Pericytes: cell biology and pathology

Pericytes are perivascular cells with multifunctional activities which are now being elucidated. The functional interaction of pericytes with endothelial cells (EC) is now being established, using current molecular and cytochemical techniques. The detailed morphology of the pericyte has been well described. Pericytes extend long cytoplasmic processes over the surface of the EC, the two cells making interdigitating contacts. At points of contact, communicating gap junctions, tight junctions and adhesion plaques are present. Pericytes appear to show both structural and functional heterogeneity. The coverage of EC by pericytes varies considerably between different microvessel types and the location of pericytes on the microvessel is not random but appears to be functionally determined. Interaction between pericytes and EC is important for the maturation, remodelling and maintenance of the vascular system via the secretion of growth factors or modulation of the extracellular matrix. There is also evidence that pericytes are involved in the transport across the blood-brain barrier and the regulation of vascular permeability. The long-standing view that pericytes are the microvessel equivalent of larger vessel smooth muscle cells and are contractile is being reassessed using current methods. An important role for pericytes in pathology, and neuropathology in particular, has been indicated in hypertension, diabetic retinopathy, Alzheimer's disease, multiple sclerosis and CNS tumour formation.

Contractile proteins in pericytes at the blood-brain and blood-retinal barriers

Evidence from a variety of sources suggests that pericytes have contractile properties and may therefore function in the regulation of capillary blood flow. However, it has been suggested that contractility is not a ubiquitous function of pericytes, and that pericytes surrounding true capillaries apparently lack the machinery for contraction. The present study used a variety of techniques to investigate the expression of contractile proteins in the pericytes of the CNS. The results of immunocytochemistry on cryosections of brain and retina, retinal whole-mounts and immunoblotting of isolated brain capillaries indicate strong expression of the smooth muscle isoform of actin (alpha-SM actin) in a significant number of mid-capillary pericytes. Immunogold labelling at the ultrastructural level showed that alpha-SM actin expression in capillaries was exclusive to pericytes, and endothelial cells were negative. Compared to alpha-SM actin, non-muscle myosin was present in lower concentrations. By contrast, smooth muscle myosin isoforms, were absent. Pericytes were strongly positive for the intermediate filament protein vimentin, but lacked desmin which was consistently found in vascular smooth muscle cells. These results add support for a contractile role in pericytes of the CNS microvasculature, similar to that of vascular smooth muscle cells.

Immunological targeting of the endothelial barrier antigen (EBA) in vivo leads to opening of the blood-brain barrier

The role of the endothelial barrier antigen (EBA) in the blood-brain barrier (BBB) of the rat is not fully understood. Pathological conditions which show BBB disruption and leakage of plasma proteins are associated with reduced EBA expression in brain endothelial cells (ECs). However, it is not known if the reduction in EBA is the primary event or is secondary to protein extravasation. We hypothesized that immunological targeting of EBA in vivo would lead to opening of the BBB. To test this hypothesis, a monoclonal antibody (anti-EBA) was intravenously injected in anaesthetized experimental rats. Control animals received intravenous injections of phosphate-buffered saline (PBS) or non-specific antibodies (anti-human cytokeratin, anti-Salmonella bacterial antigen, or anti-pan endothelial cell antigen). Two groups of rats were used, each included experimental and control animals. The first group was used for immunocytochemical detection of EBA in brain ECs and rat albumin in brain parenchyma. In the second group, the permeability of the BBB to horseradish peroxidase (HRP) was tested. Experimental animals, injected with anti-EBA antibody, showed extensive leakage of HRP and albumin in the grey and white matter of the brain. Immunocytochemistry of experimental brains showed that the intravenously injected anti-EBA became bound to ECs and was detected in tissue sections. Control animals did not show leakage of HRP or albumin, and EBA distribution was normal. This study demonstrated for the first time, that immunological 'neutralisation' of EBA leads to opening of the BBB, and provided direct evidence for the importance of EBA in maintaining the integrity of the BBB in the rat.

The blood-nerve barrier: enzymes, transporters and receptors--a comparison with the blood-brain barrier

The blood-brain barrier (BBB) has been much more extensively investigated than the blood-nerve barrier (BNB). Nevertheless it is clear that there are both similarities and differences in the molecular and morphophysiological characteristics of the two barrier systems. A number of enzymes, transporters and receptors have been investigated at both the BNB and BBB, as well as in the perineurium of peripheral nerves, which is also a metabolically active diffusion barrier. While there have been few systematic comparisons of the distribution of these molecules in both the BNB and BBB, it is apparent from the data available, reviewed in this article, that their distribution also supports the concept of the BNB and BBB having some features in common but also showing distinct identities. These similarities and differences cannot simply be accounted for by the presence of the inductive influences of astrocytes at the BBB and absence at the BNB. Whether the Schwann cell also has the capacity to induce some BNB properties remains to be determined.

Endothelial glycoconjugates: a comparative lectin study of the brain, retina and myocardium

There is evidence that the endothelial cell (EC) glycocalyx is a significant determinant of vascular permeability, acting as a charge-size filter to permeant molecules. We have therefore examined its oligosaccharide composition in 3 classes of microvessel with differing permeabilities. EC in rat brain, retina and myocardium were labelled with a panel of lectins and subjected to a semiquantitative analysis. Surprisingly, no substantial differences were evident for any lectin labelling between the 3 microvessel types despite their marked morphophysiological diversity. In particular, all showed substantial sialic acid expression, with Maackia amurensis (MAA) labelling sialic acid in an alpha2-3 linkage to beta-galactose and Sambucus nigra (SNA) recognising sialic acid in an alpha2-6 linkage to beta-galactose. Arachis hypogaea (PNA) binding after neuraminidase digestion indicated the presence of Gal beta1-3GalNAc attached to terminal sialic acid. The results therefore show that the sequences NeuNAc alpha2-3Gal beta1-3GalNAc and NeuNAc alpha2-6Gal beta1-3GalNAc are strongly expressed in the 3 microvessel types irrespective of their permeability properties. This homogeneity suggests that these lectin ligands may be involved in a common set of EC functions, e.g. cell:cell and cell:matrix interactions. However, we cannot rule out the possibility that glycocalyx differences may exist between vessels in the paracellular cleft which may alter its filtration properties.

