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The Normal Pituitary Gland

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The Normal Pituitary Gland 1 THE NORMAL PITUITARY GLAND The adenohypophysis is a red-brown epithelial GROSS ANATOMY gland; the neurohypophysis is a firm gray neural The human pituitary gland, or hypophysis, structure that is composed of axons of hypo- is a small bean-shaped organ that lies in the thalamic neurons and their supporting stroma sella turcica, or hypophysial fossa, a concave (figs. 1-4, 1-5). structure in the superior aspect of the sphenoid The adult human pituitary gland measures bone at the base of the brain (figs. 1-1–1-3). approximately 13 mm transversely, 9 mm The gland is well protected by the bony sella. anterior-posteriorly, and 6 mm vertically. It Lateral to the sella are the cavernous sinuses, weighs approximately 0.6 g. The female gland is which contain the internal carotid arteries and somewhat larger than the male gland; this can the oculomotor, trochlear, abducens, and first be documented on magnetic resonance imag- division of the trigeminal nerves; inferior and ing (MRI) where a difference of up to 2 mm in anterior is the sphenoid sinus; superior is the height is seen (1,2). The pituitary gland of preg- hypothalamus; and superoanterior is the optic nant and postpartum women is larger (1,3) and chiasm. The bilaterally symmetric gland has heavier (4); the increased size is due to marked two parts: the adenohypophysis and the neuro- prolactin cell hyperplasia during pregnancy and hypophysis. As their names suggest, these two lactation, which increases the weight to 1 g or parts are structurally and functionally different. more. Postlactational involution occurs but the Figure 1-1 ANATOMY OF THE PITUITARY GLAND Sagittal section through the midline shows the pituitary gland within the sella turcica, attached to the hypothalamus by the pituitary stalk. The gland is situated immediately posterior to the sphenoid sinus. (Section I, plate 4 from Netter FH, Forsham PH, eds. Endocrine system and selected metabolic diseases. The CIBA collection of medical illustrations, Vol 4. New York: Colorpress; Switzerland: CIBA; 1970:6.) 1 Tumors of the Pituitary Gland Figure 1-2 ANATOMY OF THE PITUITARY GLAND A view of the base of brain shows the pituitary gland immediately posterior to the optic chiasm and anterior to the tuber cinereum. (Section I, plate 4 from Netter FH, Forsham PH, eds. The CIBA collection of medical illustrations, Vol. 4. Endocrine system and selected metabolic diseases. New York: Colorpress; Switzerland: CIBA; 1970:6.) Figure 1-3 ANATOMY OF THE PITUITARY GLAND Frontal section shows the pituitary gland in relationship to the cavernous sinuses and their contents. (Section I, plate 5 from Netter FH, Forsham PH, eds. The CIBA collection of medical illustrations, Vol. 4. Endocrine system and selected metabolic diseases. New York: Colorpress; Switzerland: CIBA; 1970:7.) 2 The Normal Pituitary Gland Figure 1-4 NORMAL PITUITARY GLAND The anterior lobe is composed of tan soft tissue (left) and the posterior lobe of firm white tissue (right). The pituitary stalk is at the top. (Fig. 1-4 from Fascicle 22, Third Series.) Figure 1-5 NORMAL PITUITARY GLAND Horizontal cross section shows the anterior lobe, composed of tan soft tissue (bottom); the posterior lobe, composed of firm white tissue (top); and cysts of the remnant intermediate lobe between them. gland does not return to its pregestational size. eminence, or infundibulum; the neural stalk, or The glands of multiparous women are heavier infundibular stem; and the posterior lobe of the than those of nulliparous women (5). There is pituitary, or infundibular process. a slight to moderate size and weight reduction The adenohypophysis comprises about 80 with advancing age in both sexes (1,3). percent of the pituitary gland. It is composed of The neurohypophysis is composed of nerve three parts: the pars distalis, the pars intermedia, fibers from hypothalamic nuclei that project and the pars tuberalis (fig. 1-6). The pars distalis downward. These give rise to the median is the largest portion of the gland; it is generally 3 Tumors of the Pituitary Gland Figure 1-6 ANATOMIC COMPONENTS OF THE PITUITARY GLAND (Fig. 1-6 from Fascicle 22, Third Series.) known as the anterior lobe or the pars glandu- 5 percent of patients have hyperprolactinemia laris. The pars intermedia, or intermediate lobe, which may be caused by a coexistent prolactin- is rudimentary in the human pituitary; it is the producing pituitary adenoma but is often idio- vestigial posterior limb of the Rathke pouch (see pathic and has been attributed to distortion of the Embryology) and is found in an underdeveloped infundibular stalk and reduction in hypothalamic form adjacent to the residual cleft of the pitu- tonic inhibition (9). itary. The pars tuberalis is an upward extension Other minor anatomic variations in the size of the adenohypophysial cells that surround the and shape of the pituitary gland and its relation lower hypophysial stalk (6); it is also known as to surrounding structures appear to have no the pars infundibularis. endocrine significance (4). The hypophysis is enveloped by dura mater, Vascular