infundibular stem.

The Posterior Pituitary and Water Metabolism

The neurohypophysis, also called the posterior pituitary or neural lobe , is the ventral extension of hypothalamic tissue derived from a developmental down growth of the neuroectoderm forming the floor of the third cerebroventricle. It weighs approximately 0.10–0.15 g in humans and is well developed at birth, having been present since the fifth month of intrauterine life. In addition to containing glial elements called pituicytes, the posterior pituitary is composed of unmyelinated nerve fibers and axon terminals of neurons whose cell bodies reside primarily in the supraoptic and paraventricular hypothalamic nuclei. These hypothalamo-neurohypophyseal fibers deliver the two primary posterior pituitary hormones, oxytocin (OT) and arginine vasopressin (AVP), to the neural lobe in association with specific proteins, the neurophysins, once thought to be carrier proteins but now known to be portions of the OT and AVP precursor molecules. The neurons produce either OT or AVP, and under some circumstances both, and recent studies indicate that in addition to one of these two hormones, other neuropeptides, such as corticotropin-releasing hormone (CRH) and nesfatin-1, and neurotransmitters are also produced in OT- or AVP-containing cells. The phenomenon of colocalization of neuromodulatory agents has aroused a great deal of clinical interest in the role of neuropeptides such as OT and AVP in brain function. Both OT- and AVP-containing nerve fibers, originating in the supraoptic and paraventricular nuclei, project to a variety of other brain structures that are thought to be the sites of their observed central nervous system actions, and to the vicinity of the hypophyseal portal vessels in the median eminence. Release from these fibers of both OT and AVP explains the high levels of these hormones in portal blood and provides the framework for the actions of OT and AVP as modulators of anterior pituitary function. The arterial blood supply of the posterior pituitary is via the inferior (and to some degree the superior) hypophyseal arteries, which originate from the cavernous and postclinoid portions of the internal carotid artery.