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.