A comparison of blood-brain barrier and blood-nerve barrier endothelial cell markers

A number of major properties of endothelial cells (EC) at the blood-brain barrier (BBB) have been shown to be astrocyte-dependent. Whether analogous properties at the blood-nerve barrier (BNB) are induced and maintained by Schwann cells has not been investigated. As a preliminary investigation we have undertaken a comparative study of six EC membrane markers at the BBB and BNB and perineurium. Employing immunoblotting and immunocytochemistry the relative distribution between rat brain cortex and sciatic nerve was determined for the glucose transporter (GLUT-1), the transferin receptor (OX-26), the endothelial barrier antigen (EBA) and the OX-47 antigen. Using enzyme cytochemistry the same comparison was made for gamma-glutamyl transpeptidase (GGTP) and alkaline phosphatase. By immunocytochemistry GLUT-1 was uniformly strongly represented in brain EC, nerve EC and perineurium. OX-26 was strongly positive in brain EC but present only in trace quantities in nerve EC and perineurium. EBA similarly showed strong positivity in brain EC and trace amounts in nerve EC but was absent from perineurium. OX-47 was present moderately in brain EC and perineurium but absent from nerve EC. Quantitative immunoblotting of brain and sciatic nerve homogenates showed statistically significant differences in the level of expression of EBA and OX-26 between the two tissues. Enzyme cytochemistry showed that GGTP was strongly positive in brain EC but absent from nerve EC and perineurium. Alkaline phosphatase stained strongly in brain and nerve EC and was absent from perineurium. In summary the six membrane markers were heterogeneously represented in nerve compared with brain. This pattern of distribution in the nerve cannot simply be accounted for by the absence of astrocytes and their inductive influences. Any inductive influences of Schwann cells require investigation.

Substance P-induced enhanced permeability of dura mater microvessels is accompanied by pronounced ultrastructural changes, but is not dependent on the density of endothelial cell anionic sites

Experimental data point to a determinant role for endothelial cell (EC) anionic sites in the regulation of vascular permeability. Previous studies have shown that EC anionic sites density is reduced in conditions of enhanced permeability. The pathophysiology of migraine and vascular headache encompasses dilatation of dural vessels and extravasation of plasma proteins. The current study was carried out to determine if the density of EC anionic sites is reduced in enhanced permeability of dural vessels. Enhanced permeability was chemically induced in rats by intravenous injection of substance P and was tested by assessing leakage of horseradish peroxidase (HRP). Anionic sites were labelled with cationic colloidal gold and their density was quantified from electron microscopy negatives. Experimental animals showed increased leakage of HRP from dural vessels. However, anionic sites in EC membranes (luminal and abluminal) showed no statistical differences when their mean densities in experimental and control animals were compared. The results indicate that in this model, factors other than the density of anionic sites may be important determinants in the permeability of dural vessels. Such factors may include structural alteration of ECs consistent with an increased permeability. In this study pronounced ultrastructural changes in ECs were noted in experimental animals including widening of intercellular junctions and an increase in the number of EC gaps and vesicles.

Cerebral and pial microvessels: differential expression of gamma-glutamyl transpeptidase and alkaline phosphatase

Pial microvessels have several important blood-brain barrier (BBB) characteristics in common with cerebral microvessels, despite lacking their astrocytic ensheathment. We have therefore determined whether they have the same distribution of two enzymes, gamma-glutamyl transpeptidase (GGTP) and alkaline phosphatase, both of which are known to be astrocyte-dependent. GGTP was absent from all rat pial microvessels but strongly present in brain cortical capillaries. Alkaline phosphatase was heterogeneously expressed in pial microvessels, including capillaries, but strongly positive in brain cortical capillaries. Diffusible, inductive factors produced by astrocytes could account for these differences in enzyme distribution between the two vessel types. Furthermore, differences in expression between the two markers may reflect their differing sensitivities to the astrocytic factors. Caution is urged in the common usage of the pial microvessel as a model system in BBB studies.

Corneal glycoconjugates: an ultrastructural lectin-gold study

The oligosaccharide chains of cell surface and extracellular matrix glycoconjugates are essential for the biological properties of these molecules. We have, therefore, investigated carbohydrate residues in the rat cornea using biotinylated lectin-gold probes. Fixed corneas were removed and embedded in Lowicryl HM20 or LR White. Ultrathin sections were incubated in one of the lectins: Triticum vulgare (WGA), Canavalia ensiformis (Con A), Griffonia simplicifolia (GS-1), Limax flavus (LFA) and Allomyrina dichotoma (Allo A), followed by streptavidin-gold, or the sections were incubated in cationic colloidal gold. Semi-quantification of gold labelling was determined for corneal endothelium, Descemet's membrane, stroma and epithelium from electron micrographs. WGA and Con A binding sites were expressed either moderately or strongly throughout the cornea, suggesting a preponderance of alpha-mannose and N-acetylglucosamine residues. A particular concentration of these sugars was found in Descemet's membrane. In contrast, GS-1 (specific for alpha-galactose) and Allo A (specific for beta-galactose) labelled all regions weakly. Sialic acid residues, as defined by LFA labelling and the expression of neuraminidase-sensitive cationic colloidal gold binding sites, were sparsely distributed throughout the stroma, Descemet's membrane and endothelium. In contrast, sialoglycoconjugates were found in significant concentrations in the epithelium. Electron microscopy proved useful in providing new information on the cellular and subcellular localization of these lectin binding sites.

Is the pial microvessel a good model for blood-brain barrier studies?

