Short Portal Vessels Research Articles

Suckling-Induced Change in Oxytocin but not in α-Msh Concentrations of the Median Eminence, The Neural-, Intermediate- and Anterior Lobes of the Pituitary Gland

Previous reports have implicated that pituitary-derived prolactin (PRL) is secreted from two distinct zones of mammotropes within the anterior lobe (AL). The inner zone (AL-IZ), located adjacent to the neuro-intermediate lobe (NIL), is supposed to be involved in the rapid and massive discharge of PRL from the pituitary gland due to suckling stimulus. Whereas the outer-zone (AL-OZ) gives the basal secretion and it does not play a role in the acute secretory response during nursing. Anatomically, the AL-IZ has an intimate contact with the NIL because the blood passing through the short portal vessels (SPV) bathes it first. Based on this fact it would be hypothesized that locally released and/or produced compounds, like OXY and a-MSH, can be delivered to the AL-IZ. In conjunction, OXY and a-MSH have already been implicated to play a role in the regulation of PRL release during suckling. Therefore, the purpose of this study was to examine the possible local transportation of these hormones into the median eminence and various regions of the pituitary gland of lactating rats. We have measured the concentrations of OXY and α-MSH from tissue samples of nonsuckled (NS) and 10 or 30 min after suckling (S) was initiated using specific RIAs. It has been shown that there are no changes in the concentration of OXY and α-MSH in the AL-IZ and AL-OZ due to suckling stimulus. In contrast, our data provide compelling evidence that OXY is transported into the IL, which can be further increased by suckling stimulus. These data suggest that blood transfusing NL passes through the IL before it is drained into the cavernous sinus, which opens the road for OXY into the general circulation. In addition, our data have unequivocally shown a lack of local delivery of either α MSH or OXY into the AL that raises serious doubt about their possible role in PRL secretion during suckling stimulus.

Role of the hypophyseal neurointermediate lobe in the dynamic release of prolactin

The dynamic release of prolactin (PRL) occurs in a region of the anterior pituitary lobe (AP) that receives its blood supply primarily from the hypophyseal neurointermediate lobe (NIL) via the short portal vessels. This relationship, coupled with a growing body of supportive evidence, suggests that the hypophysiotropic signal mediating the massive discharge o f AP PRL derives from the NIL rather than the hypothalamic median eminence, as previously believed. Moreover, a major component of this signal appears to be a-MSH.

The vascularization of the pituitary gland of the chicken (Gallus domesticus). A scanning electron microscope study of vascular corrosion casts.

Microvascular corrosion casts of the pituitary gland of one to nine-day-old chickens (Gallus domesticus, white leghorn hybrids) were analysed with the scanning electron microscope. Results show that the chicken median eminence is supplied by branches of the infundibular and the neural-lobe arteries. They form a flat two-dimensional primary capillary plexus, which lacks any capillary loops and is continuous with the plexus in the neural lobe. The capillaries occupy about 60% of the total area of the median eminence. A subependymal plexus is present, showing no contact with the plexus of the median eminence. The chicken neural lobe consists of many hollow buds. These buds are supplied by branches of the neural-lobe artery, which gives rise to a two-dimensional capillary meshwork similar to that of the median eminence. An anterior group of portal vessels, comprising 14-16 vessels with a mean diameter of 37 microns, and a posterior group of portal vessels, comprising 2-4 shorter and slightly thinner vessels, arise from the median eminence are the sole blood supply for the adenohypophysis (distal lobe). Anterior portal vessels supply the cephalic lobe and the most rostral area of the caudal lobe, and posterior portal vessels supply the caudal lobe of the distal lobe. There are no short portal vessels connecting the neural lobe with the distal lobe. The (sinusoidal) capillary bed of the distal lobe is laminated. The chicken hypophysis drains into the cavernous sinus, which empties into the cerebral carotid veins. Within the period studied (days 1-9 after hatching) no age-related changes were found.

Topography of short portal vessels in the rat pituitary gland: a scanning electron-microscopic and morphometric study of corrosion cast replicas.

