Divisions of the Pituitary Gland and its Relationships to the Hypothalamus
The pituitary gland (hypophysis) is a midline structure at the base of the hypothalamus, to which it is connected through the pituitary stalk. The gland is approximately bean shaped, measures 6 mm (superior-inferior dimension) by 9 mm (anterior-posterior dimension) by 13 mm (transverse dimension), and weighs 500 to 600 mg.
The gland is composed of the adenohypophysis and the neurohypophysis, which are embryologically, anatomically, and functionally distinct. The former derives from the Rathke’s pouch, an ectodermal outgrowth of the primordial stomodeum, whereas the latter derives from the neural ectoderm and can be considered an extension of the hypothalamus.
The adenohypophysis comprises the pars distalis, the pars intermedia, and the pars tuberalis (a small portion of the adenohypophysis wrapped around the neurohypophysis in the stalk). The pars distalis is also known as the anterior lobe (or pars glandularis), whereas the pars intermedia is poorly developed in humans. The pars intermedia contains a connective tissue trabecula sepa- rating the anterior and posterior lobe as well as a narrow cleft or several small cysts at the site of the embryonic Rathke’s pouch. During development, populations of stem cells differentiate into distinct groups of adenohypophyseal secretory cells under the influence of specific transcription factors.
Several differentiated cell types arise from a common stem cell precursor, including somatotrophs, lactotrophs, mammosomatotrophs, and thyrotrophs. Somatotrophs secrete growth hormone, constitute about 50% of the cell population in the adenohypophysis, and are mainly present in the lateral wings of the anterior lobe. Lactotrophs secrete prolactin, account for approximately 9% of adenohypophyseal cells, and are concentrated in the posterolateral areas of the anterior lobe.
Mammosomatotrophs secrete both growth hormone and prolactin.
Thyrotrophs secrete thyrotropin, constitute about 5% of adenohypophyseal cells and are concentrated in the anteromedial areas of the pars distalis.
Corticotrophs synthesize pro-opiomelanocortin, which is cleaved into several proteolytic fragments, including corticotropin. Corticotrophs account for approximately 20% of cells in the adenohypophysis and are chiefly present in the midportion of the anterior lobe as well as the pars intermedia. In older individuals, some corticotrophs are also present in the adjacent neurohypophysis.
Gonadotrophs constitute approximately 10% of cells in the adenohypophysis, are distributed throughout the anterior lobe and pars tuberalis, and secrete both follicle-stimulating hormone and luteinizing hormone. Other, nonsecretory cell populations in the adenohy-pophysis include follicular and folliculostellate cells.
The neurohypophysis comprises the neural stalk, itself subdivided into the median eminence and the infundibular stem (also known as infundibulum), and the infundibular process (posterior lobe, pars nervosa or neural lobe). The neurohypophysis contains neuronal axons whose cell bodies are present in the supraoptic and paraventricular hypothalamic nuclei. Most of these axons terminate in the posterior lobe, with a minority of the axon terminals being located in the median eminence and the infundibulum. Antidiuretic hormone and oxytocin are synthesized in cell bodies of neurons in the supraoptic and paraventricular nuclei, are transported down the axons and secreted by exocytosis from axon terminals in response to nerve impulses. Pituicytes are glial cells supporting the axon terminals in the neurohypophysis.
The pituitary gland receives a rich blood supply, commensurate with its role as an endocrine organ. Its blood supply derives from paired branches of the superior and inferior hypophyseal arteries, which are branches of the internal carotid arteries. Branches of the superior hypophyseal arteries form the primary plexus of the hypophyseal portal capillary system in the median eminence and infundibulum, where they are apposed to numerous nerve terminals of nerve axons originating in the hypothalamus. Upon excitation, these neurons secrete several distinct releasing and inhibitory hormones into the portal system, which travel down the pituitary stalk in portal veins to reach the secondary plexus of the hypophyseal portal capillary system present in the adenohypophysis, and either stimulate or inhibit hormone secretion in a cell-specific manner. Branches of the inferior hypophyseal arteries directly supply the posterior lobe and anastomose with branches of the superior hypophyseal vessels. Blood from the adenohypophysis and neurohypophysis leaves the pituitary through several hypophyseal veins that drain into the cavernous sinuses.