Pial microvessels have commonly been used as model systems for studying blood-brain barrier (BBB) properties instead of cerebral cortical microvessels. Since pial microvessels are relatively accessible they have been especially employed in electrophysiological and pharmacological studies. Measurements of electrical resistance across endothelial cells (EC) as a measure of their barrier properties have been made exclusively from pial microvessels in in vivo BBB studies. Similarly the observed responses of microvessels to the application of pharmacological agents have commonly been made on pial microvessels as representative of BBB vasculature. In this review the properties of pial and cerebral microvessels are compared to determine whether the use of the pial microvessel as a model for BBB studies is valid. Similarities are described in their ultrastructural features, permeability to electron dense tracers and molecular characteristics. Measurements of electrical resistance from pial microvessels are compared with measurements from cerebral EC monolayers in tissue culture and indirect determinations for cerebral microvessels in situ. Two notable differences between pial and cerebral microvessels are described in the adult nervous system. Tight junctions between cerebral EC appear to consist of a uniform population. In pial microvessels however tight junctions consist of two populations in one the inter-EC tight junctions resemble those between cerebral EC, with fusion of adjacent EC membranes. In the second population the inter-EC tight junctions differ with a discernible gap between adjacent EC membranes. The distribution of the endothelial barrier antigen (EBA) is uniform between EC of cerebral microvessels. By contrast EC of pial microvessels from a heterogeneous population for EBA expression which is related to the proximity of the EC to the astrocytic glia limitans. The role of astrocytes in the induction and maintenance of the BBB characteristics is briefly reviewed. The possible significance of the lack of an astrocytic ensheathment of pial microvessels is assessed. In summary, caution is urged in employing pial microvessels in BBB studies and the need for more information on possible pial microvessel heterogeneity is stressed.

Molecular characteristics of pial microvessels of the rat optic nerve. Can pial microvessels be used as a model for the blood-brain barrier?

Pial microvessels have commonly been used in studies of the blood-brain barrier because of their relative accessibility. To determine the validity of using the pial microvessel as a model system for the blood-brain barrier, we have extended the comparison of pial and cerebral microvessels at the molecular level by a partial characterization of the glycocalyx of pial endothelial cells, in view of the functional importance of anionic sites within the glycocalyx. Rat optic nerves were fixed by vascular perfusion. Anionic sites on the endothelium were labelled with cationic colloidal gold by means of post- and pre-embedding techniques. The effects of digestion of ultrathin sections on subsequent gold labelling was quantified following their treatment with a battery of enzymes. Biotinylated lectins, viz. wheat germ agglutinin and concanavalin A with streptavidin gold, were employed to identify specific saccharide residues. The results demonstrate that the luminal glycocalyx of pial microvessels is rich in sialic-acid-containing glycoproteins. Neuraminidase, which is specific for N-acetylneuraminic (sialic) acid, and papain (a protease with a wide specificity) significantly reduce cationic colloidal gold binding to the luminal endothelial cell plasma membrane. Wheat germ agglutinin (with an affinity for sialic acid) binds more to the luminal than abluminal plasma membrane, whereas concanavalin A, which binds mannose, binds more to the abluminal surface. Similar results have been obtained for cerebral cortical endothelial cells. With respect to these molecular characteristics, therefore, the pial and cortical microvessels appear to be the same. However, since the two vessel types differ in other respects, caution is urged regarding the use of pial microvessels to investigate the blood-brain barrier.

Ontogeny of four blood-brain barrier markers: an immunocytochemical comparison of pial and cerebral cortical microvessels

Pial and cortical microvessels possess many blood-brain barrier (BBB) properties in common, including impermeability to electron dense tracers, high transendothelial electrical resistance and specialised endothelial cell ultrastructural features. To compare pial and cortical microvessels further, a developmental, immunocytochemical study was undertaken of 4 BBB markers in the rat: OX-47, EBA, GLUT-1 and s-laminin. The appearance of the markers was monitored from embryonic d 16, to postnatal and adult stages. Each of the 4 markers appeared simultaneously in both pial and cortical vessels. GLUT-1 and OX-47 were present in endothelial cells of the BBB from E 16 to the adult. EBA and s-laminin appeared from postnatal d 7 through to the adult. Pial microvessels lack the ensheathment of astrocytes which may be involved in the induction and/or maintenance of BBB markers in the cortex. It is possible that astrocyte-derived factors diffusing from the brain surface are responsible for induction of BBB properties in the pial microvessels.

Dural microvessels: molecular properties of their luminal anionic sites

(1) Neurogenic inflammation has been implicated in the pathogenesis of the vascular headaches of migraine and cluster headaches. (2) Dural blood vessels are both pain-sensitive and show neurogenic plasma extravasation. (3) Endothelial cell (EC) surface anionic sites appear to be a determinant of vascular permeability. We therefore examined the anionic sites of dural EC to determine whether they are different from those of pial and parenchymal vessels. Luminal anionic sites of rat optic nerve EC were labelled with cationic colloidal gold (CCG) and cationic ferritin (CF) and examined by electron microscopy. Employing a battery of enzymes, the effects of digestion of ultrathin sections on subsequent labelling with CCG was quantified using image analysis software. In addition, a gold-labelled lectin, wheat-germ agglutinin (WGA), was employed to locate specific saccharide residues. Of the enzymes with a narrow specificity, only neuraminidase substantially reduced CCG binding. Of the proteolytic enzymes, papain was most effective in reducing labelling. These results show that the luminal EC anionic sites are chiefly composed of sialoglycoproteins. The labelling with biotinylated WGA-streptavidin gold was similar to that with CCG without enzyme digestion. This suggests that WGA is binding to N-acetylneuraminic (sialic) acid residues and not to the neutral N-acetylglucosamine (since CCG would not label uncharged molecules). These results do not differ significantly from those for pial and parenchymal EC. It is therefore likely that factors other than anionic site molecular composition account for the susceptibility of dural vessels to neurogenic plasma extravasation. The relevance of these observations in an experimental animal model to the human clinical condition remains to be determined.