We applied scanning electron microscopy combined with imaging and morphometric techniques to analyze the dorsal topography and morphology of short portal vessels linking the capillary beds of the pituitary neural and anterior lobes in adult male albino rats. The pituitary microvasculature was replicated by intracarotid injection of Batson's No. 17 compound producing plastic casts that were advantageous for comprehensive morphometric analyses using an imaging device. The analysis revealed the existence of two types of portal vessels having quantitatively different morphological properties. The bilateral venular plexus of 3-4 vessels located at the base of the infundibular stalk (each venule measuring 300 microns in length and 32 microns in diameter) appears to be the major part of the short portal system in the dorsum of the rat pituitary gland. Narrower capillary-like shunt vessels (6.8 microns in diameter), of about the same length as the venules, were situated throughout other subregions of the intermediate lobe cleft. The short portal vessels of both types made direct anastomoses with the capillary networks in the neural and anterior lobes. The neural lobe capillaries were twice as numerous (1324 per mm2), and only half as wide (6.2 microns), as the sinusoidal capillaries in the anterior lobe (density of 637 per mm2; diameter of 13.7 microns). The topographical position of the portal venular system suggests that the caudolateral subregions of the pituitary neural and anterior lobes have a functional relationship dependent on rapid interlobe transfer of neurohumoral factors such as hormones via the portal blood. This process appears to be supplemented throughout the rest of the cleft between the two lobes by a small number of capillary shunts that supply the epithelial cell lobules of the intermediate lobe in situ. The findings collectively indicate that this portal system provides a constant stream of neurohumoral information that is shared moment-by-moment between the pituitary neural and anterior lobes.

Microcirculatory patterns in adult rat cerebral hypophysis: a scanning electron microscope study of replicated specimens.

The blood vascular bed of the cerebral hypophysis in the adult rat was replicated completely or incompletely by arterial injection of different amounts of methacrylate resin, to be observed with a scanning electron microscope. Complete replication confirmed our previous findings (Murakami et al., 1987) on the distribution and structure of the vascular beds in and around the hypophysis of the rat. One long major and several minor portal routes (vide infra) were reproduced sufficiently together with the systemic veins of the posterior lobe. Incomplete replication demonstrated that resin flows: 1) via the long portal vessels from the median eminence and neural stalk to the anterior lobe; 2) via the accessory long portal vessels from the subependyma to the anterior lobe; 3) via the short portal vessels from the posterior lobe to the anterior lobe; 4) via the neuro-intermedial portal vessels from the posterior lobe to the intermediate lobe; 5) via the intermedio-distal portal vessels from the intermediate lobe to the anterior lobe; and 6) via the tuberal portal vessels from the tuberal lobe to the anterior lobe. Incomplete replication also demonstrated that resin in the median eminence and neural stalk is drained preferentially into the anterior lobe via the long portal vessels, and that resin in the posterior lobe is drained mainly into the systemic veins. We were unable to demonstrate a retrograde resin flow from the anterior lobe to the median eminence, subependyma, neural stalk, intermediate lobe and posterior lobe, nor an ascending resin flow from the posterior lobe to the median eminence and subependyma. Also failing to be noted were an ascending resin flow from the hypophysis to the hypothalamus and a descending resin flow from hypothalamus to the hypophysis.

Developmental study of neuropeptide Y-like immunoreactivity in the neurohypophysis and intermediate lobe of the rhesus monkey (Macaca mulatta).

The purpose of this study was to examine the development and distribution of neuropeptide Y-immunoreactive fibers in the neurohypophysis of the rhesus monkey (Macaca mulatta) throughout life and the relationship of these fibers to the hypothalamo-hypophyseal portal vasculature. In rhesus monkeys, which varied in age from fetal life to 34 years, neuropeptide Y-immunoreactive fibers were present at all ages examined. In adult monkeys, varicose neuropeptide Y-labeled fibers were concentrated in the upper infundibular stem in association with capillary loops of the portal vasculature and the long portal vessels. Other fibers travelled down the infundibular stem and were distributed at the junction of the lower infundibular stem and infundibular process in the vicinity of the short portal vessels. In the infundibular process, neuropeptide Y-immunoreactive fibers were concentrated along the border of the intermediate lobe. Other stained fibers were sparsely distributed in the infundibular process and were often associated with small vessels. Neuropeptide Y-immunoreactivity was also located in a few fibers and cells of the intermediate lobe. Very few labeled fibers were seen in the fetal neurohypophysis, but their number increased gradually during the first postnatal year. At two years of age, a high density of stained fibers was observed, especially in the infundibular process. The number of axons in the infundibular process was lower at 12 years and continued to decline until 34 years of age. Neuropeptide Y may modulate hormone release at these sites and may also be released directly into vessels in the infundibular process. The close association of neuropeptide Y-labeled fibers with capillaries of the portal vasculature strongly suggests that neuropeptide Y is released into the portal blood of monkeys throughout life and may influence hormone secretion from the anterior pituitary gland.

Correlation of spatial differences in concentrations of prolactin and growth hormone cells with vascular pattern in the female mouse adenohypophysis.

