Ultrastructure of venules in the cat brain

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

How to Perform Intra-Operative Contrast-Enhanced Ultrasound of the Brain-A WFUMB Position Paper

Intra-operative ultrasound has become a relevant imaging modality in neurosurgical procedures. While B-mode, with its intrinsic limitations, is still considered the primary ultrasound modality, intra-operative contrast-enhanced ultrasound (ioCEUS) has more recently emerged as a powerful tool in neurosurgery. Though still not used on a large scale, ioCEUS has proven its utility in defining tumor boundaries, identifying lesion vascular supply and mapping neurovascular architecture. Here we propose a step-by-step procedure for performing ioCEUS analysis of the brain, highlighting its neurosurgical applications. Moreover, we provide practical advice on the use of ultrasound contrast agents and review technical ultrasound parameters influencing ioCEUS imaging.

Solving an Old Dogma: Is it an Arteriole or a Venule?

There are very few reliable methods in the literature to discern with certainty between cerebral arterioles and venules. Smooth muscle cells (SMC) and pericytes are present in both arterioles and venules, so immunocytochemistry for markers specific to intramural cells (IMC) is unreliable. This study employed transmission electron microscopy (TEM) and a canine brain to produce robust criteria for the correct identification of cerebral arterioles and venules based on lumen:vessel wall area, tested against the less accurate lumen diameter:vessel wall thickness. We first used morphology of IMC to identify two distinct groups of vessels; group 1 with morphology akin to venules and group 2 with morphology akin to arterioles. We then quantitatively assessed these vessels for lumen:vessel wall area ratio and lumen diameter:wall thickness ratio. After assessing 112 vessels, we show two distinct groups of vessels that can be separated using lumen:vessel wall area (group 1, 1.89 -10.96 vs. group 2, 0.27-1.57; p < 0.001) but not using lumen diameter:vessel wall thickness where a substantial overlap in ranges between groups occurred (group 1, 1.58-22.66 vs. group 2, 1.40-11.63). We, therefore, conclude that lumen:vessel wall area is a more sensitive and preferred method for distinguishing cerebral arterioles from venules. The significance of this study is wide, as cerebral small vessel disease is a key feature of vascular dementia and understanding the pathogenesis relies on correct identification of vessels.

The importance of comorbidities in ischemic stroke: Impact of hypertension on the cerebral circulation

Comorbidities are a hallmark of stroke that both increase the incidence of stroke and worsen outcome. Hypertension is prevalent in the stroke population and the most important modifiable risk factor for stroke. Hypertensive disorders promote stroke through increased shear stress, endothelial dysfunction, and large artery stiffness that transmits pulsatile flow to the cerebral microcirculation. Hypertension also promotes cerebral small vessel disease through several mechanisms, including hypoperfusion, diminished autoregulatory capacity and localized increase in blood-brain barrier permeability. Preeclampsia, a hypertensive disorder of pregnancy, also increases the risk of stroke 4-5-fold compared to normal pregnancy that predisposes women to early-onset cognitive impairment. In this review, we highlight how comorbidities and concomitant disorders are not only risk factors for ischemic stroke, but alter the response to acute ischemia. We focus on hypertension as a comorbidity and its effects on the cerebral circulation that alters the pathophysiology of ischemic stroke and should be considered in guiding future therapeutic strategies.

Innervation of the brain, intracerebral Schwann cells and intracerebral and intraventricular schwannomas

The cerebral vasculature and the choroid plexus are innervated by peripheral nerves. The anatomy of the vascular supply to the brain and its related perivascular nerves is reviewed. Intracerebral and intraventricular schwannomas most likely come from neoplastic transformation of Schwann cells investing the perivascular nerves and nerves within the choroid plexus.