Optic nerve microvessels: a partial molecular definition of cell surface anionic sites

Incorporated in the luminal glycocalyx of vascular endothelia (EC) are negatively charged microdomains (anionic sites). These sites are considered functionally important (a) in their interaction with circulating blood constituents, and (b) as a determinant of vascular permeability. The molecular composition of these EC sites, described for a number of tissues, has demonstrated a heterogeneity dependent on their anatomical location. Luminal anionic sites have not been characterized for EC of optic nerve. Optic nerves were removed from Sprague-Dawley rats previously fixed by vascular perfusion. EC anionic sites were labelled with the probes cationic colloidal gold (CCG) and cationic ferritin (CF), using the pre- and post-embedding techniques, and examined by electron microscopy. The effects of enzyme digestion of ultrathin sections on subsequent CCG labelling were determined using a battery of enzymes in association with the post-embedding technique. CCG labelling was quantified following each enzyme treatment using image analysis software. The biotinylated lectin wheat germ agglutinin (WGA) with streptavidin gold was also used to localize specific monosaccharide residues. The luminal front of intraneural EC showed a uniform labelling with CCG and CF which was greater than on the abluminal surface. Extracellular matrix components and basal laminae were moderately labelled. Digestion of tissue sections with heparitinase and trypsin had no significant effect on subsequent CCG labelling. Proteinase K was less effective than papain but both produced a significant reduction. Neuraminidase almost completely eliminated labelling. CCG binding to the luminal plasma membrane of optic nerve EC can be significantly reduced with proteolytic and glycolytic enzymes. The results demonstrate that sialoglycoproteins principally constitute these luminal EC anionic sites. Biotinylated WGA-streptavidin gold, which detects both N-acetylneuraminic (sialic) acid and N-acetylglucosamine, gave a similar pattern of labelling to CCG alone on the luminal versus abluminal EC fronts. These findings suggest that WGA is binding predominantly to N-acetylneuraminic acid residues since CCG would not label the neutral (uncharged) N-acetylglucosamine.

Distribution of a putative endothelial barrier antigen in the ocular and orbital tissues of the rat

BACKGROUND

A rat endothelial barrier antigen (EBA) recognised by a monoclonal antibody has been shown to be expressed strongly by endothelial cells of brain capillaries possessing a blood-brain barrier and only weakly expressed by fenestrated brain vessels.

METHODS

In this study immunocytochemical methods for light and electron microscopy were used to study EBA distribution in the eye and orbital tissues of the rat.

RESULTS

Blood-ocular barrier vessels in the optic nerve, retina, iris, and some vessels in th choroid and ciliary body were immunopositive for EBA. By pre-embedding immunocytochemistry for electron microscopy the antigen was observed on the luminal endothelial cell surface.

CONCLUSION

Surprisingly, some non-barrier vessels in the ciliary body and choroid expressed EBA suggesting that it may play a broader role in endothelial properties than previously recognised. The functional significance of EBA remains to be elucidated.

Differential expression of an endothelial barrier antigen between the CNS and the PNS

A monoclonal antibody to an antigen (EBA) expressed by neural endothelial cells (EC) was used to investigate any difference in the distribution of EBA between the CNS and PNS. Pre-embedding ultrastructural cytochemistry of rat sciatic and optic nerves was undertaken using anti-EBA, detected with a silver-enhanced gold-conjugated secondary antibody. LM immunocytochemical localisation of EBA was also performed using an HRP-conjugated secondary antibody. EC of pial and parenchymal optic nerve vessels were strongly immunopositive for EBA. Vessels of the dura were negative. At the EM level EBA was observed on the EC luminal surface. In contrast, EC of sciatic nerve were either negative or only weakly immunopositive. The molecular characteristics and function of EBA are largely unknown. Therefore the functional significance of the present findings remains to be determined.

Molecular characterization of anionic sites on the luminal front of endoneurial capillaries in sciatic nerve

The distribution of anionic microdomains has been described in cerebral vessels and more recently in capillaries of peripheral nerve. Evidence is accumulating that these sites play a role in the barrier function of vascular endothelia in the PNS and CNS. The chemical nature of anionic sites has been at least partly determined for cerebral vessels but not in peripheral nerve. This study reports our preliminary investigations to determine the nature of endothelial anionic sites in sciatic nerve. The effects of digestion of ultra-thin sections of nerve with a battery of proteolytic and glycolytic enzymes (papain, trypsin, proteinase K, hyaluronidase, heparinase, heparitinase and neuraminidase) on the distribution of anionic sites was determined using the label, cationic colloidal gold. Papain, a proteolytic enzyme of broad specificity, succeeded in removing the majority of cationic colloidal gold-binding sites on the luminal surface of vascular endothelia. In contrast trypsin and proteinase K were less effective, reflecting their narrower specificity. Hyaluronidase, heparinase and heparitinase did not significantly affect cationic colloidal gold-labelling. However, a considerable reduction in cationic colloidal gold-binding occurred following neuraminidase digestion. These results suggest that, as in cerebral vessels, sialic acid-containing glycoproteins are largely responsible for the negatively charged domains on the luminal membrane of endothelial cells in peripheral nerve.

Blood-nerve barrier: ultrastructural and endothelial surface charge alterations following nerve crush

Nerve crush results in an enhanced vascular permeability of the endoneurial vessels distal to the lesion. Vascular permeability at the blood-nerve barrier (BNB) to serum proteins is influenced by many factors, including anionic surface charge, endothelial vesicular transcytosis and the presence or absence of fenestrated vessels. Using mice and rats, the present ultrastructural investigation examined the effect of nerve crush (axonotmesis) on: (1) the distribution of endothelial anionic sites and (2) the appearance of fenestrations in endoneurial vessels after 4 and 14 day intervals as demonstrated with cationic probes. Transient anionic fenestrations developed in a minority of mouse endoneurial vessels in 4-day crushed nerves, but were not found in 14-day crushed nerves of mice nor in crushed nerves of rats. The known increase in the permeability of endoneurial vessels in rats and mice was not associated with reduced luminal labelling with cationic ferritin at physiological pH. At pH 2.0 the labelling of glycocalyx moieties (such as sialic acid) with cationic colloidal gold was disrupted in some epi- and endoneurial vessels of 4-day rats, but in a greater proportion after 14 days. The enhanced permeability of the BNB during degeneration and regeneration is related to the formation of anionic fenestrations in endoneurial vessels of mice and to the reduced and uneven distribution of endothelial glycocalyx moieties that are anionic at pH 2.0 in rats.