Adult female mice of the DDY/S strain were used to study the distribution of PRL or GH cells and the vasculature of the anterior pituitary lobe. Electron microscopy was used to quantify PRL or GH cells in horizontal sections. Most parenchymal cells were either PRL or GH cells, and both types of cells were present in all regions. The densities of PRL cells in the rostral and caudal areas were significantly greater than that of GH cells. The density of GH cells was greater in the anterolateral wings. Thus, the spatial differences in concentrations of PRL and GH cells were reversed. The vasculature was studied with scanning electron microscopy of vascular casts and with stereoscopy of pituitary glands injected with India ink. The adenohypophysis was supplied by long and short portal vessels. The long portal vessels originated from the primary capillary plexus on the median eminence and the upper portion of the pituitary stalk, and they supplied rostral regions of the adenohypophysis. Most of the short portal vessels connected caudal areas of the anterior lobe with the posterior lobe, crossing the surface of the intermediate lobe. The blood in the short portal vessels may flow from the posterior lobe toward the anterior lobe. Thus, within the rostral and caudal areas, which are supplied by long and short portal vessels, respectively, PRL cells predominated; the anterolateral wings where GH cells predominated were far from these regions. These data suggest that the anatomical pattern of the blood supply may account in part for the spatial distribution of PRL and GH cells.

Formation of a direct arterial blood supply to the anterior pituitary gland following complete or partial interruption of the hypophyseal portal vessels.

If regions of the anterior pituitary gland received systemic blood via a direct arterial blood supply these regions would escape hypothalamic regulation and thus be a sequela in endocrine disorders. Since, in the untreated rat, all of the blood supply to the anterior pituitary gland is via the hypophyseal portal vessels, we hypothesized that partial interruption of the portal vessels could provoke the establishment of a direct arterial blood supply (arteriogenesis). We utilized the injection of polystyrene microspheres (15 or 9 micron diameter) into the left ventricle of the heart to test this hypothesis. Microspheres are trapped in the first capillary plexus they reach since they are too large to traverse the capillaries. No microspheres reached the anterior pituitary gland of control rats, a finding consistent with the fact that the anterior pituitary gland receives all of its blood supply via the hypophyseal portal blood vessels. Microspheres were observed in the primary portal capillary plexus in the infundibulum (median eminence), infundibular stalk (pituitary stalk), and infundibular process (pars nervosa), the first capillary plexus which they reached. A lesion of the medial basal hypothalamus (MBH) which destroyed the long portal vessels did not result in arteriogenesis since few, if any, microspheres were observed in the anterior pituitary gland. We confirmed, using vascular casts, that these lesions resulted in the long-term destruction of the primary portal capillaries in the infundibulum and of the long portal vessels. In MBH-lesioned animals it appears that all of the blood supply of the anterior pituitary gland is via short portal vessels arising from the infundibular stem and process.(ABSTRACT TRUNCATED AT 250 WORDS)

The Posterior Pituitary

Removal of the posterior pituitary from anesthetized male rats results in a prompt and significant increase in circulating prolactin that is reversed by the injection of dopamine. Posterior pituitary extracts, which contain high concentrations of endogenous dopamine, inhibit prolactin secretion from isolated anterior pituitary cells. This inhibition is prevented by incubation of the cells with the dopamine receptor antagonist (+)-butaclamol. The data show that posterior pituitary dopamine reaches the anterior pituitary via the short hypophysial portal vessels and participates in the regulation of prolactin secretion.

Can the pituitary secrete directly to the brain? (Affirmative anatomical evidence).

Vascular casts of 10 rhesus monkey pituitary glands and three vascular casts of the rhesus monkey cavernous sinus were examined by scanning electron microscopy. A continuous neurohypophyseal capillary bed was found uniting the infundibulum, infundibular stem, and infundibular process. The neurophypophysis was supplied by three groups of arteries: superior hypophyseal, middle hypophyseal, and inferior hypophyseal. Numerous anastomoses were found between individual arteries, and some hypophyseal arteries formed anastomotic links between different portions of the circle of Willis. Veins located at the caudal pole of the infundibular process, capillaries linking the infundibulum to the hypothalamus, and portal vessels extending from the infundibulum to the adenohypophysis provided efferent vascular pathways from the neurohypophysis. The adenohypophysis received no direct arterial supply; its entire afferent vascular supply was provided by portal vessels. Lateral hypophyseal veins were not found; small adenohypophyseal veins joined larger neurohypophyseal veins to form confluent pituitary veins which extended to the cavernous sinus. The capacity of the venous connections draining the adenohypophysis directly to the cavernous sinus appeared small when compared to that of of the long portal vessels supplying the adenohypophysis. However, many of the short portal vessels interposed between the adenohypophysis and the infundibular stem and process were well arranged to function as alternative efferent routes from the adenohypophysis. The limited potential for venous drainage directly to the cavernous sinus suggests that blood leaves the adenohypophysis by other routes; blood carried via long portal vessels from the infundibulum to the adenohypophysis may return to the neurohypophyseal capillary bed via short portal vessels. This anatomical study suggests that hypothalamic and adenohypophyseal secretions are conveyed to the capillary bed of the neurohypohysis. These secretions may leave the neurohypophysis via any of seven potential routes: one efferent route is directed to the adenohypophysis, another route is directed to the systemic circulation, but five of the potential efferent routes are directed toward the brain.

