THE FINE STRUCTURAL LOCALIZATION OF NUCLEOSIDE PHOSPHATASE ACTIVITY IN THE BLOOD-BRAIN BARRIER

Tissue Non-specific Alkaline Phosphatase (TNAP) in Vessels of the Brain

The microvessels of the brain represent around 3-4 % of the brain compartment but constitute the most important length (400 miles) and surface of exchange (20 m(2)) between the blood and the parenchyma of brain. Under influence of surrounding tissues, the brain microvessel endothelium expresses a specific phenotype that regulates and restricts the entry of compounds and cells from blood to brain, and defined the so-called blood-brain barrier (BBB). Evidences that alkaline phosphatase (AP) is a characteristic feature of the BBB phenotype that allows differentiating capillary endothelial cells from brain to those of the periphery have rapidly emerge. Thenceforth, AP has been rapidly used as a biomarker of the blood-brain barrier phenotype. In fact, brain capillary endothelial cells (BCECs) express exclusively tissue non-specific alkaline phosphatase (TNAP). There are several lines of evidence in favour of an important role for TNAP in brain function. TNAP is thought to be responsible for the control of transport of some compounds across the plasma membrane of the BCECs. Here, we report that levamisole-mediated inhibition of TNAP provokes an increase of the permeability to Lucifer Yellow of the endothelial monolayer. Moreover, we illustrate the disruption of the cytoskeleton organization. Interestingly, all observed effects were reversible 24 h after levamisole removal and correlated with the return of a full activity of the TNAP. This reversible effect remains to be studied in details to evaluate the potentiality of a levamisole treatment to enhance the entry of drugs in the brain parenchyma.

Synergism between different transport systems stimulates the uptake of neutral amino acids by isolated brain microvessels

The polar long-chain amino acids glutamine and methionine can be transported across the endothelial cells of brain microvessels either by an L-system which operates by a facilitated diffusion, exchanging mechanism, or by a concentrating, energy-dependent A-system. The presence of glutamine and/or of methionine can induce a synergism between the two transport systems which results, by a transstimulation mechanism, in a net increased uptake of neutral hydrophobic aminoacids. The methionine analog S-methylthiocysteine, which is the mixed disulfide resulting from the combination of methanethiol with cysteine, behaves similarly to methionine in stimulating the uptake of neutral hydrophobic amino acids. The same transstimulating effect can even be obtained in collagenase-treated, A-system-deprived microvessels by inducing the direct formation of S-methylthiocysteine within the cytoplasmic compartment of the endothelial cells.

Effects of different oxidizing agents on neutral amino acid transport systems in isolated bovine brain microvessels

Using isolated bovine brain microvessels as an in vitro model of the blood-brain barrier (BBB) we have evaluated the role of free radical generating solutions on some amino acid transport systems operating on the endothelial cell membrane. Fe(2+)/ascorbate, phenylhydrazine and CuSO(4) did not affect any of the transport system tested, while exposure of bovine brain microvessels to tert-butylhydroperoxide (t-BHP) caused a reduced capacity to take up small neutral amino acids via the Na(+)-dependent A-system. The presence of glucose during t-BHP treatment did not prevent this inhibition, which was partially counteracted when the isolated microvessels were incubated with 5mM inosine before the oxidative stress. Incubation of the isolated capillaries with 5mM dithiothreitol, after exposure to t-BHP, resulted in a 50% recovery of the alpha-methylaminoisobutyrate (MeAIB) uptake by the A-system. Treatment with t-BHP, which had no effect on the L-system of neutral amino acid transport, caused a significant decrease of the intracellular levels of ATP, of glutathione (GSH), and of gamma-glutamyltranspeptidase (GGT) activity, while no significant modification of hexokinase (HK) or of alkaline phosphatase (ALKP) activities were observed. Oxidative damage of the BBB appears therefore to impair essentially the metabolic pathways which ensure the energy requirement for the endothelial cells, thus inhibiting the energy-dependent amino acid transport system "A".

Age-related changes in the blood-brain barrier

Aging of the cerebral microcirculation results in significant alteration in the blood-brain barrier (BBB). The barrier function appears to remain intact in older animals, although it may be more susceptible to disruption by external factors (hypertension) and drugs (haloperidol). While overall transport processes do not change with age, aging animals and humans have altered BBB function of select carrier mediated transport systems including the transport of choline, glucose, butyrate and triiodothyronine. These age-related changes are the result of either alteration in the carrier molecules or the physiochemical properties of the cerebral microvessels. At the present time, it is not known whether changes in the BBB contribute to the age-related neurodegenerative diseases or are merely epiphenomena of aging.