Recent Advances in High-Resolution MR Application and Its Implications for Neurovascular Coupling Research

The current understanding of fMRI, regarding its vascular origins, is based on numerous assumptions and theoretical modeling, but little experimental validation exists to support or challenge these models. The known functional properties of cerebral vasculature are limited mainly to the large pial surface and the small capillary level vessels. However, a significant lack of knowledge exists regarding the cluster of intermediate-sized vessels, mainly the intracortical, connecting these two groups of vessels and where, arguably, key blood flow regulation takes place. In recent years, advances in MR technology and methodology have enabled the probing of the brain, both structurally and functionally, at resolutions and coverage not previously attainable. Functional MRI has been utilized to map functional units down to the levels of cortical columns and lamina. These capabilities open new possibilities for investigating neurovascular coupling and testing hypotheses regarding fundamental cerebral organization. Here, we summarize recent cutting-edge MR applications for studying neurovascular and functional imaging, both in humans as well as in animal models. In light of the described imaging capabilities, we put forward a theory in which a cortical column, an ensemble of neurons involved in a particular neuronal computation is spatially correlated with a specific vascular unit, i.e., a cluster of an emerging principle vein surrounded by a set of diving arteries. If indeed such a correlation between functional (neuronal) and structural (vascular) units exist as a fundamental intrinsic cortical feature, one could conceivably delineate functional domains in cortical areas that are not known or have not been identified.

Regulation of cerebral vasculature in normal and ischemic brain

We outline the mechanisms currently thought to be responsible for controlling cerebral blood flow (CBF) in the physiologic state and during ischemia, focusing on the arterial pial and penetrating microcirculation. Initially, we categorize the cerebral circulation and then review the vascular anatomy. We draw attention to a number of unique features of the cerebral vasculature, which are relevant to the microcirculatory response during ischemia: arterial histology, species differences, collateral flow, the venous drainage, the blood-brain barrier, astrocytes and vascular nerves. The physiology of the arterial microcirculation is then assessed. Lastly, we review the changes during ischemia which impact on the microcirculation. Further understanding of the normal cerebrovascular anatomy and physiology as well as the pathophysiology of ischemia will allow the rational development of a pharmacologic therapy for human stroke and brain injury.

Immunocytochemical basis for a meningeo-glial network

Evidence is presented here for a cellular network that courses through all layers of meninges, the vasculature of both the brain and meninges, and extends into the brain parenchyma. Confocal mapping of calcium-binding protein S100beta immunoreactivity (S100beta-ir) and of the intermediate filament vimentin-ir through serial sections of the meningeal-intact adult rat brain revealed this network. In all tissues examined, S100beta-ir and vimentin-ir were primarily colocalized, and were found in cells with elongated processes through which these cells contacted one another to form a network. The location of labeling and the morphology of the cells labeled were consistent with the possibility that this network consists of fibroblasts in the meninges and the walls of large blood vessels, of pericytes at the level of capillaries, and of ependymocytes and a population of astrocytes in the brain parenchyma. At many sites along the borders of the brain parenchyma itself and of the brain blood vessels, it was possible to detect S100beta-ir and vimentin-ir cell processes that cross the basal laminae. This suggested the probable means by which the S100beta-ir cells of the extraparenchymal tissues anatomically contact the cells that express the same markers in the brain. Privileged anatomical relationships of the S100beta/vimentin network with the glial fibrillary acidic protein (GFAP) astrocytes further suggested that, together, they form the structural basis for a general meningeo-glial network. This organization challenges the current model of brain architecture, calls for a reconsideration of the role of meninges and vascular tissues, and appears to reflect the existence of hitherto unsuspected systems of communication.