Schwann cell endocytosis: a role in nerve regeneration?

Schwann cell plasma membrane vesicles have been shown to increase in numerical density after nerve injury but their function is unclear. In this study, ultrastructural tracers were micro-injected in vivo into crushed rat sciatic nerves after various time intervals to ascertain whether plasma membrane vesicles of Schwann cells are involved in the uptake and utilization of molecules from the endoneurium during axonal regeneration and remyelination. Horseradish peroxidase (HRP), a tracer of fluid-phase endocytosis, was taken up by macrophages and fibroblasts but remained external to Schwann cells throughout the study. After 14-16 days of crush injury, HRP was present within vessel lumina and in cytoplasmic vesicles of pericytes and vascular endothelia. Low-density lipoprotein-gold, which is primarily internalized by receptor-mediated endocytosis, and bovine serum albumin-gold, proposed as a tracer for fluid-phase endocytosis, were internalized by macrophages and fibroblasts but were not taken up by Schwann cells. Although Schwann cells formed pits in the plasma membrane and vesicles were evident in the cytoplasm, none of the tracers used were internalized by Schwann cells. It is suggested that Schwann cell plasmalemmal and cytoplasmic vesicles have a cellular role unrelated to endocytosis or alternatively the Schwann cell basal lamina may function as a diffusion barrier to the tracers employed.

Distribution of anionic sites on the perineurium

The distribution of anionic sites on the perineurial basal lamina (BL) and plasmalemma of dorsal root ganglia and sciatic nerves was determined using cationic ferritin (CF) and cationic gold (CCG). The probes were applied to the tissue before and after resin embedding and visualised by electron microscopy. There were no apparent differences in charge distribution between the 2 tissues. At physiological pH a strong anionic charge was distributed evenly over the BL as demonstrated by pre-embedding labelling with CF; the plasmalemma was only moderately anionic. A similar application of CCG at pH 2.0 revealed a quasi-regular distribution of anionic sites (presumably due to acidic carbohydrate moieties) on the BL, whilst CCG-labelling of L. R. White sections indicated a differential distribution of these moieties on the BL of the inner and outer perineurial lamellae. Cationic ferritin (12 nm diameter) crossed the BL and entered perineurial caveolae, but CCG (effective diameter of 15 nm) did not, suggesting that the BL is a size-restrictive filter. These results are discussed with regard to the ultrastructure and function of the BL of other tissues and the possible role of perineurial BL charge as a determinant of perineurial permeability.

Influence of fatty acid content of lysophosphatidyl choline on its myelinotoxic properties

The efficacy of lysophosphatidyl choline (LPC) type I and type IV in producing demyelination was assessed in rat tibial and sural nerve. By light and electron microscopy, a greater myelinolytic activity was demonstrated with type I, and concomitantly electrophysiology showed a more severe conduction block. In teased nerve preparations and 1-microns thin sections, demyelinated fibres were more frequent with LPC type I. At 1 h after injection, electron microscopy showed much more extensive myelin lysis in the form of fine vesicular debris. By 6 days, completely demyelinated fibres were much more common and associated Schwann cells contained either small quantities or no myelin debris. With type IV LPC, cytopathological changes were more extensive at 1 h. A minority of Schwann cells showed swollen hydropic cytoplasm and degradation of organelles. Axonal retraction from the myelin sheath occurred in occasional fibres, and in a few unmyelinated fibres axoplasm showed organelle depletion and increased granularity. By 6 days, Schwann cells still contained large quantities of gross myelin debris and had often retracted to expose extensive areas of axolemma. The findings suggest that the two types of LPC have different myelinolytic actions, which may be related to their different fatty acid content. A possible role for the two types of LPC in 'bystander demyelination' is considered.

Blood-nerve barrier: distribution of anionic sites on the endothelial plasma membrane and basal lamina of dorsal root ganglia

Previous investigations of the blood-nerve barrier have correlated the greater permeability of ganglionic endoneurial vessels, compared to those of nerve trunks, with the presence of fenestrations and open intercellular junctions. Recent studies have demonstrated reduced endothelial cell surface charge in blood vessels showing greater permeability. To determine the distribution of anionic sites on the plasma membranes and basal laminae of endothelial cells in dorsal root ganglia, cationic colloidal gold and cationic ferritin were used. Electron microscopy revealed the existence of endothelial microdomains with differing labelling densities. Labelling indicated that caveolar and fenestral diaphragms and basal laminae are highly anionic at physiological pH, luminal plasma membranes and endothelial processes are moderately charged and abluminal plasma membranes are weakly anionic. Tracers did not occur in caveolae or cytoplasmic vesicles. In vitro tracer experiments at pH values of 7.3, 5.0, 3.5 and 2.0 indicated that the anionic charge on the various endothelial domains was contributed by chemical groups with differing pKa values. In summary, the labelling of ganglionic and sciatic nerve vessels was similar except for the heavy labelling of diaphragms in a minority of endoneurial vessels in ganglia. This difference is likely to account in part for the greater permeability of ganglionic endoneurial vessels. The results are discussed with regard to the blood-nerve and -brain barriers and vascular permeability in other tissues and a comparison made between the ultrastructure and anionic microdomains of epi-, peri- and endoneurial vessels of dorsal root ganglia and sciatic nerves.

Blood-nerve barrier: distribution of anionic sites on the endothelial plasma membrane and basal lamina

The distribution of anionic sites on the cell membranes and basal laminae of vascular endothelial cells in the rat sciatic nerve was investigated using cationic ferritin (CF) and cationic colloidal gold (CCG). Nerves fixed by perfusion followed by immersion were chopped into 400 microns thick slices and incubated in CF or embedded in LR White resin for staining with CCG. Using electron microscopy, the distribution of these tracers was investigated. The results indicated that microdomains of various charge densities exist. Diaphragms of caveolae and transendothelial channels, and luminal endothelial processes are highly anionic, the basal laminae of endothelial cells and pericytes and luminal membranes are medium and abluminal membranes least anionic. Inter-endothelial tight junctions were unlabelled and not penetrated by CF. These structures are thought to represent charge and size filters that control permeability of the vasa nervorum. The distribution of these charge-size filters is discussed in terms of the blood-nerve barrier, a physiological property present in the endo- but absent in the peri- and epineurial vessels.