Distribution of activity of alkaline phosphatase and Mg-dependent adenosine triphosphatase in the cranial dura mater-arachnoid interface zone of the rat

The distribution of the activity of alkaline phosphatase and Mg-dependent adenosine triphosphatase was studied in the encephalic dura mater-arachnoid borderline (interface) zone of albino Wistar rats. Intense clustering of electron-dense granules that indicated alkaline phosphatase activity was observed in the inner dural cells, the neurothelial cells, the outermost row of the outer arachnoidal cells and in the intercellular cleft between the latter two (the so-called electron-dense band). The remainder of the outer arachnoidal cells contained almost no reaction product. Mg-adenosine triphosphatase activity was distributed differently; a lack of reaction product was observed not only in the outer arachnoidal cells, but also in the zone occupied by the electron-dense band. The data confirm histochemically the barrier properties of the dura mater-arachnoid interface zone.

Isolated brain microvessels as in vitro equivalents of the blood-brain barrier: selective removal by collagenase of the A-system of neutral amino acid transport

On treatment with collagenase, brain microvessels, together with several protein components, lose some enzymatic activities such as alkaline phosphatase and gamma-glutamyltranspeptidase, whereas no change occurs in the activities of 5'-nucleotidase and glutamine synthetase. The energy-requiring "A-system" of polar neutral amino acid transport is also severely inactivated, whereas the L-system for the facilitated exchange of branched chain and aromatic amino acids is preserved. In the collagenase-digested microvessels, this leads to loss of the transtimulation effect of glutamine on the transport of large neutral amino acids, because such transtimulation is due to a cooperation between the A- and L-systems. By contrast, NH4+ maintains (and even enhances) its ability to stimulate the L-system of amino acid transport, presumably through glutamine synthesis within the endothelial cells.

Intraendothelial accumulation of calcium in the hippocampus and thalamus of rats after systemic kainic acid administration

The accumulation of calcium in the hippocampal and thalamic vascular endothelium and the perivascular space was detected histochemically by means of the pyroantimonate technique 30, 60 and 120 min after systemic kainic acid administration. An increased number of calcium pyroantimonate deposits was found in the endothelial mitochondria 60 min after kainate injection. The mitochondria were swollen at this time and vacuoles containing deposits were observed. After 120 min a pronounced perivascular glial swelling was conspicuous, besides the numerous endothelial mitochondrial deposits. The swollen glial processes contained a large number of pyroantimonate deposits. It seems likely that the transendothelial calcium transport processes are accompanied by intraendothelial calcium accumulation and mitochondrial calcium sequestration.

Phosphatase activities in human glioma cells as revealed by light and electron microscopy--a preliminary study

Alkaline phosphatase (ALPase) and Mg2+-activated ATPase (Mg2+-ATPase) activities were demonstrated in human brain tumors by light and electron microscopy. Four cases of glioma, i.e., two cases of astrocytoma, grade II, and two cases of glioblastoma, were used as materials. At the light microscopic level, Mg2+-ATPase activity was observed in the capillary wall and glial cells of both astrocytoma and glioblastoma. ALPase activity was restricted to the capillary wall. Its activity was stronger in glioblastoma than in astrocytoma. By electron microscopy, in astrocytoma, reaction product representing Mg2+-ATPase activity was distributed in the plasma membranes of endothelial cells and pericytes. Activity was primarily localized at the abluminal surface of endothelial cells and the surface of pericytes facing endothelium. The plasma membrane of glial cells was also positive. ALPase activity revealed essentially the same distribution pattern in blood vessels as above. In glioblastoma, on the other hand, activities of both phosphatases were markedly positive on the luminal surface of the plasma membrane of endothelial cells. They were much stronger than those along the abluminal endothelial surface. Phosphatase activities in brain tumor appear to change in localization pattern in association with glioma malignancy. This might reflect a functional aspect of changes in blood-brain barrier in glioma.