The vasculature of the peripheral portion of the human eighth cranial nerve

The vasculature of the peripheral portion of the human eighth cranial nerve (VIIIN) was investigated by light and transmission electron microscopy. Arterioles and venules running longitudinally around the VIIIN formed the extrinsic vascular system. The anatomical relationship between these extrinsic vessels and the VIIIN sheath was similar to that between blood vessels on the surface of the brain and the pia mater. In the endoneurium, postcapillary venules and large capillaries were sparsely distributed and longitudinally arranged, and these microvessels formed the intrinsic microvascular system, which was supported by the extrinsic vascular system via anastomosing vessels. The ultrastructural features of the internal auditory artery and its main branches were the same as those of other intracranial arteries. Ultrastructural study also revealed myo-endothelial junctions in anastomosing arterioles, and endothelio-pericytic junctions in extrinsic and anastomosing venules. Microvascular endothelial cells were connected by tight junctions in both the vestibular ganglion and the rest of the VIIIN. These features of the vasculature were considered to be effective for maintenance of the endoneurial fluid and regulation of the circulation in the peripheral portion of the human VIIIN.

Vasomotor effects of neurotransmitters and modulators on isolated human pial veins

Vasomotor reactivity of human pial veins, obtained in conjunction with neurosurgical operations, was studied in vitro. The effect of transmitters in nerves previously recognized in these vessels, as well as that of neuromodulators, was characterized. A comparison of these effects with their effects in the nearby pial arteries of the same patients was made. It was found that the veins were equipped with more sensitive alpha-adrenergic receptors (lower EC50 values) than the arteries. The reverse was found for 5-hydroxytryptamine. Acetylcholine, which causes an endothelium-dependent dilation of pial arteries, contracted the veins despite an apparently intact endothelium. Considering the lower maximum values in veins, responses to histamine, the neuropeptides calcitonin gene-related peptide, bradykinin, and neuropeptide Y; and prostaglandins (PGE1 and PGF2 alpha) were principally the same in the arteries and veins. The dilatory responses to vasoactive intestinal polypeptide and substance P were less pronounced in veins than in arteries. The veins only transiently contracted to a depolarizing potassium solution; calcium influx promotors and inhibitors, as well as calcium-free solution, did not affect the contractile ability of the vein, contrasting to the reactivity of the artery. This clearly indicates that the veins are not substantially dependent upon calcium influx for their acute contractile ability.

Role of veins and cerebral venous pressure in disruption of the blood-brain barrier

The goal of this study was to determine whether increases in cerebral venous pressure contribute to, and may account for, disruption of the blood-brain barrier during acute hypertension and hyperosmolar stimuli. We studied the relation between pial venous pressure and disruption of the blood-brain barrier during acute arterial hypertension, superior venae cavae occlusion, and superfusion with hyperosmolar arabinose. Sprague-Dawley rats were studied using intravital fluorescent microscopy and fluorescein-labelled dextran (mol. wt. = 70,000). Disruption of the blood-brain barrier was characterized by the appearance of microvascular leaky sites and quantitated by the clearance of fluorescein dextran. We measured pressure (servo null) in pial arterioles and venules 40-60 micron in diameter. Acute hypertension, occlusion of the superior venae cavae, and hyperosmolar arabinose produced leaky sites primarily in venules. Acute hypertension increased arteriolar pressure and also venular pressure, from 7 +/- 1 (mean +/- SE) to 28 +/- 2 mm Hg. Clearance of fluorescein dextran increased from 0.03 +/- 0.01 to 2.90 +/- 0.40 ml/sec X 10(-6). Occlusion of the superior venae cavae increased pial venous pressure from 7 +/- 1 to 30 +/- 3 mm Hg, and clearance of fluorescein dextran, from 0.02 +/- 0.01 to 3.10 +/- 0.59 ml/sec X 10(-6). In contrast to acute hypertension, there was a decrease in arterial and pial arteriolar pressure during occlusion of the superior venae cavae. Thus, similar increases in venous pressure during acute hypertension and superior venae cavae occlusion, despite directionally opposite changes in arterial and arteriolar pressure, produced similar disruption of the blood-brain barrier.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for sprouting specificity following medial septal lesions in the rat

