BLOOD SUPPLY OF THE PITUITARY GLAND
The pituitary gland receives its arterial blood supply from two paired systems of vessels: from above come the right and left superior hypophysial arteries, and from below arise the right and left inferior hypophysial arteries. Each superior hypophysial artery divides into two main branches—the anterior and posterior hypophysial arteries passing to the hypophysial stalk. Communicating branches between these anterior and posterior superior hypophysial arteries run on the lateral aspects of the hypophysial stalk; numerous branches arise from this arterial circle. Some pass upward to supply the optic chiasm and the hypothalamus. Other branches, called infundibular arteries, pass either superiorly to penetrate the stalk in its upper part or inferiorly to enter the stalk at a lower level. Another important branch of the anterior superior hypophysial artery on each side is the artery of the trabecula, which passes downward to enter the pars distalis. The trabecula is a prominent, compact band of connective tissue and blood vessels lying within the pars distalis on either side of the midline. At its central end the trabecula is contiguous with the mass of connective tissue, which is interposed between the pars distalis and the lower infundibular stem. Peripherally, the components of the trabecula spread out to form a ﬁbrovascular tuft. On approaching the lower infundibular stem, the artery of the trabecula gives off numerous straight parallel vessels to the superior portion of this area and thus constitutes the “superior artery of the lower infundibular stem.” The “inferior artery of the lower infundibular stem” is derived from the inferior hypophysial arterial system. The artery of the trabecula is of large caliber throughout its course; it gives off no branches to the epithelial tissue through which it passes. It is markedly tortuous and is always surrounded by connective tissue.
The inferior hypophysial arteries arise as a single branch from each internal carotid artery in its intracavernous segment. Near the junction of the anterior and posterior lobes of the pituitary, the artery gives off one or more tortuous vessels to the dural covering of the pars distalis and ﬁnally divides into two main branches— a medial and a lateral inferior hypophysial artery. The infundibular process is surrounded by an arterial ring formed by the medial and lateral branches of the paired inferior hypophysial arteries. From this arterial ring, branches are given off to the posterior lobe and to the lower infundibular stem. Components of the superior and inferior hypophysial arterial systems anastomose freely.
The epithelial tissue of the pars distalis receives no direct arterial blood. The sinusoids of the anterior lobe receive their blood supply from the hypophysial portal vessels, which arise from the capillary beds within the median eminence and the upper and lower portions of the infundibular stem. Blood is conveyed from this primary capillary network through hypophysial portal veins to the epithelial tissue of the anterior lobe. Here, a secondary plexus of the pituitary portal system is formed, leading to the venous dural sinuses, which surround the pituitary, and to the general circulation. Some of the long hypophysial portal veins run along the surface of the stalk, chieﬂy on its anterior and lateral aspects. Most of the long hypophysial portal vessels leave the neural tissue to run down within the pars tuberalis, but a few remain deep within the stalk until they reach the pars distalis. The short hypophysial portal veins are embedded in the tissue surrounding the lower infundibular stem. They supply the sinusoidal bed of the posterior part of the pars distalis, and the long portal veins supply its anterior and lateral regions. Vascular tufts, comprising the primary capillary network in the median eminence and infundibular stem, are intimately related to the great mass of nerve ﬁbers of the hypothalamo hypophysial tract running in this region. On excitation, these nerve ﬁbers liberate into the portal vessels, releasing hormones (e.g., growth hormone–releasing hormone, corticotropin releasing hormone, gonadotropin-releasing hormone, thyrotropin releasing hormone) and inhibitory factors (e.g., somatostatin, prolactin inhibitory factor [dopamine]), which are conveyed to the sinusoids of the pars distalis. Extensive occlusion of the hypophysial portal vessels or of the capillary beds of the hypophysial stalk may lead to ischemic necrosis of the anterior pituitary because these hypophysial portal vessels are the only afferent channels to the sinusoids of the pars distalis.