Lysophosphatidylcholine-induced incipient demyelination: involvement of a new tubular structure

Demyelination was induced in the rat sciatic and tibial nerves by microinjection with lysophosphatidylcholine (LPC). Accompanying early myelin lysis (1-24 h) was the formation of vesicles and tubular structures. The tubules which are novel structures have a diameter range of 24-27 nm, a centre-to-centre spacing 30-50 nm and may extend for 3 microns in length. In this form they are arranged as a monolayer in the periaxonal space. As demyelination progressed and the periaxonal space widened the tubules increased in number and became more irregularly arranged. The tubules are apparently derived from the myelin lamellae/Schwann cell plasma membrane, while the axolemma remains intact.

Diffusion barrier properties of the perineurium: an in vivo ionic lanthanum tracer study

While the perineurium as a diffusion barrier has been extensively investigated by light and electron microscopy, such studies have been largely restricted to the use of protein tracers. In the present study the permeability of the perineurium to a physiologically more relevant ionic tracer has been assessed. In vivo the rat sural or tibial nerve was either microinjected with lanthanum nitrate solution for endoneurial application or bathed in the lanthanum solution for epineurial application. The findings generally demonstrated an effective barrier to the tracer which failed to penetrate the inner layers of the perineurium. Only at the highest lanthanum concentration and longest time intervals employed did trace quantities occasionally penetrate the barrier and then only in the presence of some cytopathological changes to the outermost perineurial cells. The usefulness of the microinjection method was limited by the slight but unavoidable trauma to the perineurium. The findings are related to those of other studies which have used electron dense tracers, also to studies using physiological including electrophysiological techniques and morphological including freeze-fracture methods.

The vasa nervorum: microcorrosion casts for scanning electron microscopy

Studies on the vasa nervorum have a long history, not least because of their beneficial application in surgical practice and in understanding the pathogenesis of some neuropathies. In the present study a method is described for the preparation of microcorrosion casts of the vasa nervorum suitable for examination by scanning electron microscopy. The results confirm the findings of earlier investigations but also demonstrate the advantages of an immediate three-dimensional representation of the vascular architecture together with the additional magnification and resolving power of electron microscopy.

Filipin-sterol complexes in disrupted myelin in the rat

Using filipin as a cytochemical probe for cholesterol we have compared the distribution of filipin labelling in mildly disrupted myelin and normal myelin. The myelin lamellae in rat sciatic nerve were separated either by hypotonic saline (0.035-0.07 M) or nerve section (24-32 h) before aldehyde fixation and filipin treatment. Myelin separation was assessed in ultrathin sections and filipin distribution in freeze-fracture replicas. In separated myelin lamellae filipin labelling was similar throughout the myelin sheath while in normal control myelin filipin occurred most in the outer (abaxonal), least in the inner (adaxonal) and intermediate in the middle lamellae. It is concluded that this heterogeneous filipin labelling in normal myelin is a result of diffusion gradients to filipin within the myelin sheath and that in vivo cholesterol is uniformly distributed throughout all the lamellae of the myelin sheath. The site of the diffusion barrier to filipin within normal myelin is considered.

Lysophosphatidyl choline-induced demyelination. A freeze-fracture study

Focal demyelination was produced in the rat sciatic nerve by microinjection of lysophosphatidyl choline (LPC). The demyelinating lesion was examined over the following 48 h using the freeze-fracture technique to examine myelin, Schwann cell and axonal membranes. Myelin lamellae were replaced by myriad spherical or oval membranous vesicles. The axonal and Schwann cell plasma membranes remained intact and the latter showed a large increase in caveolae-associated pores in some nerve fibres. The lysis of myelin lamellae and membranous vesicle formation are related to the known action of LPC on myelin and its membrane fusogenic properties. The importance of calcium ion influx and membrane protein aggregation and depletion in vesiculation are discussed.

The blood-nerve barrier: an in vivo lanthanum tracer study

The permeability of the blood-nerve barrier was investigated using ionic lanthanum as an electron-dense tracer. The rat sural nerve was microinjected in vivo with lanthanum nitrate solution either into the endoneurial space or into the epineurium. Five to sixty minutes after injection the sural nerves were fixed by vascular perfusion or immersion. Using electron microscopy, lanthanum tracer was observed to be associated with endoneurial vessels in the perivascular spaces, in the inter-endothelial clefts and within the lumina. Furthermore, tracer was present in the spaces between adjacent endothelial cell layers and within vesicles and caveolae of endothelial cells. Epineurial vessels showed a similar distribution of tracer deposits but in greater quantities in inter-endothelial cell spaces and vessel lumina. The results are considered to demonstrate an absence of a blood-nerve barrier to ions as exemplified by lanthanum and are compatible with data from physiological experiments. The blood-nerve and blood-brain barriers are contrasted in their permeability to ions, their related fine structure and their physiological roles.

Distribution of filipin-sterol complexes in the unmyelinated nerve fibre

The filipin-sterol technique was employed, together with freeze-fracture, to investigate the fine structure of the unmyelinated nerve fibre in the peripheral nerve. No heterogeneity was observed in the distribution of filipin labelling either in the Schwann cell plasma membrane or along the axolemma. The distribution of labelling is contrasted with that in the myelinated nerve fibre and related to the relative morphology and electrophysiology of the two types of fibre.