Damage to the rat septohippocampal pathway results in the growth of sympathetic axons from nearby blood vessels into the denervated hippocampal formation. Sympathohippocampal sprouting exhibits lesion specificity--that is, only injury to the septohippocampal projection elicits the sprouting response. Whether other perivascular fibers sprout in response to septohippocampal injury (response specificity) has been addressed in the present study. Using cathecholamine histofluorescence and acetylcholinesterase histochemical techniques, we determined the distribution and incidence of perivascular sympathetic and nonsympathetic fibers associated with parahippocampal blood vessels in normal rats and in rats sustaining medial septal lesions. We found that sympathetic fibers are more numerous than acetylcholinesterase-positive fibers at all septotemporal levels of the hippocampal formation and that both types are very rare at dorsal hippocampal levels in normal rats. Following medial septal lesions, however, there is a tremendous increase in the number of perivascular sympathetic fibers at dorsal hippocampal levels but no change in the number of acetylcholinesterase-positive fibers. Electron microscopic observations indicate that the increase in perivascular fibers is due to increases in the number of sympathetic axonal fascicles as well as the number of axons per fascicle. Furthermore, both light and electron microscopic data suggest that parahippocampal veins are normally not accompanied by perivascular fibers but are associated with sympathetic fibers following medial septal lesions. These results indicate that sympathetic sprouting in response to septohippocampal denervation exhibits specificity not only in terms of the lesion which elicits such sprouting but also in terms of the types of fibers that respond to the lesion.

Junctional transfer in cultured vascular endothelium: II. Dye and nucleotide transfer

Vascular endothelial cultures, derived from large vessels, retain many of the characteristics of their in vivo counterparts. However, the observed reduction in size and complexity of intercellular gap and tight junctions in these cultured cells (Larson, D.M., and Sheridan, J.D., 1982, J. Cell Biol. 92:183) suggests that important functions, thought to be mediated by these structures, may be altered in vitro. In our continuing studies on intercellular communication in vessel wall cells, we have quantitated the extent of junctional transfer of small molecular tracers (the fluorescent dye Lucifer Yellow CH and tritiated uridine nucleotides) in confluent cultures of calf aortic (BAEC) and umbilical vein (BVEC) endothelium. Both BAEC and BVEC show extensive (and quantitatively equivalent) dye and nucleotide transfer. As an analogue of intimal endothelium, we have also tested dye transfer in freshly isolated sheets of endothelium. Transfer in BAEC and BVEC sheets was more rapid, extensive and homogeneous than in the cultured cells, implying a reduction in molecular coupling as endothelium adapts to culture conditions. In addition, we have documented heterocellular nucleotide transfer between cultured endothelium and vascular smooth muscle cells, of particular interest considering the prevalence of "myo-endothelial" junctions in vivo. These data yield further information on junctional transfer in cultured vascular endothelium and have broad implications for the functional integration of the vessel wall in the physiology and pathophysiology of the vasculature.

A method for the preparation of cerebral blood vessels

Elements of the cerebral vascular system of the rat have been prepared by simultaneous perfusion with fibrinogen and thrombin. The resulting fibrin fills and serves to support the vessels which can then easily be excised from the brain and prepared for light and electron microscopy. The structure and ultrastructure of cerebral blood vessels is well preserved by this technique.

A possible paracellular route for the resolution of hydrocephalic edema

Considering the possibility of a paracellular route for edema resolution we studied the microvasculature of the subependymal and subcortical white matter in hydrocephalic rats. Normal adult rats were used as controls. After injection of kaolin suspension into the cisterna magna, the animals were killed at intervals of 1, 2, 4, and 8 weeks. In hydrocephalic rats at 1 week after kaolin injection, widening of the interendothelical cleft between the tight junction (dehiscence) was seen in 27 of 76 (35%) vessels. At 2 weeks after kaolin injection, the number of the dehiscences had increased (39/7:56%) and some were enlarged, forming interendothelial blisters. At 4 weeks in hydrocephalic rats, both dehiscences and blisters were still prominent (45/73:63%) and at 8 weeks the dehiscences were still prominent, but the number of the blisters had decreased (25/81:31%). The blisters and dehiscences were most pronounced in the corpus callosum and occipital regions. Following i.v. injection of horseradish peroxidase, the interendothelial dehiscences and blisters were completely devoid of the marker substance. These findings indicate that in obstructive hydrocephalus the tight junctions may constitute part of a paracellular pathway for the resorption of interstitial edema fluid.