Adenylate cyclase in the microvessels of the rat brain. A histochemical study with light and electron microscopy

The presence of adenylate cyclase (AC) in the microvessels of the rat brain was studied by a new histochemical method for light and electron microscopy. The method is based on the precipitation of strontium and the subsequent conversion of the formed strontium salt into lead phosphate. Isoproterenol and 5-guanylylimidodiphosphate were used as enzyme activators. In the light microscope, the final reaction product was detected in the choroid plexus as well as in the walls of the microvessels in the brain parenchyma. In the electron microscope, both the luminal and abluminal endothelial membrane as well as the basal lamina of the parenchymal microvessels displayed reaction product. The observations demonstrate that isoproterenol-stimulated AC is located in the endothelium of the rat brain microvessels and suggest that the enzyme may play a role in the receptor-mediated regulation of endothelial functions.

Local cerebral glucose metabolism in rats with chronic portacaval shunts

Local cerebral glucose utilization was measured with the autoradiographic [14C]deoxyglucose method in rats at 2 days and 1, 4, 8, and 12 weeks after the construction of a portacaval shunt, and in weight-matched controls. Local glucose utilization in brain was altered in shunted animals, but the magnitude and direction of the changes differed among 36 neuroanatomical structures, depending upon the length of time that the animals had been shunted. In rats shunted for 4 weeks or less, glucose utilization did not differ from control (30 of 36 structures) or was decreased (6 structures). The largest decreases of glucose utilization, noted at 1 week, occurred in the parietal (-25%) and frontal cortices (-28%) and subcortical white matter (-50%. In rats shunted for 8 weeks, however, glucose utilization was higher than control in many brain structures (13 of 36), and after 12 weeks it was higher than control in most structures (25 of 36). Only the parietal cortex did not follow this trend; it exhibited a decreased rate of glucose utilization in rats shunted for 8 weeks (-21%) that normalized in animals shunted for 12 weeks. Portal-systemic shunting of blood increased arterial blood ammonia concentrations to twice the control value of 85 +/- 3 microM in animals shunted for 1 week, and to approximately 2.5 times control in animals shunted for 4-12 weeks. Kety-Schmidt measurements of cerebral blood flow and arterial-venous (torcular) differences for ammonia across the brains of control rats and rats with portacaval shunts for 8 weeks revealed an increased cerebral uptake of ammonia in the shunted animals. The late-developing morphological changes known to occur in astrocytes and the delayed increases in local glucose consumption in rats with portacaval shunts may be related, and represent an adaptive response to sustained hyperammonemia.

The blood-brain barrier following experimental subarachnoid hemorrhage. Part 2: Response to mercuric chloride infusion

Under controlled physiological conditions, fresh blood was injected into the cisterna magna of 10 adult cats to produce subarachnoid hemorrhage (SAH). Damage to the blood-brain barrier (BBB) was induced 30 minutes after SAH by the intracarotid injection of a 6 x 10(-5)M solution of mercuric chloride (HgCl2). A control series of five cats received the same injection of HgCl2. Intravenously injected Evans blue dye was used to indicate areas of BBB damage. The lesions were confirmed by fluorescence microscopy. All control animals showed BBB damage in the hemisphere injected with HgCl2. Of the animals in the test group with SAH, 90% were free from lesions. When lesions were present, the distribution differed from that in the control group. These results bear a similarity to the reported absence of HgCl2 lesions during the acute stages after total cerebral ischemia. This suggests that the cellular components of the BBB participate in a general metabolic inhibition following SAH.

Cytochemical study on enzyme activity associated with cerebrospinal fluid secretion in the choroid plexus and ventricular ependyma

Normal adult albino and Sprague-Dawley rats, under intraperitoneal Nembutal anesthesia, were used to demonstrate enzymatic activity in the choroid plexus and ventricular ependyma. The brain tissues were perfused or immersed with cold 2% glutaraldehyde and 8% sucrose in 0.1 M cacodylate buffer (pH 7.2-7.4) for 30 min and washed overnight in the same buffer solution., The choroid plexus (lateral and fourth ventricles) and ventricular ependyma (lateral ventricle) were trimmed from the fixed and washed brain tissues, which were frozen and sectioned. For histo- and cyto-chemical study, the sections were immersed in the following incubation media; for Na+, K+-ATPase (ouabain-sensitive, K+-dependent, p-nitrophenylphosphatase: p-NPPase) according to the one-step method of Mayahara et al. (1978): for Mg2+- ATPase, Wachstein-Meisel's incubation medium (1957); for adenylate cyclase (AC), following Araki and Saito's lead citrate method (1979). The cytochemical findings gave the following results. In the choroid plexus, the ouabain-sensitive electron-dense reaction products of NA+, K+-ATPase (p-NPPase) were strongly positive in the microvilli and along the inner surface of microvilli, without showing any Mg2+-ATPase and AC activities, and all three enzymatic activities were positive along the basal plasmalemmas and negative along the lateral and apical (not including microvilli) plasmalemmas. In the ventricular ependyma, Na+,K+-ATPase (P-NPPase) activity was not found, and the reaction product of AC was observed on the apical plasmalemmas and those of Mg2+-ATPase along the basal plasmalemmas. These cytochemical findings are helpful in understanding the regulation of cerebrospinal fluid production through Na+, K+-ATPase (p-NPPase) and cyclic AMP (AC).

Biosynthesis of prostacyclin in rat cerebral microvessels and the choroid plexus

Microvessels, predominantly capillaries, were isolated from rat cerebrum by a modification of published procedures. The morphology and purity of the preparations were monitored by light and electron microscopy and by enrichment in alkaline phosphatase, gamma-glutamyl transpeptidase, and prostacyclin, synthetase. A reversed-phase high-pressure liquid chromatographic method was used in the purification of prostaglandins after extraction from aqueous incubation solutions. Prostacyclin synthesis in brain is localized in cerebral blood vessels and capillaries. The endogenous biosynthetic capacity of the isolated cerebral capillary fractions for prostacyclin, measured as its chemically stable breakdown product, 6-keto-prostaglandin F1 alpha, was 11 ng/mg protein/10 min. Choroid plexus and intact surface vessels synthesized 6-keto-prostaglandin F1 alpha at 37 and 35 ng/mg protein/10 min, respectively. The prostacyclin-synthesizing enzyme of the cerebral capillaries also converted the exogenously added prostaglandin endoperoxides to 6-keto-prostaglandin F1 alpha. Comparison of the synthesis of prostaglandins 6-keto-F1 alpha, E2, and F2 alpha showed that 6-keto-prostaglandin F1 alpha was the major prostaglandin formed in the microvessels, in the larger surface vessels, and in the choroid plexus. Prostaglandin D2 was not detected. Prostacyclin synthesis by the cerebral vasculature is similar to that in other blood vessels and cultured human endothelial cells. Possible physiological roles or prostacyclin in the cerebral microvasculature are discussed with special regard to the autoregulation of cerebral blood flow.

The enzyme cytochemistry of the intracellular organelles in the rat choroid plexus epithelial cell

Electron microscopic cytochemical studies on the rat choroid plexus epithelium have revealed enzymatic sites for the activities of acid phosphatase, glucose-6-phosphatase and thiamine pyrophosphatase on different organelles. Only the activity of acid phosphatase has been previously described. Acid phosphatase, glucose-6-phosphatase and thiamine pyrophosphatase were respectively situated mainly in the lysosomes, in the endoplasmic reticulum an nuclear envelope, and in the Golgi complex. These three enzymes can thus be considered as marker enzymes for their respective organelles in the choroid plexus epithelial cells as well as in other tissue cells. The possible function of these enzymes in the choroid plexus epithelial cells is also briefly discussed.

Cerebral blood flow and ultrastructural studies of the brain capillaries in the early state of hypertension in SHR+

The 133Xe single bolus injection technique was used for measuring the cerebral blood flow (CBF) in spontaneously hypertensive (SHR) and normotensive rats. Their age was from 5 to 25 weeks and corresponded to the time before the initiation of the arterial blood pressure (BP) increase to the constant phase of hypertension. CBF in SHR 5 and 10 weeks of age was significantly higher than in all other groups being 155 +/- 7.2 and 151 +/- 9.3 ml/min/100 g, respectively. At 15 weeks of age the localization of the reactive products of ATP and CTP became scanty and spread over the endothelial cytoplasm. Further, at 25 weeks an enlarged number of empty pinocytic vesicles and astrocytic foot processes filled with flocculent electron dense material was found. These data suggest that the initial high CBF preceding the increase of BP can initiate the irreversible disorder of endothelial cells in brain capillaries.

Ultrastructural cytochemical studies of cerebral microvasculature in scrapie infected mice

Alkaline phosphatase, 5'-nucleotidase nucleoside diphosphatase and thiamine pyrophosphatase activities were studied by cytochemical method applied to electron microscopy of brain microvasculature in normal and scrapie infected mice. In control mice, the major location of all phosphatases studied was the luminal plasma membrane of the endothelial cells. In scrapie infected mice, changes in activity and distribution of the above mentioned phosphatases manifested themselves in the appearance of the reaction product on the abluminal side of the vessel wall. Our data presents evidence that following scrapie infection, these enzymes change their specific localization along the endothelial cell membranes. These enzymatic changes may serve as useful indicators of some alterations in the mammalian blood-brain barrier following infection by scrapie agent in the mouse.

