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.