[Hemato-encephalic barriers. Morphologic data]

Within the cranio-spinal cavity we can consider three compartments: blood, cerebro-spinal fluid and nervous parenchyma and thus, three barriers (Blood-Cerebro-Spinal Fluid, Blood-Brain, Cerebro-Spinal Fluid-Brain). The morphological studies of these barriers were performed with exogenous tracers such as horseradish peroxidase, cytochrome C and ferritin or endogenous tracers such as autologous antiperoxidase immunoglobulins. 1. The blood-brain barrier is exogenous and endogenous tracers proof. It is found on the level of the brain capillary endothelium with tight junctions and rare plasmalemmal vesicles. 2. The blood-cerebro-spinal fluid barrier is found on the level of choroid plexus and of leptomeningeal vessel. In the former, the tracer is stopped by the tight junctions (zonula occludens type) of the choroid plexus epithelium. Besides, there is no morphological evidence of transepithelial passage from blood to cerebro-spinal fluid. In the later, the barrier is, almost always, found on the level of the vascular endothelium. 3. The parenchymatous-cerebro-spinal fluid interface cannot be called a barrier because the diffusion of the tracers is not restricted either by the astrocytic marginal layer or by the ependyma. The circumventricular organs other than choroid plexus are morphologically characterized by the free diffusion of tracers in their perivascular connective space. Subcommissural organs capillaries alone behave like those of the brain. The spinal cord capillaries, in opposition to those of the brain, are characterized by a perivascular connective space, for 40 p. 100 of them. The significance of this fact is still unknown.

Cerebral veins: fluorescence histochemistry, electron microscopy, and in vitro reactivity

Pial veins, choroid plexus veins, and the cerebri magna vein were investigated with regard to their ultrastructural organization, adrenergic nerve supply, and in vitro reactivity. The vessel walls consisted of a continuous layer of endothelial cells, large amounts of collagenous material, and occasional pericytes. Smooth muscle cells were observed only in a few specimens from the cerebri magna vein. All veins were surrounded by adrenergic nerve fibres. Potassium (124 mM) and noradrenaline (10(-5) - 10(-4) M) induced small contractions (0.2-0.5 mN) of isolated veins during in vitro conditions. The magnitude of these responses was less than one-tenth of that obtained in small pial arteries.

Basement membrane surfaces and perivascular compartments in normal human brain and glial tumours. A scanning electron microscope study

The relationship of perivascular tissues to arteries and veins in normal brain and glial tumors were investigated by light microscopy and by scanning and transmission electron microscopy. Vessels and perivascular tissues were separated through various planes by careful tearing of fixed tissue blocks of brain and tumour. Mirror surfaces of torn blocks were examined by scanning electron microscopy and the identity of the vessels and other structure confirmed by transmission electron microscopy. Perivascular glial basement membranes remained adherent to arterial adventitia in both normal brain and in tumour so that the torn surface around the vessel exposed perivascular glial processes attached to the vessel walls. A clear plane of separation of perivascular glial basement membrane from the adventitia of veins was achieved in glial tumours. Mirror surfaces showed the smooth undulating sheet of basement membrane separated from the fine fibrillary connective tissue of the vessel wall. Tears in the basement membrane revealed the perivascular glial processes. The structure of the perivascular basement membrane is discussed in relation to its role as an attachment site for perivascular glial and as an impedence to inflammatory cell migration into brain parenchyma.