Localisation of gonadotropin-releasing and thyrotropin-releasing hormones in human brain by radioimmunoassay.

Localization of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH) in the human brain by radioimmunoassay is reported. The hypothalami and other brain regions were obtained during routine autopsies from 3 22-40 year old females and 12 41-95 year old males. GnRH concentration was highest in the preoptic area (340 pg/mg tissue) and declined progessively toward the caudal region which contained negligible amounts of GnRH. The highest concentrations of TRH were found in the posterior nucleus (158 pg/mg tissue) and the premammillary area. Exceedingly high concentrations of both hormones were in the pituitary stalk (1430 pg/mg and 855 pg/ml tissue for GnRH and TRH respectively). Total content of GnRH in the hypothalamus was 40.5 ng and in the pituitary stalk 58.6 ng. TRH content in the hypothalamus was 61.7 ng and in the pituitary stalk 35 ng. No apparent differences were observed in relation to age or sex. The absence of TRH in the lower 1/3rd of the pituitary stalk implies that it reaches the anterior pituitary by was of the long portal vessels whereas GnRH which is evenly distributed along the whole pituitary stalk and infundibular stem is transported via the long and short portal vessels.

Anterior hypophysial function in the posterior-hypophysectomized rat: normal regulation of the adrenal system.

Because the posterior pituitary gland is joinedto the anterior pituitary by the short portal vessels and con-tains both corticotropin and corticotropin-releasing factors, it could play a role in regul

Effect of selective surgical interruption of the anterior pituitary's blood supply on ACTH release.

The acute effect of selective surgical interruption of the anterior pituitary’s blood supply on the release of ACTH has been investigated in surgically stressed rats. The corticosterone secretory rate was used to evaluate the release of ACTH. In intact, sham-operated animals, corticosterone secretion was near maximal throughout a 50-min observation period. Removal of the pituitary greatly suppressed the secretion of corticosterone. Posterior lobectomy or transection of the portal vessels in the upper stalk had no apparent effect on ACTH release. This finding was interpreted to mean that the posterior pituitary and post-chiasmatic eminence are not obligatory sources of CRF. Transection of the pituitary stalk greatly suppressed ACTH release. This finding supports the view that the caudal hypophysial arteries do not provide the anterior pituitary with a significant amount of blood via the short portal vessels. Transection of the peduncular arteries or the stalk portal vessels in the lower stalk suppressed AC...

THE VOLUMES OF PARS DISTALIS, PARS INTERMEDIA AND INFUNDIBULAR PROCESS OF THE PITUITARY GLAND OF THE RAT, WITH SPECIAL REFERENCE TO THE EFFECT OF STALK SECTION

The pituitary stalk was transected in rats by a subtemporal approach, and a non‐irritant plate was inserted between its cut ends. The pituitary glands were studied at intervals ranging from 24 hr. to 41 weeks later. The glands were sectioned serially and the volumes of the various lobes, and of the infarct or scar in pars distalis, were measured. A similar study was made of pituitaries of normal rats.The immediate effect of stalk section was a large infarct, which initially occupied from 29 to 78 per cent of pars distalis, but in most rats involved more than half of the lobe. After a few days the infarcted area began to shrink, eventually becoming a narrow sear. As the sear contracted pars distalis became greatly flattened from above down and the cells of the surviving parenchyma became small and undifferentiated. No substantial regeneration of parenchyma occurred, and pars distalis remained much smaller in volume than in normal rats of corresponding sex and body weight. The very small scar seen a few weeks after stalk section gave little idea of the size of the original infarct. After stalk section there was a rapid and marked atrophy of the infundibular process. There was no hypertrophy of pars intermedia.The distribution of the infarcted and surviving areas of pars distalis after stalk section is explained by the two different groups of portal vessels which supply these areas, namely, the long portal vessels from the stalk, which are transected at operation, and the short portal vessels from the infundibular process, in which the circulation remains unimpaired. Variations in size of the initial infaret may be due to anatomical variations in the extent of the territories supplied by the long and short portal vessels respectively.