Arachnoid villi affected by subarachnoid pressure and haemorrhage. Scanning electron microscopic study in the dog

Cerebrospinal fluid outflow: a review of the historical and contemporary evidence for arachnoid villi, perineural routes, and dural lymphatics

Cerebrospinal fluid (CSF) is produced by the choroid plexuses within the ventricles of the brain and circulates through the subarachnoid space of the skull and spinal column to provide buoyancy to and maintain fluid homeostasis of the brain and spinal cord. The question of how CSF drains from the subarachnoid space has long puzzled scientists and clinicians. For many decades, it was believed that arachnoid villi or granulations, outcroppings of arachnoid tissue that project into the dural venous sinuses, served as the major outflow route. However, this concept has been increasingly challenged in recent years, as physiological and imaging evidence from several species has accumulated showing that tracers injected into the CSF can instead be found within lymphatic vessels draining from the cranium and spine. With the recent high-profile rediscovery of meningeal lymphatic vessels located in the dura mater, another debate has emerged regarding the exact anatomical pathway(s) for CSF to reach the lymphatic system, with one side favoring direct efflux to the dural lymphatic vessels within the skull and spinal column and another side advocating for pathways along exiting cranial and spinal nerves. In this review, a summary of the historical and contemporary evidence for the different outflow pathways will be presented, allowing the reader to gain further perspective on the recent advances in the field. An improved understanding of this fundamental physiological process may lead to novel therapeutic approaches for a wide range of neurological conditions, including hydrocephalus, neurodegeneration and multiple sclerosis.

Acute Cerebral Artery Constriction in the Spontaneously Hypertensive Rat following Blood and Plasma Administration into the Subarachnoid Space

The purpose of the present study was to demonstrate, using a vascular casting technique, acute vasoconstrictive changes in the cerebral vasculature 1 h following whole-blood or plasma infusion into the subarachnoid space of conscious spontaneously hypertensive rats. Vascular casts from animals infused (over 20 min) with 0.45 ml of heparinized autologous arterial blood or plasma exhibited incomplete filling, while casts from saline-infused controls exhibited virtually no filling defects. Significant elevations in intracranial pressure were noted in blood, but not in plasma- or saline-infused rats. Two characteristic forms of constriction occurred, depending upon the vessel lumen diameter. Vessels with lumen diameters >100 &mgr;m were flattened longitudinally with deep endothelial nuclear imprints, while smaller vessels had focal circular constrictions resembling beads. Arterial cast filling terminated in vessels with lumen diameters from 70 to 120 &mgr;m with focal signs of constriction at or near the point of cast termination. The results indicate that the presence of both blood and plasma in the subarachnoid space produces acute small-artery constriction. This phenomenon is due to a noncellular blood component and does not correlate with increases in intracranial pressure. Copyright 1996 S. Karger AG, Basel

[Morphological study of human arachnoid granulations with reference to their classification]

Stereomicroscopic and microscopic study showed human arachnoid granulations with different morphology that we classified in simple and lobate. Simple granulations were small and completely involved by fibrous capsule that delimited a continuous subdural space from the pedicle to the apex. Lobate granulations were bigger than the simple; in the apex the fibrous capsule was thinner than in other regions, and fused with granulation periphery causing interruption of subdural space. Simple granulations might be an initial development stage; lobate granulations would represent a higher development stage, with ideal morphologic structure for absorption of the CSF.

Arachnoid granulation affected by subarachnoid hemorrhage

The purpose of this study was to investigate using light microscopy the fibrocellular components of arachnoid granulations affected by mild and severe subarachnoid hemorrhage. The erythrocytes were in the channels delimited by collagenous and elastic bundles and arachnoid cells, showing their tortuous and inter-communicating row from the pedicle to the fibrous capsule. The core portion of the pedicle and the center represented a principal route to the bulk outflow of cerebrospinal fluid and erythrocytes. In the severe hemorrhage, the fibrocellular components are disorganized, increasing the extracellular channels. We could see arachnoid granulations without erythrocytes, which cells showed big round nucleus suggesting their transformation into phagocytic cells.

Sub-pial infiltration of blood products following experimental subarachnoid haemorrhage

The authors studied the trans-pial penetration of blood products into the cerebral cortex of cats within the first hour following subarachnoid hemorrhage (SAH). Twenty-two animals were subjected to SAH by cisternal injection of blood, and 13 were used as controls. The brains were fixed in situ and the histological sections were examined under the fluorescence microscope. The cerebral parenchyma of the control animals displayed a homogeneous greenish autofluorescence. In the animals with SAH, the subarachnoid blood showed a yellow autofluorescence that also penetrated into the superficial cortex. The possible clinical implications of the observation are discussed.

Scanning electron microscope study of human arachnoid villi

The functional morphology of human arachnoid villi obtained from surgical biopsy specimens has been studied by scanning electron microscopy (SEM). On SEM examination, the villi appeared to be distended, as if functioning normally. The endothelial cells constituting the cerebrospinal fluid (CSF)-blood interface were covered by numerous microvilli, uniformly oriented along the major axis of the villus. Examination for cell-to-cell contact revealed only occasional areas of tight adherence between adjacent endothelial cells, while widened intercellular spaces were frequently observed. Generally corresponding to the apex of the villus, points of emergence of endothelium-lined hollow structures were identified; these may represent apical openings of open pathways from the subarachnoid space to the venous system. Ultrastructural arrangements consistent with a closed system of CSF reabsorption were also observed. Large cells maximally distended and protruding into the sinus lumen were commonly seen; these were interpreted as the result of the formation of giant vacuoles within the endothelium covering the villus. This study has provided ultrastructural evidence for both closed and open systems of CSF reabsorption. Ultrastructural findings, such as gaps between endothelial cells and tubule-like endothelium-lined structures as previously identified in animals and observed in man by transmission electron microscopy, were demonstrated in human biopsy specimens by SEM.

Polarization microscopic investigation of subarachnoid fibrosis after subarachnoid haemorrhage

The author used the polarization microscope and topo-optical reactions such as phenol reaction, collagen-specific sulphation, and picrosirius red F 3 BA staining to investigate the nature of subarachnoid fibrosis after experimental subarachnoid haemorrhage. It is strongly suggested that the subarachnoid fibrotic deposits--which here analysed in a previous study by scanning electronmicroscopy--contain a high amount of collagen as a main component both three weeks and three months after the SAH.