Supply a layer of dense connective tissue that lines the sella turcica. The diaphragma sellae, a reflection The blood supply of the human pituitary of the dura that constitutes the roof of the sella gland is a complex portal system that originates turcica, has a small central opening for the hy- in the hypothalamus (fig. 1-11). This hypophy- pophysial stalk, the connection to the hypothala- sial portal circulation carries hypothalamic mus (fig. 1-7). The sellar diaphragm protects the stimulatory and inhibitory hormones from the pituitary gland from the pressure of cerebrospinal infundibulum to adenohypophysial cells, there- fluid (CSF). Defective development or absence of by playing a major role in the regulation of ad- this structure causes the empty sella syndrome in enohypophysial hormone secretion (10–15). which increased CSF pressure results in enlarge- The arterial supply of the median eminence ment of the sella turcica and compression of the and posterior pituitary is derived from two pituitary gland (figs. 1-8–1-10). In severe cases, or, in some individuals, three paired arteries the entire gland is only a thin layer of tissue at which arise from the intracranial portions of the bottom of the sella turcica. This lesion is usu- the internal carotid arteries: superior, middle, ally unassociated with functional hypophysial and inferior hypophysial arteries. The superior abnormalities (7,8), however, approximately hypophysial arteries branch into an external 4 The Normal Pituitary Gland Figure 1-7 NORMAL DIAPHRAGMA SELLAE Normal sella turcica viewed from above shows an intact dia- phragm and pituitary gland with the pituitary stalk emerging from the sellar diaphragm. (Fig. 1-7 from Fascicle 22, Third Series.) Figure 1-8 EMPTY SELLA SYNDROME Magnetic resonance imaging (MRI) identifies an enlarged sella turcica (arrows) in which the pituitary parenchyma is compressed at the bottom; the space is filled with cerebrospinal fluid (left, sagittal view; right, coronal view). (Courtesy of Dr. S. Ezzat, Ontario, Canada.) and an internal plexus. The external plexus is of a central muscular artery surrounded by composed of small arteries which surround the a spiral of capillaries; the arteriole feeds the upper half of the stalk and give rise to a mesh capillaries through small orifices surrounded of capillaries. The internal plexus forms the by muscular sphincters. Flow through these gomitoli, unique vascular structures, 1 to 2 mm complex structures in the infundibulum and in length and 0.1 mm in width, composed proximal hypophysial stalk proceeds through 5 Tumors of the Pituitary Gland Figure 1-9 EMPTY SELLA SYNDROME A widely opened sellar dia- phragm allows increased pressure from cerebrospinal fluid within to compress the pituitary. (Fig. 1-9 from Fascicle 22, Third Series.) Figure 1-10 EMPTY SELLA SYNDROME The pituitary gland is atten- uated along the bottom of the enlarged sella. (Fig. 1-10 from Fascicle 22, Third Series.) the portal vessels to adenohypophysial capillar- nal surface of the pituitary stalk in the subarach- ies. Although their function is not certain, the noid space and give rise to the subcapsular artery complexity of these gomitoli suggests that they and the artery of the fibrous core. These arteries regulate the rate of blood flow to the anterior provide a minor contribution to the blood sup- pituitary gland, thereby influencing the trans- ply of the adenohypophysis, then return upward port of hypothalamic regulatory hormones to along the pituitary stalk as the long stalk arteries the adenohypophysis. In some individuals, the to anastomose with the neurohypophysial capil- middle hypophysial arteries form the trabecular, lary bed. The inferior hypophysial arteries enter or loral, arteries, which descend along the exter- the sella turcica just beneath its diaphragm and 6 The Normal Pituitary Gland Figure 1-11 BLOOD SUPPLY OF THE HYPOTHALAMUS AND PITUITARY (Plate XVII from Scheithauer BW. The hypothalamus and neurohypophysis. In: Kovacs K, Asa SL, eds. Functional endocrine pathology. Boston: Blackwell Scientific; 1991:170-244.) supply the pituitary capsule, the neural lobe, occurs within the neurohypophysial capillary and the lower pituitary stalk. In the intralobar bed, resulting in the mixing of blood derived groove, they divide into ascending and descend- from different portal vessels (14,15). The adeno- ing branches, which form an arterial circle about hypophysis receives the majority of its blood the neural lobe. A branch to the lower pituitary from portal vessels via the neural lobe, but, stalk, the communicating artery, anastomosis in addition, some arterial blood is directed to with the trabecular arteries. The capillaries of the adenohypophysis via two branches of the the neurohypophysis are fenestrated and lie inferior hypophysial artery: the capsular artery, outside the blood-brain barrier. which supplies the connective tissue of the Early studies suggested that the long portal pituitary capsule and penetrates to the superfi- vessels that arise in the infundibulum carry cial cell rows of the adenohypophysis, and the 70 to 90 percent of the pituitary blood flow artery of the fibrous core.
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