Morphology

The pituitary gland is located in a small depression in the sphenoid bone, the sella turcica, just beneath the hypothalamus, and is connected to the hypothalamus by a thin stalk called the infundibulum. It is a compound organ consisting of a neural or posterior lobe derived embryologically from the brain stem, and a larger anterior portion, the adenohypophysis, which derives embryologically from the primitive foregut. The cells located at the junction of the two lobes comprise the intermediate lobe, which is not readily identifiable as an anatomic entity in humans (Fig. 1).

Histologically, the anterior lobe consists of large polygonal cells arranged in cords and surrounded by a sinusoidal capillary system. Most of the cells contain secretory granules, although some are only sparsely granulated. Based on their characteristic staining with standard histochemical dyes and immunofluorescent stains, it is possible to identify the cells that secrete each of the pituitary hormones. It was once thought that there was a unique cell type for each of the pituitary hormones, but it is now recognized that some cells may produce more than one hormone. Although particular cell types tend to cluster in central or peripheral regions of the gland, the functional significance, if any, of their arrangement within the anterior lobe is not known.

The posterior lobe consists of two major portions: the infundibulum, or stalk, and the infundibular process, or neural lobe. The posterior lobe is richly endowed with nonmyelinated nerve fibers that contain electron-dense secretory granules. The cell bodies from which these fibers arise are located in the bilaterally paired supraoptic and paraventricular nuclei of the hypothalamus. These cells are characteristically large compared to other hypothalamic neurons and hence are referred to as magnocellular. Secretory material synthesized in cell bodies in the hypothalamus is transported down the axons and stored in bulbous nerve endings within the posterior lobe. Dilated terminals of these fibers lie in proximity to the rich capillary network whose fenestrated endothelium allows secretory products to enter the circulation readily.

The vascular supply and innervation of the two lobes reflect their different embryological origins and provide important clues that ultimately led to an understanding of their physiologic regulation. The anterior lobe is sparsely innervated and lacks any secretomotor nerves. This fact might argue against a role for the pituitary as a relay between the central nervous system and peripheral endocrine organs, except that communication between the anterior pituitary and the brain is through vascular, rather than neural, channels.

The anterior lobe is linked to the brain stem by the hypothalamo-hypophysial portal system through which it receives most of its blood supply (Fig. 2). The superior hypophysial arteries deliver blood to an intricate network of capillaries, the primary plexus, in the median eminence of the hypothalamus. Capillaries of the primary plexus converge to form long hypophy-seal portal vessels, which course down the infundibular stalk to deliver their blood to capillary sinusoids interspersed among the secretory cells of the anterior lobe. The inferior hypophysial arteries supply a similar median eminence neurohypophysis <[ infundibulum neural lobe posterior lobe

anterior lobe

-pars intermedia-

FIGURE 1 Midsagittal section of the human pituitary gland indicating the nomenclature of its various parts.

anterior lobe

> adenohypophysis

-pars intermedia-

FIGURE 1 Midsagittal section of the human pituitary gland indicating the nomenclature of its various parts.

FIGURE 2 Vascular supply of the human pituitary gland. Note the origin of long portal vessels (LPV) from the primary capillary bed and the origin of short portal vessels (SPV) from the capillary bed in the lower part of the stalk. Both sets of portal vessels break up into sinusoidal capillaries in the anterior lobe. SHA and IHA, superior and inferior hypophysial arteries, respectively; AT, trabecular artery that forms an anastomotic pathway between SHA and IHA; V, venous sinuses. (Redrawn from Daniel PM, Prichard MML. Observations on the vascular anatomy of the pituitary gland and its importance in pituitary function. Am Heart J 1966;72:147.)

FIGURE 2 Vascular supply of the human pituitary gland. Note the origin of long portal vessels (LPV) from the primary capillary bed and the origin of short portal vessels (SPV) from the capillary bed in the lower part of the stalk. Both sets of portal vessels break up into sinusoidal capillaries in the anterior lobe. SHA and IHA, superior and inferior hypophysial arteries, respectively; AT, trabecular artery that forms an anastomotic pathway between SHA and IHA; V, venous sinuses. (Redrawn from Daniel PM, Prichard MML. Observations on the vascular anatomy of the pituitary gland and its importance in pituitary function. Am Heart J 1966;72:147.)

capillary plexus in the lower portion of the infundibular stem. These capillaries drain into short portal vessels, which supply a second sinusoidal capillary network within the anterior lobe. Nearly all of the blood that reaches the anterior lobe is carried in the long and short portal vessels. The anterior lobe receives only a small portion of its blood supply directly from the paired trabecular arteries, which branch off the superior hypophysial arteries. In contrast, the circulation in the posterior pituitary is unremarkable. It is supplied with blood by the inferior hypophysial arteries. Venous blood drains from both lobes through a number of short veins into the nearby cavernous sinuses.

The portal arrangement of blood flow is important because blood that supplies the secretory cells of the anterior lobe first drains the hypothalamus. Portal blood can thus pick up chemical signals released by neurons of the central nervous system and deliver them to secretory cells of the anterior pituitary. As might be anticipated, because hypophysial portal blood flow represents only a tiny fraction of the cardiac output, only minute amounts of neural secretions are needed to achieve biologically effective concentrations in pituitary sinusoidal blood when delivered in this way. More than 1000 times more secretory material would be needed if it were dissolved in the entire blood volume and delivered through the arterial circulation. This arrangement also provides a measure of specificity to hypothalamic secretion, because pituitary cells are the only ones exposed to concentrations that are high enough to be physiologically effective.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment