Functional maturation of the capillary wall in the fetal and neonatal rat heart: permeability characteristics of developing myocardial capillaries

The development of the functional components of the myocardial capillary wall was characterized by time-course studies of transendothelial transport of intravascularly injected probes of graded size from 16 days of gestation in the fetal rat to seven days postpartum. Despite the morphological changes occurring in the developing endothelial cells, the interaction of the probes was similar throughout the developmental period studied. The carbon particles were retained within the capillary lumina without any association with interendothelial junctions or with plasmalemmal vesicles. Carbon also was associated with coated vesicles. In contrast to carbon, ferritin was localized sequentially, over 60 sec of circulation, in plasmalemmal vesicles on the lumenal surface, in the cytoplasm, and on the ablumenal surface of the endothelial cells as well as in the interstitial space. Ferritin was located also in coated pits and vesicles and, after 90 sec of circulation, in multivesicular bodies. Within 30 sec of circulation, reaction product of myoglobin was located in plasmalemmal vesicles, coated vesicles, and transendothelial cell channels. Also within 30 sec, myoglobin partially filled the interendothelial space from the capillary lumina to the level of the tight junction. At all developmental ages studied, the interendothelial cell junctions appeared structurally tight and were impermeable to all of the probes. Once ferritin or myoglobin had reached the ablumenal space, the basal lamina did not appear to restrain the passage of the probes. Plasmalemmal vesicles are the capillary structures which transendothelially transport ferritin and myoglobin in developing myocardial capillaries.     

Plasma membrane-associated vesicles in retinal capillaries of the rat

The structure and function of abluminal vesicles in endothelial cells of rat retinal capillaries was examined using glutaraldehyde-tannic acid fixation and the hemeproteins--horseradish peroxidase, microperoxidase, and lactoperoxidase--as tracers. Numerous vesicles, delimited by a tannic acid-positive membrane, were distributed along the abluminal front. Other vesicles were arranged in clusters and chains or tubule-like structures. Such vesicles were not found in the vicinity of the capillary lumen. When the retina was exposed to hemeproteins, either in vitro or after intravitreal injection, the abluminal vesicles became labeled with tracer reaction product. Apparently "free" vesicles and tubules seen in tangential sections through the basal lamina were also labeled, suggesting that they were in continuity with the plasma membrane in another plane of section. No enzyme reaction product was present in the capillary lumen. Peroxidase-positive multivesicular bodies were observed, suggesting that some protein was endocytosed and directed to lysosomes where it was presumably degraded. The results suggest that abluminal endothelial vesicles represent pits or invaginations of the plasma membrane and, as such, are not involved in the transendothelial transport of protein from the perivascular space to the capillary lumen. Tannic acid treatment revealed a population of similar vesicles associated with the plasma membrane of pericytes. After exposure to hemeproteins, enzyme reaction product was localized in these vesicles and in a few multivesicular bodies. The results suggest that the majority of these vesicles are in continuity with the plasma membrane and are not involved in endocytosis.     

Myocardial capillaries in the fetal and the neonatal rat: a morphometric analysis of the maturing myocardial capillary bed

Developing myocardial capillaries from 16-day-gestation fetus to adult undergo several morphological changes including a thinning of the lateral extensions of the capillary endothelial cells, the formation of a basal lamina, and an increase in the number of plasmalemmal vesicles. A decrease in the extracellular space, an increase in the number of capillaries, and a decrease in the capillary diameter were also observed during the developmental period. In view of these ultrastructural changes, a morphometric analysis was made on the developing myocardial wall to demonstrate specific quantitative changes. The volumes which were occupied by capillary endothelial cells, capillary lumina, extracellular space, and myocardial myocytes within a reference volume of myocardium were measured; and we found that 8% of the reference myocardial volume was occupied by capillary endothelial cells, 85% was occupied by myocardial myocytes, 4% was occupied by capillary lumina, and, except for a significant change in extracellular space at 16 days gestation, 3% was occupied by extracellular space. Each volume ratio was found to be nearly constant throughout the studied period. In contrast to this constancy in the volume ratios, other parameters which were measured demonstrated significant changes during the developmental period studied. These overall changes include a 135% increase in capillary density, a 63% increase in luminal surface area of capillary endothelial cells, a 24% decrease in capillary diameter, a 12% decrease in diffusion distance, and a 35% decrease in the diameter of the erythrocyte population.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endothelial plasmalemmal vesicles have a characteristic striped bipolar surface structure