The development of Schmidt-Lanterman incisures: an electron microscope study

The development of Schmidt-Lanterman incisures was investigated in the rat sural nerve during an active phase of postnatal myelination (5-21 days post partum). Two distinct populations of incisures were recognised and the following nomenclature for their developmental stages is proposed. Primary incisures which appear ab initio in myelination and always extend across the whole radial thickness of the myelin sheath but initially around only part of its circumference. Consequently they appear in transverse section as sectoral incisures (occupying less than half the circumference) and in longitudinal section as asymmetric incisures (involving one side only of the myelin sheath). Secondary incisures appear later, in regions of a compact myelin sheath, initially traversing only part of its radial thickness but commonly occupying its whole circumference. Thus they usually appear in transverse section as circumferential incisures and in longitudinal section as symmetric incisures (involving both sides of the myelin sheath). Less commonly secondary incisures may form in a sector of the myelin sheath but still in regions of compact myelin and thus appear asymmetric in longitudinal section and sectoral in transverse section. Secondary incisures appear mainly adaxonally in the earlier stages examined and mainly abaxonally in the later stages. The maturation of primary and secondary incisures into the radially and circumferentially complete incisure characteristic of the mature myelinated nerve fibre is described. The above mechanisms of incisural formation are contrasted with mechanisms previously suggested to occur during normal development and remyelination and related to the plasticity and ultrastructure of the myelin sheath.

Filipin-sterol complexes at Schmidt-Lanterman incisures

Employing the freeze-fracture technique, the distribution of filipin-sterol complexes was determined for membranes of peripheral nerve myelin. A heterogeneous distribution of complexes was observed with the greatest abundance on membranes associated with the cytoplasmic channels of Schmidt-Lanterman and longitudinal incisures. In addition there was an irregular network of well-labelled membrane bands in compact myelin. The results are related to a possible role for these channels and bands in the biochemical turnover of cholesterol in myelin.

Filipin-sterol complexes at nodes of Ranvier

Using the filipin-sterol technique, regional heterogeneity in the axonal and Schwann cell plasma membranes was investigated at the node of Ranvier and paranodes. Filipin-sterol complexes were abundant at the nodal axolemma but infrequent throughout the paranodal axolemma. The paranodal Schwann cell plasma membrane was rich in complexes which extended over the nodal Schwann cell microvilli. There were no regional differences in filipin labelling of the nodal-paranodal Schwann cell plasma membrane in relation to features such as paranodal cytoplasmic columns or mesaxonal furrows. However, the paranodes of adjacent Schwann cells were sometimes markedly different from each other in the amount of filipin labelling. The extent to which filipin labelling is indicative of cholesterol membrane content is discussed and the findings are related to current concepts of distribution, mobility and interaction of protein and lipid in biomembranes, with particular reference to the nodal axolemma.

Distribution of filipin-sterol complexes in the myelinated nerve fiber

Using filipin as a cytochemical probe to reveal the distribution of cholesterol, myelinated peripheral nerve fibers were examined in freeze-fracture replicas. Filipin-sterol complexes were most abundant in the Schwann cell and axonal plasma membranes. In the Schwann cell plasma membrane there was no heterogeneity in complex distribution in relation to the subjacent cytoplasmic network. In myelin lamellae there was a decrease in complexes from outer to inner lamellae and some aggregation of complexes in individual lamellae. The density of complexes in cytoplasmic organelles varied from absent in mitochondria to high in lysosome-like bodies. The results are interpreted in terms of the related biochemical composition and biophysical properties of cell membranes, with particular reference to the myelinated nerve fiber. The influence of diffusion barriers and gradients on the formation of complexes by filipin is considered.

Axonal microtubules: a computer-linked quantitative analysis

Employing current computer-aided morphometric techniques, axonal microtubule density was determined for the rat sural nerve. Analysis of extensive data showed that while microtubule number increases with axon size, the increase is not directly proportional. Thus the relationship between microtubule density and axonal size is inversely related, so that microtubule density is greater in smaller axons than in larger axons. When a proximal and distal site, separated by 2 cm, were compared for microtubule density there was no significant difference, using pooled data for all fibre diameters. The results are interpreted in terms of our present knowledge of axonal-microtubule quantitative relationships, which is reviewed.

Axonal degeneration in large and small nerve fibres. An electron-microscopic and morphometric study

Using computer-aided morphometric methods, axonal degeneration following nerve crush was analysed to reassess whether small fibres degenerate before large fibres or vice versa, or simultaneously. Axonal microtubule density was used as the criterion for determining the extent of fibre degeneration. Axonal areas and axonal microtubule numbers were recorded from a large sample of myelinated fibres in the right unoperated rat sural nerve and distal to crush in the left sural nerve. Both samples were divided into small and large fibre groups, according to axonal areas. Statistical analysis of the data confirmed a significant loss of microtubules from the left crushed nerve fibres but no significant difference in the relative loss of microtubules from small and large fibres. It is concluded, therefore, that in Wallerian degeneration, axonal breakdown, as assessed by microtubule loss, occurs simultaneously in small and large fibres. The findings are related to the electrophysiological changes which occur in Wallerian degeneration.

Nodes of Ranvier and Schmidt-Lanterman incisures: an in vivo lanthanum tracer study

The permeability of the tight junctional system of myelin, at the juxtanodal myelin terminal loops and Schmidt-Lanterman incisures, was investigated using the ionic tracer lanthanum (a) in vivo followed by fixation, (b) concurrently with fixation, (c) following fixation. Employing the same methods the juxtanodal membrane complex formed between myelin loops and axolemma was also tested. The results of this study demonstrate that the periaxonal space (between axon and Schwann cell) is apparently accessible to lanthanum via the myelin loop-axolemmal junction, irrespective of the mode of exposure of myelinated fibres to the tracer. Similarly, the tight junctions between adjacent myelin terminal loops apparently do not prevent lanthanum penetration either in living or in fixed nerves. By contrast the tracer obtained access to the extracellular space within incisures only in vivo. The results are interpreted in terms of the permeability of nodes and incisures in vivo to physiologically important ions and related to current concepts of the electrophysiology of the myelinated nerve fibre.

