As seen in Figure 1 the hypothalamus is located below the third ventricle of the brain, just above the median eminence. Anatomically, the hypothalamus consists of a collection of neurosecretory centers including the preoptic area, and the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. Some nuclei, such as the supraoptic and paraventricular nuclei, which secrete hormones to be released by the posterior pituitary are distinct, while others are less so. These cell groups are not homogeneous; subgroups can be identified within them. Notable is the paraventricular nucleus, which contains cells that secrete one of two pituitary releasing hormones (TRH and CRH) as well as the posterior pituitary hormones, oxytocin and vasopressin.
Fig1. Anatomy of hypothalamus and pituitary. A. This lateral view of the human brain shows the anatomical location of the hypothalamus and the anterior pituitary and their relationship to each other and nearby structures. B. Schematic diagram of the same view as A. A: Adapted with permission from Uygur et al. Transsphenoidal Surgery, 2010, Chapter 7, Imaging of the Sella and Parasellar Region; p. 44; Elsevier
The median eminence is a midline prominence arising at the rostral end of the hypothalamus and continuing into the pituitary stalk. It is the contact between the hypothalamic neurohormones that control anterior pituitary hormone secretion and the hypothalamic/ hypophysiotrophic portal circulatory system. The axons terminating in the median eminence contain, in proximity to the portal system, dense vesicles 50 to 130 nm in diameter, suggesting that they can secrete the contents of the vesicles into the capillaries of the portal system (see next section).
The pituitary is intimately connected to the hypo thalamus by the infundibular (pituitary) stalk. The anterior pituitary (adenohypophysis) consists of three parts: the pars distalis, comprising most of the gland and its hormone-producing cells; the pars intermedia; and the pars tuberalis, as shown in Figure2. Cells of the pars distalis are arranged in anastomosing cords close to fenestrated capillaries in the secondary capillary plexus of the hypophyseal portal system, described below. In humans, the pars intermedia is not well defined anatomically in comparison with other species, where it is defined by the residual edge of Rathke’s pouch.
Fig2. The pituitary gland in a midsagittal section. The two subdivisions of the pituitary gland are the anterior (darker shade) and posterior (lighter shade) lobes. The anterior lobe (adenohypophysis) is traditionally divided into three components, the pars tuberalis (that portion surrounding the stalk), the pars distalis (the majority of the anterior lobe), and the pars intermedia. In most species, but not in humans, there is a distinct pars intermedia or intermediate lobe, defined by the residual edge of Rathke’s pouch (brown streak in the figure) from which the embryonic gland was derived. In humans the cells in the intermediate lobe are dispersed and this portion of the gland is not anatomically distinct. The neurohypophysis consists of the hypothalamic neurons that produce certain hormones (not shown), the axonal processes of these neurons through the neural stalk and the posterior pituitary gland. The posterior pituitary is the subject of Chapter 4.