Capillary endothelial cells have a large population of small (65-80 nm diameter in transmission electron microscopy) vesicles of which a large fraction is associated with the plasmalemma of the luminal and abluminal side. We studied the fine structure and distribution of these plasmalemmal vesicles by high resolution scanning electron microscopy in cultured endothelial cells obtained from bovine adrenal cortical capillaries. Cell monolayers were covered with polylysine-coated silicon chips, split in high potassium buffer, fixed in aldehyde mixtures, and then treated with OsO4 and thiocarbohydrazide. After critical point drying, the specimens were coated with a thin (less than 2 nm) continuous film of chromium. On the cytoplasmic aspect of the dorsal plasmalemmal fragments seen in such specimens, plasmalemmal vesicles appear as uniform vesicular protrusions approximately 70-90 nm in diameter, preferentially concentrated in distinct large fields in which they occur primarily as single units. Individual plasmalemmal vesicles exhibit a striped surface fine structure which consists of ridges approximately 10 nm in diameter, separated by furrows and oriented as meridians, often ending at two poles on opposite sides of the vesicles in a plane parallel to the plasmalemma. This striped surface structure is clearly distinct from the cage structure of coated pits found, at low surface density, on the same specimens. The cytoplasmic aspect of the plasmalemma proper is covered by a fibrillar infrastructure which does not extend over plasmalemmal vesicles but on which the latter appear to be anchored by fine filaments.     

Immunocytochemical localization of heparan sulphate proteoglycan in the rat submandibular gland

Summary Heparan sulphate proteoglycan is the predominant proteoglycan synthesized by the parenchymal cells of the rat submandibular gland. A polyclonal antibody was used to localize this proteoglycan in the adult rat submandibular gland. Localization was accomplished by indirect immunoperoxidase cytochemistry at the light and electron microscopic levels. Heparan sulphate proteoglycan was localized in a continuous, linear pattern in the lamina densa of the basement membrane surrounding all of the epithelial components of the gland as well as the basement membrane of the capillaries and small arterioles in the glandular stroma. In addition, heparan sulphate proteoglycan was seen in vesicles and pits along the acinar cell basal plasmalemma adjacent to the basement membrane and in the endoplasmic reticulum and Golgi apparatus of the acinar cells.     

Distribution of endothelial vesicles in the microvasculature of skeletal muscle and brain cortex of the rat,as demonstrated by tannic acid tracer analysis

Summary Ultrathin serial sectioning and labeling with tannic acid have demonstrated that most plasmalemmal vesicles of rat vascular endothelial cells are not free, but rather are conjoined in three dimensions to form racemose invaginations from the cell surfaces. To elucidate the distribution of vesicles in these microvascular endothelial cells, we have examined terminal arterioles, capillaries and post-capillary venules of rat skeletal muscle and brain cortex, using tannic acid labeling and stereological methods, and have determined the proportions of free vesicles and the vesicles of luminal and abluminal invaginations, as well as the numerical density of vesicles. In the case of capillaries, regional differences in distribution have also been studied. The ratio of free vesicles is 6–7% and is constant throughout the muscle microvasculature. The distribution (proportions and numerical densities) of vesicles in the brain and muscle microvascular endothelial cells shows regionally distinctive patterns. In rapid-frozen, freeze-substituted endothelial cells, there are almost as many fused vesicles as seen in chemically fixed cells. Therefore, aldehydes do not seem to induce membrane fusion, and the distribution of vesicles seems to be preserved by chemical fixation. The structure and function of plasmalemmal vesicles are discussed.     

Specific binding sites for albumin restricted to plasmalemmal vesicles of continuous capillary endothelium: receptor-mediated transcytosis

The interaction of homologous and heterologous albumin-gold complex (Alb-Au) with capillary endothelium was investigated in the mouse lung, heart, and diaphragm. Perfusion of the tracer in situ for from 3 to 35 min was followed by washing with phosphate-buffered saline, fixation by perfusion, and processing for electron microscopy. From the earliest time examined, one and sometimes two rows of densely packed particles bound to some restricted plasma membrane microdomains that appeared as uncoated pits, and to plasmalemmal vesicles open on the luminal front. Morphometric analysis, using various albumin-gold concentrations, showed that the binding is saturable at a very low concentration of the ligand and short exposure. After 5 min, tracer-carrying vesicles appeared on the abluminal front, discharging their content into the subendothelial space. As a function of tracer concentration 1-10% of plasmalemmal vesicles contained Alb-Au particles in fluid phase; from 5 min on, multivesicular bodies were labeled by the tracer. Plasma membrane, coated pits, and coated vesicles were not significantly marked at any time interval. Heparin or high ionic strength did not displace the bound Alb-Au from vesicle membrane. No binding was obtained when Alb-Au was competed in situ with albumin or was injected in vivo. Gold complexes with fibrinogen, fibronectin, glucose oxidase, or polyethyleneglycol did not give a labeling comparable to that of albumin. These results suggest that on the capillary endothelia examined, the Alb-Au is adsorbed on specific binding sites restricted to uncoated pits and plasmalemmal vesicles. The tracer is transported in transcytotic vesicles across endothelium by receptor-mediated transcytosis, and to a lesser extent is taken up by pinocytotic vesicles. The existence of albumin receptors on these continuous capillary endothelia may provide a specific mechanism for the transport of albumin and other molecules carried by this protein.     

The blood-brain barrier in a reptile, Anolis carolinensis

An electron microscopic study was made of the ultrastructure and permeability of the capillaries in the cerebral hemispheres of the lizard, Anolis carolinensis. The brain of Anolis is vascularized by a loop-type pattern consisting exclusively of arteriovenous capillary loops. The ultrastructure of the endothelium and the arrangement of the various layers from the capillary lumen to the central nervous tissue is similar to that of mammals. The endothelial cells form a continuous layer around the lumen and are joined by tight interendothelial junctions. The basal lamina of the endothelium is also continuous and encloses pericyte processes. The cells of the nervous tissue rest directly on the basal lamina of the capillary and are separated from each other by a 200 Å space. Intravenously injected horseradish peroxidase (MW 40,000) and ferritin (MW 500,000) were used to study the permeability of the capillaries. The entry of horseradish peroxidase and ferritin into the intercellular spaces of the brain is restricted by the tightness of the interendothelial junctions. No vesicular transport of either tracer occurs; however, ferritin does enter the endothelial cells in vacuoles. No tracer molecules are present in the basal lamina, pericytes, or nervous tissue. The different responses of the endothelial cell to the tracers used in this study suggest that endocytotic activities of endothelial cells involve different processes. Vacuoles formed by marginal folds, vacuoles formed by endothelial surface projections or deep invaginations of the plasma membrane, 600–800 Å vesicles, and coated vesicles all seem to differ in the nature of the substances which they endocytose.     

Electron microscopic research on the capillary permeability of the primary portal plexus in rats in the prenatal period

Permeability of portal capillaries to intravascularly injected ionic lanthanum, ferritin and horse-radish peroxidase has been examined in rats on the 18th fetal day, and on days 1 and 9 of postnatal life. For several minutes, tracer molecules pass through the capillary wall and reach the median eminence. In the case of immature capillaries, the materials pass freely through the endothelial cells, and to a lesser extent are transferred via occasional plasmalemmal vesicles and fenestrae. As the maturation of capillaries proceeds their permeability via plasmalemmal vesicles and fenestrae increases considerably due to a gradual rise in the number of these structures. The plasmalemma of differentiated endothelial cells becomes impermeable to all the tracers. Only ionic lanthanum appears to penetrate through transendothelial channels and intercellular junctions between adjacent endothelial cells.     

Structural and functional aspects of porcine endometrial capillaries on days 13 and 15 after oestrus or mating.

The permeability of subepithelial capillaries in porcine endometrium was studied during midcycle and early pregnancy. Gilts were slaughtered on Day 13 or Day 15 of the oestrous cycle or pregnancy, 15 min after injection through the ear vein of 11.1 MBq of 125I-labelled human albumin in phosphate-buffered saline. The radioactivity of endometrial strips taken along the mesometrial and antimesometrial aspects of the uterine horn varied on average from 270 to 701 c.p.m./g and no difference (P greater than 0.05) was found between reproductive status, days of slaughter or sampling sites. The majority of the subepithelial capillaries showed ultrastructural evidence of increased vascular permeability, such as marked thinning of the capillary walls, especially on the side proximal to the epithelial basal lamina, multilayering and partial disparition of the endothelial basal lamina and abundant endothelial vesicles. Fenestrated pores were observed, but were rare. There was no obvious difference between reproductive status, days of sampling or sampling sites inside the uterus, suggesting that on Days 13-15 after oestrus the ultrastructural characteristics of porcine endometrial capillaries are little affected by the presence of attaching blastocysts and supporting the results obtained with radioactive albumin. Ferritin injected directly into a uterine artery of one gilt on Day 15 of pregnancy was carried through the capillary wall by endothelial vesicles, showing ultrastructural evidence of increased permeability.     

Vesicular transport in capillary endothelium: does it occur?

A revised picture of the organization of endothelial plasmalemmal vesicles is presented. Three-dimensional reconstructions of endothelial segments from frog mesenteric capillaries and rat heart capillaries based on ultrathin serial sectioning have shown that plasmalemmal vesicles are not true vesicles but parts of an elaborate system of invaginations of the surface membrane. The revised picture probably applies to capillary endothelia in general. The absence of free cytoplasmic vesicles implies that vesicular transport is unlikely to occur. A reinterpretation of previous studies of vesicular transport shows that they are equally compatible with the present view that plasmalemmal vesicles are static elements of invaginations of the endothelial surface membrane.     

The plasmalemmal vesicular system in striated muscle capillaries and in pericytes

By ultrathin serial sectioning of frog mesenteric capillaries it was recently demonstrated that the many apparently free vesicles in electron microscope (EM) sections of endothelial cells may be artefacts due to conventional (500–700 Å thick) sectioning (Frøkjaer-Jensen, 1980). The vesicles were found to be part of two sets of invaginations of the cell surfaces; one set connected to the lumen, the other to the interstitium. The present study extends this view to comprise the vesicle organization in frog striated muscle capillaries. By analysis of the three-dimensional organization of the plasmalemmal vesicles in 21 ultrathin serial sections (120–150 Å) of two muscle capillaries it is demonstrated that less than 1% of the about 70% apparently free vesicles seen in conventional thin sections of the same capillaries in fact represent truly free vesicular units. By analysis of 15 conventional EM cross-sections of capillaries from the frog cutaneous-pectoris muscle containing plasmaproteins in high concentration it is furthermore demonstrated that 48% of the total vesicle population connect to the lumen at the time of fixation. This organization of the vesicular system seems incompatible with the concept that macromolecules are transferred across the capillary wall by vesicular transport or by a series of fusions and fissions between individual cytoplasmic vesicles but is compatible with the notion that macromolecules exchange across capillary walls by means of passive processes such as diffusion and convection through rare ‘large pores’. The study emphasizes that any attempts to classify vesicles in conventional thin sections as ‘luminal’, ‘cytoplasmic’ and ‘abluminal’ is impossible and may lead to erroneous interpretations of vesicle involvement in transcapillary exchange of macromolecules. The rare occurrence of transendothelial channels compared to the number of vesicle invaginations suggests that the main function of the vesicular system relates to functions other than transport.     

Fibronectin and its relation to the basal lamina and to the cell surface in the chicken blastoderm

Summary The ultrastructural distribution of fibronectin immunoreactivity was investigated in the chicken embryo during late gastrulation. Sites of binding of anti-fibronectin antibodies were ascribed to the basal lamina and associated structures, and to the cell surface. The fibronectin-rich basal lamina was resolved into (1) a lamina densa, which appears as a continuous, dense sheet, (2) a lamina lucida, consisting of anchoring cords between lamina densa and epithelial cells, and (3) a lamina intima, closely juxtaposed to the cell surface. Cell-surface labelling was also observed in mesoblast cells, and along the dorsal side of the deep-layer cells. The ventral side of the latter cells was poorly stained in the endophyllic crescent, except in coated pits, and more regularly stained at the level of definitive endoblast. Some structures associated with the basal lamina reacted intensely with anti-fibronectin antibodies. These are (1) the interstitial bodies, which are aggregates of extracellular material, and (2) a kind of fibril or tubule, embedded in a fibronectin matrix and mainly found in the endophyllic crescent. Some intracellular labelling was found in most deep-layer cells, in few epiblast cells, never in mesoblast cells. These results extend previous studies on the localization of fibronectin, and correlate its presence and surface topology with its postulated role in migration of mesoblast cells on the basal lamina which, chemically, constitutes an appropriate substrate.     

Laminin ultrastructural immunolocalization in rat testis during ontogenesis

Summary The synthesis of one of the main glycoproteins of the basement membrane, the laminin, was demonstrated by ultrastructural immunolocalization during rat foetal (16th day to 20th day of gestation) and postnatal development of the testis. The lamina densa, part of seminiferous tubular basement membrane, is labeled uniformly at all studied stages. The lamina lucida is not well defined before the postnatal stages, at which times discrete immunostaining extends from the lamina densa to the adjacent seminiferous epithelial cells (spermatogonia and Sertoli cells). The extracellular matrix around the peritubular cells is not labeled before birth. Intracellular immunostaining was detected as early as the 16th day of gestation in both Sertoli cells and cells around the seminiferous tubules which will transform later into peritubular cells. It was located in rough endoplasmic reticulum (RER) cisternae and secretory vesicles. After 18–20 days of postnatal life, the immunostaining faints progressively. Some positive material is seen in the RER of the gonocytes at all studied stages.Sertoli cells and peritubular cells are the main producing cells of laminin after the 16th of gestation. The laminin secreted by gonocytes may play an important role in adhesion of gonocytes to the lamina densa and adjacent Sertoli cells before their transition from basal compartment to adluminal compartment.     

Laminin ultrastructural immunolocalization in rat testis during ontogenesis

The synthesis of one of the main glycoproteins of the basement membrane, the laminin, was demonstrated by ultrastructural immunolocalization during rat foetal (16th day to 20th day of gestation) and postnatal development of the testis. The lamina densa, part of seminiferous tubular basement membrane, is labeled uniformly at all studied stages. The lamina lucida is not well defined before the postnatal stages, at which times discrete immunostaining extends from the lamina densa to the adjacent seminiferous epithelial cells (spermatogonia and Sertoli cells). The extracellular matrix around the peritubular cells is not labeled before birth. Intracellular immunostaining was detected as early as the 16th day of gestation in both Sertoli cells and cells around the seminiferous tubules which will transform later into peritubular cells. It was located in rough endoplasmic reticulum (RER) cisternae and secretory vesicles. After 18-20 days of postnatal life, the immunostaining faints progressively. Some positive material is seen in the RER of the gonocytes at all studied stages. Sertoli cells and peritubular cells are the main producing cells of laminin after the 16th of gestation. The laminin secreted by gonocytes may play an important role in adhesion of gonocytes to the lamina densa and adjacent Sertoli cells before their transition from basal compartment to adluminal compartment.     

The structure of lymphatic capillaries in lymph formation.

The lymphatic vascular system consists of endothelial lined vessels which begin as blind-end tubes or saccules that are located within the connective tissue areas. This system serves as a one-way drainage apparatus for the removal of diffusible substances as well as plasma proteins that escape the blood capillaries. If permitted to accumulate, these escaped components would deplete the circulatory system of its plasma colloids and disrupt the balance of forces responsible for the control of fluid movement and the exchange of gases and fluids across the blood vascular wall. The lymphatic capillaries are strategically placed and anatomically constructed to permit a continuous and rapid removal of the transient interstitial fluids, plasma proteins, and cells from the interstitium. Structurally the lymphatic capillaries consist of a continuous endothelium that is extremely attenuated over major aspects of its diameter, except in the perinuclear region which bulges into the lumen. These vessels lack a continuous basal lamina and maintain a close relationship with the adjoining interstitium by way of anchoring filaments. The adjacent cells are extensively overlapped and lack adhesion devices in many areas. When electron-opaque tracers are injected intravenously (i.e., horseradish peroxidase and ferritin), subsequent electron microscopic examination of tissues reveals the presence of tracer particles within the interstitium and the lymphatic capillary lumen. These particles gain access into the lymphatic capillaries via two major pathways: 1) the intercellular clefts of patent junctions and 2) plasmalemmal vesicles (pinocytotic vesicles). Another salient feature of the lymphatic endothelial cell includes the presence of numerous cytoplasmic filaments, which are similar in morphology to the actin filaments observed in a variety of cell types. The ultrastructural features of the lymphatic capillaries are discussed in relation to their role in the removal of interstitial fluids and particulate matter, and in the formation of lymph.     

The ultrastructure of the renal corpuscle of a lizard

The few and small renal corpuscles of the lizard Podarcis (= Lacerta) taurica are composed of a tuft of three to four capillaries (glomerulus), Bowman's capsule and mesangium. The thin interdigitated capillary endothelial cells are, in most regions, in contact with the mesangium. In some regions, however, they rest on a bilaminate basement membrane with an electron-dense lamina densa and a less dense lamina rara. Bowman's capsule is composed of visceral and parietal layers. The epithelial cells (podocytes) of the visceral layer bear trabeculae connected to pedicels with microvilli. The pedicels rest on a bilaminate basement membrane which in some regions has a double-layered densa with connecting bands. Generally, this basement membrane is thicker than that of the capillary endothelial cells. The mesangium is composed mostly of irregular satellite cells with large nuclei and cytoplasmic processes, but also has smaller cells with kidney-shaped nuclei and cytoplasmic processes containing microfilaments. The mesangium cells are embedded in a collagenous matrix which extends to invade the area between the epithelial basement membrane and the capillary endothelium. These observations are discussed in relation to the structure and function of vertebrate renal corpuscles with special reference to the mesangium.     

Cytochemical localization of alkaline phosphatase and Na+, K+-ATPase activities in the blood-brain barrier of Rana esculenta

The ultrastructural distribution of alkaline phosphatase and Na+, K+-ATPase on the brain capillaries in Rana esculenta was investigated. Alkaline phosphatase activity appears both on the luminal and abluminal walls of the endothelial capillary cells; Na+, K+-ATPase is, instead, only present on the abluminal side. This different enzymatic distribution indicates that endothelial cells of the brain capillaries are polarized and the luminal and abluminal endothelial membranes are functionally different. The role of these two enzymatic activities is discussed in relation to the blood-brain barrier.     

Impermeability of hagfish cerebral capillaries to horseradish peroxidase

Summary Brain capillaries and their permeability to intravenously injected horseradish peroxidase, HRP, (MW: 40,000) were examined electron-microscopically in an attempt to find a structural explanation for the poorly developed blood-brain barrier in the hagfish, Myxine glutinosa. In particular, it was the aim of this study to examine the role of the numerous endothelial vesicles and tubules in the transport of this tracer between blood and brain. Many of the vesicles and tubules were found to be in continuity with the luminal or abluminal surfaces, but tubules generating channels through the endothelial cells were never observed. The cleft between adjacent endothelial cells was obliterated by punctate junctions. HRP, which was allowed to circulate for up to 35 min, was not found in the basal lamina or in the surrounding brain parenchyma. Few of the luminal vesicles and tubules were marked by the tracer. In the intercellular cleft HRP was stopped by the junctions. It is concluded that the hagfish like other vertebrates has a blood-brain barrier to HRP, and the numerous vesicles and tubules occurring in hagfish brain endothelium are not involved in the transendothelial transport of this macromolecule.     

Brain phagocytes may empty tissue debris into capillaries

The possibility that brain phagocytes may empty remnants of degenerated neurons into capillaries has been studied in frogs. Degeneration of nerve fibers was brought about by transectioning the optic tract, the tectothalamic and tectoisthmic tracts, the postoptic commissure or the radial nerve. To help identification of phagocytozed degenerated neuronal elements, the transected fibers were filled either with horseradish peroxidase (HRP) or cobaltous-lysine complex. The survival times were 3, 4, 7, 27, 47 and 70 days after the application of the markers. The HRP-labeled structures were identified in 60 μm thick sections using diaminobenzidine as chromogen, while cobalt was precipitated in the form of cobaltous sulfide. Small pieces of these sections were further processed for electron microscopy. In each area of the brain and spinal cord investigated, microglial cells and astrocytic processes containing fragments of degenerated neuronal elements could be seen close to capillaries. In some cases a microglial or astrocytic process pierced the capillary basal lamina and seemingly delivered inclusion bodies into the cytoplasm of capillary endothelial cells and pericytes. In the inclusion bodies, which were usually large vesicles, fragments of HRP or cobalt-labeled or unlabeled membranes with a foamy appearance, or condensed myelin lamellae could be observed. These vesicles protruded the luminal membrane of the endothelial cell that was disrupted in some cases suggesting that the content of the inclusion body was discharged into the lumen of the capillary. These results give support to Penfield's hypothesis (1925) that glial cells may empty phagocytozed materials into capillaries.