Innervation of the cerebral veins as compared with the cerebral arteries: a histochemical and electron microscopic study

The distribution of nerve fibers in the cerebral veins was studied by catecholamine fluorescence simultaneously with acetylcholinesterase (AChE) histochemistry. A comparison of nerve fibers in the cerebral arteries was made. The ultrastructure of terminal boutons in the veins fixed with potassium permanganate was also studied. In the adventitia of the cerebral artery, green fluorescent aminergic fibers and brownish AChE-reactive (probably cholinergic) fibers were observed. In contrast, the cerebral venous system showed no AChE-positive fibers. Catecholamine fluorescent varicose fibers were detected in the dural sinus, the internal cerebral vein, and the superficial vein of Labbé. The highest density of aminergic fibers was found in the dural sinus and the second highest in the internal cerebral vein. Most of the terminal boutons in the adventitia of the cerebral veins were found adjacent to a muscle-like cell and showed only cored vesicles under electron microscopy. Results of our study suggest that the cerebral venous system has a neurogenic innervation, mainly from aminergic fibers, which is different from the neurogenic supply to the cerebral arterial system.

Intercellular junctions and transfer of small molecules in primary vascular endothelial cultures

The ultrastructure of gap and tight junctions and the cell-to-cell transfer of small molecules were studied in primary cultures and freshly isolated sheets of endothelial cells from calf aortae and umbilical veins. In thin sections and in freeze-fracture replicas, the gap and tight junctions in the freshly isolated cells from both sources appeared similar to those found in the intimal endothelium. Most of the interfaces in replicas had complex arrays of multiple gap junctions either intercalated within tight junction networks or interconnected by linear particle strands. The particle density in the center of most gap junctions was noticeably reduced. In confluent monolayers, after 3-5 days in culture, gap and tight junctions were present, although reduced in complexity and apparent extent. Despite the relative simplicity of the junctions, the cell-to-cell transfer of potential changes, dye (Lucifer Yellow CH), and nucleotides was readily detectable in cultures of both endothelial cell types. The extent and rapidity of dye transfer in culture was only slightly less than that in sheets of freshly isolated cells, perhaps reflecting a reduced gap junctional area combined with an increase in cell size in vitro.

Cortical blood vessels of the human brain

The study is divided into two parts. (a) Superficial or pial vessels: Arterioles and venules at the gyrus surface as well as their mode of penetration into or emergence from nervous tissue is described. The absence of pial capillaries is noted. Arterial and venous anastomoses are described whereas arteriovenous anastomoses were not encountered. In particular, the relationship of superficial vessels to the arachnoid was studied. (b) Intracortical vessels: Arteries and veins were divided into 5 groups according to their degree of cortical penetration. Considering its density, the vascular network of the cortex was divided into 4 vascular layers. A correlation between these layers and the cellular layers was established. Problems in distinguishing between arteries and veins, the geometric disposition of cortical vessels, different types of anastomoses and particular vascular features whose significance remains unclear, are discussed.

Comparative studies on the pre- and postterminal blood vessels in the cerebellar cortex of Rhesus monkey, cat, and rat

In the rhesus monkey, cat and rat, pial arteries give off branches which run vertically through all three layers of the cerebellar cortex. The large cortical arteries are surrounded by a perivascular space in the molecular layer. Their wall consists of several layers of smooth-muscle cells and the luminal endothelium. As the arteries reach the deeper layers of the cerebellar cortex, the number of smooth-muscle cells is reduced. In the rat, sometimes no smooth-muscle cells are detectable in the preterminal arterial vessels. If these deep arteries branch off by dichotomy of terminal vessels there occurs a gradual or complete loss of myocytes in all three species. In the cat, where cortical arteries give off branches at right angles, there is a sphincter-like accumulation of smooth-muscle cells at the opening to the smaller branch. The postterminal vessels and veins in all species exhibit the same mural structure found in capillaries. The wall consists only of an endothelium and occasional pericytes embedded in the basal lamina. Even the large veins which run to the pial veins show this simple mural structure.