The non-directional pattern of axonal changes in Wallerian degeneration: a computer-aided morphometric analysis

Wallerian degeneration was investigated to determine whether axonal changes occur progressively in a somatofugal or somatopetal direction or simultaneously along the length of the axon. Microtubule density was used as a measure of the extent of axonal degeneration and was assessed by a computer-aided analysis of electron micrographs. The left sural nerves of ten rats were crushed and 30 hours later axonal areas and axonal microtubule numbers were recorded from a large sample of axons at two sites 1 cm and 3 cm distal to the crush. The same recordings were made from the right unoperated nerve at two comparable sites. Statistical analysis of all the data provided no evidence for a somatofugal or reverse direction of degeneration. It is concluded therefore that in Wallerian degeneration axonal changes, as indicated by microtubule dissolution, occur simultaneously along the length of the axon. It is proposed that to interpret the conflicting published data on the direction of fibre degeneration, Schwann cell changes (e.g. myelin ovoid formation) and axonal changes (e.g. microtubule dissolution) should be considered independently since they have different aetiological mechanisms which may account for the differing experimental results reported.

Myelinated nerve fibres and the fate of lanthanum tracer: an in vivo study

The permeability of the marginal tight junctional system of myelin was tested in the rat employing the electron-dense tracer lanthanum nitrate. Lanthanum was either included in the fixative used for vascular perfusion (at a concentration of 20 mM) or was microinjected in vivo into the sural or tibial nerve (5, 10 and 20 mM). After 5-60 minutes, the microinjected nerves were fixed either by immersion or vascular perfusion. Lanthanum tracer was present in the intraperiod line gap of myelin, irrespective of the mode of application of the tracer, the method of fixation or the time of exposure to lanthanum. However, the tracer was present more extensively when included in the fixative compared with in vivo microinjection. Internodally, lanthanum was usually restricted to the inner, or more commonly, the outer lamellae of larger fibres, while all lamellae were usually penetrated by tracer in smaller fibres. Paranodally, compact myelin was more extensively penetrated. The periaxonal space (between axon and Schwann cell) was readily accessible to tracer. It is concluded that the marginal tight junctional system of myelin is apparently of the 'leaky' type and is permeable to ions. The findings have implications for the electrophysiology and pathophysiology of the myelinated nerve fibre.

A combined morphological and electrophysiological study of conduction block in peripheral nerve

The reliability of the electrophysiological criterion of conduction block in determining the presence of focal demyelination in a peripheral nerve has been studied in an animal model. Demyelination was produced in the rat tibial nerve by one or two closely spaced microinjections of lysophosphatidylcholine (LPC). Histological and electrophysiological data were obtained on the acute lesion (up to 6 days), and during recovery (up to 11 weeks). Single LPC injections produced a lesion of very variable severity. Double injections more reliably produced a severe lesion with marked conduction block. Slight axonal damage was occasionally seen in nerves showing severe demyelination. The ratio of amplitude of muscle action potentials evoked by stimuli proximal and distal to the sites of nerve injection was calculated to detect the development of conduction block. The post injection ratio was more than 2 standard deviations below the control mean in 86% of nerves showing signs of demyelination. No control saline injected nerves showed such evidence of conduction block. The severity of the electrophysiological abnormality did not prove a reliable indicator of the severity of the histological lesion, however. The possible reasons for this variability are discussed and it is argued that caution should be exercised when interpreting this particular electrophysiological finding in clinical practice.

Plasma membrane pores of the Schwann cell in Wallerian degeneration: a morphometric analysis

Using the freeze-fracture technique, myelinated fibres were examined from the rabbit sciatic nerve at 48 h after a proximal nerve crush. Employing computer-aided morphometric techniques the distribution of Schwann cell plasma membrane pores was analysed. In both normal control and crushed nerves membrane pores were restricted to the cytoplasmic circumferential bands and longitudinal columns, which characterize the surface of the myelinated nerve fibre, and were absent from the flat plaque-like areas delimited by the bands and columns. In approximately half of the myelinated fibres from the crushed nerves there was a five-fold increase in the density of plasma membrane pores. This response of the Schwann cell was interpreted in terms of an increase in pinocytosis and related to regenerative phenomena in the peripheral nerve.

Demyelination: a failure of cell communication?

A hypothesis is proposed that demyelination in both the CNS and PNS involves a failure of cell communication between the axon and oligodendrocyte/Schwann cell, as a primary event. The site of communication is assumed to be the paranodal myelin loop-axolemma membrane complex. It is postulated that "cross-talk" between the two cell types can be interrupted, and hence demyelination initiated, by pathophysiological changes in either the axon or myelinating cell. Experimental evidence in support of the hypothesis is cited in so far as it exists.

Effects of capsaicin applied locally to adult peripheral nerve. II. Anatomy and enzyme and peptide chemistry of peripheral nerve and spinal cord

(1) Capsaicin solution was applied for 15 min around a 1 cm length of sciatic nerve in the mid upper leg of adult rats. (2) Electron microscopic examinations of the nerve in the treated region after 14 days shows no signs of degeneration of either myelinated or unmyelinated fibres attributable to the capsaicin. (3) Fluoride resistant acid phosphatase FRAP disappears from the central terminals of the treated nerve by 7 days. (4) 1.5 mM capsaicin is sufficient to product a complete reduction of FRAP in the spinal cord. (5) The peptides substance P and cholecystokinin (CCK) are markedly depleted in the region of spinal cord terminations of the treated nerve at 14 days. (6) Substance P and CCk are not affected in spinal cord regions other than in the unmyelinated afferent terminal zone. Similarly neurotensin and neurophysin which are not present in afferent fibres are not influenced by capsaicin treatment of the sciatic. (7) It is concluded that there are chemical changes in the spinal cord terminals of fine afferents after local peripheral capsaicin.

The node of Ranvier in early Wallerian degeneration: a freeze-fracture study

Using the freeze-fracture technique, the sciatic nerve of the rat and rabbit was examined distally at 24 h after crush, with particular reference to the node of Ranvier and paranode. The paranodes, in the majority of myelinated fibres, showed a loss of the cytoplasmic circumferential bands and longitudinal columns and their associated membrane pores which characterise the normal Schwann cell surface. Axonal changes consisting of accumulations of axoplasmic organelles occurred at both the node and paranode. At the nodes large intramembraneous particles in the axolemma (E face) appeared unchanged. Nodal Schwann cell microvilli and paranodal myelin terminal loops were generally unaffected. The findings are discussed in terms of the decrease in amplitude of the action potential which occurs in early Wallerian degeneration.