Page:Tumors of the pituitary gland.djvu/19

 Tumors of the Pituitary Gland

Figure 1-11 SCHEMATIC REPRESENTATION OF THE 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 Scientifie Publica- tions, 1991:170-244.)

structures in the infundibulum and proximal hy- pophysial stalk proceeds through the portal ves- sels to adenohypophysial capillaries. Although the function of the gomitoli is not certain, their structure suggests that they regulate the rate of blood flow to the anterior pituitary, thereby influ- encing the transport of hypothalamic regulatory hormones to the adenohypophysis. In some indi- viduals, the middle hypophysial arteries form the trabecular or loral arteries, which descend along the external surface of the pituitary stalk in the subarachnoid space and give rise to the sub- capsular artery and the artery of the fibrous core; these arteries may provide a minor contribution to the blood supply of the adenohypophysis, then return upwards along the pituitary stalk as the long stalk arteries to anastomose with the neu- rohypophysial capillary bed. The inferior hypo- physial arteries enter the sella just beneath its diaphragm and supply the pituitary capsule, the neural lobe, and the lower pituitary stalk. In the intralobar groove, they divide into ascending and descending branches which form an arterial cir- cle about the neural lobe. A branch to the lower pituitary stalk, the communicating artery, anas- tomoses with the trabecular arteries. Notably, the capillaries of the neurohypophysis are fenes- trated and lie outside the blood-brain barrier.

Early studies suggested that the long portal vessels which arise in the infundibulum carry 70 to 90 percent of the blood flow while only 10 to 30 percent originates in the short portal vessels which link the infundibular stem or process to the adenohypophysis (7). However, it is now recog- nized that blood flow occurs within the neurohy- pophysial capillary bed, resulting in mixing of blood derived from different portal vessels (4,5). The adenohypophysis receives the majority of its blood from portal vessels via the neural lobe, but, in addition, some arterial blood is directed to it via two branches of the inferior hypophysial artery, the capsular artery, which serves the connective tis- sue of the pituitary capsule and penetrates to the superficial cell rows of the adenohypophysis and the artery of the fibrous core. In some individuals, the middle hypophysial artery may vascularize the adenohypophysis directly (11).

The venous drainage of the pituitary is to the cavernous sinus and from there to the inferior petrosal sinuses bilaterally. It has been reported that the volume of veins leading away from the adenohypophysis and neurohypophysis to the cavernous sinus is considerably smaller than that of the portal vessels entering the gland. This observation led to the recognition of the neuro- hypophysial capillary bed as a dynamic pool in which the short portal vessels also serve as effer- ent channels. The reversal of blood flow in this system implies that secretory products of the adenohypophysis enter the neurohypophysis and the median eminence and thereby help reg- ulate hypothalamic factors (4,5).

Pituitary capillaries are lined by fenestrated endothelium with a thin subendothelial space. Hormones released by adenohypophysial cells pass through the basement membrane of their cell of origin, the capillary basement membrane, the subendothelial space, and the endothelial cell layer to reach the bloodstream.