Isolation and characterization of brain endothelial cells: morphology and enzyme activity

Microvessels were isolated from rat brain using a double collagenase treatment which removed the endothelial basement membranes. The isolate was characterized by intact luminal and abluminal membranes and an absence of pericytes and astrocyte membranes. Minimal contamination by 5'-nucleotidase, an enzyme believed exclusively localized within the plasma membranes of neuroglia, established the purity of the isolated microvessels. Enrichment of alkaline phosphatase and gamma-glutamyl transpeptidase activity in microvessel preparations supports the endothelial localization of these enzymes.

Brain sodium uptake after choroid plexectomy

When in a (control) series of rats brain sodium uptake (BSU) -- as calculated from estimated radioactivities of brain and blood following the injection of [24Na]sodium chloride 30 min and [22Na]sodium chloride one minute before killing -- was plotted against average arterial blood pressure of the animals during the 30 min survival period, a linear dependence of BSU on blood pressure was observed. In another series of animals, in which the choroid plexuses of the fourth and both lateral ventricles had been removed, the regression line showed the same slope, but a significantly lower intercept. On the other hand, a series of rats which had been subjected to a sham operation without removal of the choroid plexuses, showed a regression line not different from the control series. The results indicate that probably the blood pressure independent component of brain sodium uptake, as represented by the intercept of the regression line, for the greater part constitutes cerebrospinal fluid secretion by the choroid plexus.

Mucosaccharide histochemistry and histoenzymorphologic observations on the epidermis of Ariosoma balearicum de la Roche (Anguilliformes, Pisces)

The epidermis of Ariosoma balearicum consists of three layers - the basal layer, the middle layer and the outer layer. In between the basal cells are found clusters of small lymphocytes which show a moderat acid phosphatase activity. The middle layer and the outer layer are composed of three types of cells - the polygonal cells, the mucous cells and the club cells. The mucosubstances within the mucous cells exhibit the properties of neuraminic acid containing mucosaccharides with vicinal hydroxyl, sulfate esters and carboxyl groupings. The superficial cells of the outermost layer are capable of secreting sulphated and carboxylated mucosubstances to form an extracellular mucous coating which in fish epidermis seems to be necessary for the accumulation of electrolytes. The club cells are generally provided with one nucleus or double nuclei which appear displaced from the center and constitute the main histological component of the middle layer. The contents of these cells, in addition to a strong protein uptake visualized by the positive reactions exhibited in the coupled tetrazonium and mercuric bromophenol blue preparations, gave positive response to neutral mucosubstances which appear to be a glycoprotein involved in the slime secretions. Alkaline phosphatase and TPP-hydrolysing enzyme which could be detectable in the basal layer and the basement membrane were correlated with the probable role that they could play in the transportation of various chemicals and nutriments through the cell membranes.

Histochemical characteristics of diphosphate nucleoside consumption in cat and rat nervous system

Consumption of diphosphate nucleosides is investigated by histochemical methods in rat and cat nervous system. Results show NDP-ase is effectively in histological sections from animals previously perfused with glutaraldehyde or formaldehyde. Histochemical reaction is increased in presence of Ca++, Mg++, Mn++ or imidazole and inhibited by L-DOPA and Noradrenaline in incubation medium. A comparative study of TPP-ase and NDP-ase activities to dilucidate the identity of both enzymes is described.

Histochemical evaluation of thiamine pyrophosphatase activity during first molar odontogenesis of the neonatal hairless mouse

Localization of thiamine pyrophosphatase activity has been evaluated in the developing first molar of the neonatal hairless mouse. Postnatal animals from parturition to five days of age were decapitated and the severed heads frozen and sectioned in a frontal plane on a cryostat. 14 micron thick sections were fixed and subsequently incubated for thiamine pyrophosphatase activity according to the method of GOLDFISCHER et al. (1971). The tissue was visualized, dehydrated, cleared and mounted. Light microscopy was utilized in evaluating thiamine pyrophosphatase activity. Thiamine pyrophosphatase activity in the first molar of the hairless mouse is presented in tabular form and compared to similar data for the Swiss albino mouse. Enzyme activity increased as the metabolic activities of various cell layers increased. Thus, thiamine pyrophosphatase activity appeared to be related to the degree of differentiation and functional completency of the odontogenic tissues in the hairless mouse.

