Gonadotropin-releasing hormone (GnRH), which controls the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) 2.
Corticotropin-releasing hormone (CRH), which controls the release of adrenocorticotrophic hormone (ACTH) 3.
Growth hormone–releasing hormone (GHRH), which regulates the release of growth hormone (GH) 4.
Thyrotropin-releasing hormone (TRH), which regulates the secretion of thyroid-stimulating hormone (TSH)
The hypothalamus is the source of all neurohypophyseal hormone production. The neural posterior pituitary can be viewed as a direct extension of the hypothalamus connected by the fingerlike infundibular stalk. The capillaries in the median eminence differ from those in other regions of the brain. Unlike the usual tight junctions that exist between adjacent capillary endothelial lining cells, the capillaries in this region are fenestrated in the same manner as capillaries outside the CNS. As a result, there is no blood–brain barrier in the median eminence.
Pituitary The pituitary is divided into three regions or lobes: anterior, intermediate, and posterior. The anterior pituitary (adenohypophysis) is quite different structurally from the posterior neural pituitary (neurohypophysis), which is a direct physical extension of the hypothalamus. The adenohypophysis is derived embryologically from epidermal ectoderm from an infolding of Rathke’s pouch. Therefore, it is not composed of neural tissue, as is the posterior pituitary, and does not have direct neural connections to the hypothalamus. Instead, a unique anatomic relationship exists that combines elements of neural production and endocrine secretion. The adenohypophysis itself has no direct arterial blood supply. Its major source of blood ﬂow is also its source of hypothalamic input—the portal vessels. Blood ﬂow in these portal vessels is primarily from the hypothalamus to the pituitary. Blood is supplied to the posterior pituitary via the superior, middle, and inferior hypophyseal arteries. In contrast, the anterior pituitary has no direct arterial blood supply. Instead, it receives blood via a rich capillary plexus of the portal vessels that originates in the median eminence of the hypothalamus and descends along the pituitary stalk. This pattern is not absolute, however, and retrograde blood ﬂow has occurred (4). This blood ﬂow, combined with the location of the median eminence outside the blood–brain barrier, permits bidirectional feedback control between the two structures.
Figure 7.3 The hypothalamic secretory products function as pituitary-releasing factors that control the endocrine function of the ovaries, the thyroid, and the adrenal glands.
The specific secretory cells of the anterior pituitary are classified based on their hematoxylin-and eosin-staining patterns. Acidophilic-staining cells primarily secrete GH and prolactin and, to a variable degree, ACTH (5). The gonadotropins are secreted by basophilic cells, and TSH is secreted by the neutral-staining chromophobes.
GnRH (also called luteinizing hormone–releasing hormone, or LHRH) is the controlling factor for gonadotropin secretion (6). It is a decapeptide produced by neurons with cell bodies primarily in the arcuate nucleus of the hypothalamus (7–9) (Fig. 7.4). Embryologically, these neurons originate in the olfactory pit and then migrate to their adult locations (10). These GnRHsecreting neurons project axons that terminate on the portal vessels at the median eminence where GnRH is secreted for delivery to the anterior pituitary. Less clear in function are multiple other secondary projections of GnRH neurons to locations within the CNS.