The fine localization of ATPases in cultures in vitro of chick embryo spinal cord

A histochemical study of the ultrastructural localization of ATPases in cultures of chick embryo spinal cord has been carried out. The localization of Ca2+ and Mg2+ activated membrane ATPases appears similar: both enzyme activities are localized on the outer surfaces of plasma membranes of all kinds of cell present in the cultures, with the exception of the membranes in direct contact with the culture medium. The results are discussed in relation to data concerning the localization and function of ATPases in vivo and in relation to the possible establishment of mechanisms of nutrient uptake and transfer in cultures of nervous tissue.

Studies of excitable membranes. II. A comparison of specializations at neuromuscular junctions and nonjunctional sarcolemmas of mammalian fast and slow twitch muscle fibers

Mammalian fast and slow twitch skeletal muscles are compared by freeze-fracture, thick and thin sectioning, and histochemical techniques using conventional and high voltage electron microscopy. Despite gross morphological differences in endplate structure visualized at relatively low magnifications in this sections, rat extensor digitorum longus (EDL) (fast twitch) and soleus (slow twitch) fibers cannot be distinguished on the basis of size, number, or distribution of molecular specializations of the pre- and postsynaptic junctional membranes exposed by freeze fracturing. Specializations in the cortex of the juxtaneuronal portions of the junctional folds are revealed by high voltage electron stereomicroscopy as a branching, ladder-like filamentous network associated with the putative acetylcholline receptor complexes. These filaments are considered to be involved in restricting the mobility of receptor proteins to the perineuronal aspects of the postynaptic membrane. Although the junctional membranes of both EDL and soleus appear similar, a differential specialization of the secondary synaptic cleft was noted. The extracellular matrix in the bottom of soleus clefts was observed as an ordered system of filamentous "combs," These filamentous arrays have not been detected in EDL junctions. Examination of the extrajunctional sarcolemmas of EDL and soleus reveal additional differences which may be correlated with variations in electrical and contractile properties. For example, particle aggregates termed "square arrays" previously described in the sarcolemmas of some fibers of the rat diaphragm were observed in large numbers in sarcolemmas of EDL fibers but were seldom encountered in soleus fibers. These gross compositional differences in the membranes are discussed in the light of functional differences between fiber types.

Isolation from bovine brain of a fraction containing capillaries and a fraction containing membrane fragments of the choroid plexus

Combined differential and density gradient centrifugation was used for the isolation of a capillary-rich fraction from the cerebral cortex and a brush border containing fraction from the bovine choroid plexus. The activities of gamma-glutamyl transpeptidase and several other marker enzymes were monitored during the fractionation procedure. Electron microscopic examination showed a membrane-rich fraction in the choroid plexus high in the gamma-glutamyl transpeptidase and 5'-nucleotidase activities. From the brain cortex, a capillary-rich fraction was obtained which was high in gamma-glutamyl transpeptidase and alkaline phosphatase activities. A histochemical examination showed gamma-glutamyl transpeptidase activity localized in the capillary walls.

Sialic acid in crude myelin fractions from rat brain

Protein- and lipid-bound sialic acid was assayed in myelin fractions isolated by four different methods from rat brain homogenates. The extent to which non-myelin cellular membranes contaminate these fractions was assessed by electron microscopy and marker-enzyme assays. Small amounts of sialic acid found in the least contaminated myelin fractions are considered to be constituents of axonal and satellite cell plasma membranes known to be present. The data are discussed with reference to the ultrastructural appearance of myelin.

The third circulation revisited

The author reviews modern information concerning the formation, flow and functions of the cerebrospinal fluid. Particular attention is given to the lymphatic-like features of the third circulation and to its importance as an internal milieu for nervous tissue.

Electron microscopic study of ATPase activity in human brain tumors

✓ Ultrastructural localization of ATPase was demonstrated in 15 human brain tumors; ATPase activity in the tumor cell was outside the cell membrane and appeared in varying degrees according to the type of tumor. Nonglial tumors such as meningiomas and chromophobe pituitary adenomas showed more intense enzyme activity than gliomas; malignant tumors such as medulloblastoma and glioblastoma multiforme showed low activity. Blood vessels in the tumor showed poor ATPase activity in both endothelium and basement membrane; the lack of ATPase in the vascular wall may contribute to the breakdown of the blood-brain barrier.