S. U L T R A S T R U C T U R E R E S E A R C H
12, 687-704 (1965)
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T h e U l t r a s t r u c t u r e of the C a p i l l a r y Fenestrae in the A d r e n a l M e d u l l a of the Rot 1 LARS-G. ELFVIN
Department of Zoology, University of California, Los Angeles, California Received December 22, 1964 A single-layered membrane that appears to be structurally highly organized has been observed to bridge capillary fenestrae in the rat adrenal medulla. The membrane measures about 20-30 ~ in thickness and is suspended from the peripheral opaque component of the triple-layered plasma membrane of the endothelial cell. In the central area of the membrane is a dense ring-like structure, the periphery of which contains some highly stained particles and which has a center similar in density to the rest of the membrane. In cross sections the innermost region can be s~en to contain some stained material extending out perpendicularly to the plane of the membrane on both sides of the fenestrae. The diameter of the fenestra is about 500 ~, the overall width of the central region being 150-200 A, and the light center being 50-100 A in diameter. The membrane structure is observed in the fenestrae, where it separates the capillary lumen from the pericapillary space. It is also observed in the walls of endothelial vesicles. The single-layered membrane is interpreted to be chemically different from the plasma membrane of the endothelial cell and to be nonlipid. There is a wealth of physiological evidence supporting the opinion that the capillary endothelium possesses specialized regions at which some of the exchange of material through the walls of the capillaries occurs (13). According to this evidence the permeability characteristics of these regions would clearly differ from those of the rest of the endothelial plasma membrane. The electron microscope has allowed a direct demonstration of specialized structures, so-called fenestrae, in the endothelium of capillaries in a variety of tissues. Most endocrine organs have been shown to contain capillaries in which the fenestrae are closed by a thin membrane. This has been demonstrated in the thyroid (2, 3) and pituitary gland (6) as well as in the cortex of the adrenal gland (24). Similar structures have also been observed in the capillaries of several other organs. z This investigation was supported by Public Health Service Research Grant no. G M 11,742 from the Institute of General Medical Sciences and by National Science Foundation Research Grant no. GB 2379. 4 5 - 6 5 1 8 2 3 J . Ultrastructure Research
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According to a recent study by Rhodin (15), the fenestrae of the glomerular capillaries in the mouse kidney consist of a membranous diaphragm about 60 A thick in which a central thickening about 100 ~ in diameter can be discerned. The thickening was suggested by Rhodin to serve as a reinforcement of the fragile membrane of the fenestrae. The diaphragm of the fenestrae was assumed to serve as an effective barrier to large molecules, but also to facilitate rapid diffusion of small molecules. In contrast to the above-mentioned observations some authors have described the fenestrae as representing true holes in the endothelial cell layer (1, 7, 8, 23). Because of the difficulties encountered in the attempts to secure a generally good fixation of the plasma membrane, the exact relationship between the membrane of the fenestrae and the endothelial plasma membrane has not been established in these earlier studies. This fact, along with the use of material that was not stained with heavy metals other than osmium tetroxide, may probably also account for the difficulty in observing the fenestral membrane at all in many previous investigations. In the present study a single-layered membrane of the fenestrae and its structural relationship to the triple-layered plasma membrane of the capillary endothelium is demonstrated in capillaries of the rat adrenal medulla. Certain other structural features of the fenestral membrane will also be described.
MATERIAL AND METHODS The electron microscopic techniques used in this study for the adrenal medulla of the rat are similar to those described in an earlier paper (4). Most of the analyzed material was taken from animals that had been perfused with glutaraldehyde (11, 19) followed by postfixation of the tissue with osmium tetroxide and embedding in Vestopal W (18). Material fixed by direct immersion in 1% osmium tetroxide solution was also studied. In the lastmentioned case the specimens were treated with 0.5% uranyl acetate for 1 hour before dehydration (10). Section staining with uranyl acetate (22) and lead citrate (14) was used throughout the study. OBSERVATIONS The vascular bed of the adrenal medulla usually displays a dilated structure after perfusion. Intravascular structures are very rarely observed. The size of the blood vessels varies; some reach remarkably large dimensions, others are quite small. Most of the medullary vessels are lined by a single layer of endothelial cells (Fig. 1). Because of the difficulties in establishing which of these vessels are of arterial and which FIG. 1. Survey picture showing part of a capillary in immediate proximity to a chromaffin cell type I (4). The basement membranes between the endothelium and the chromaffin cell have fused to a large extent to form one continuous opaque layer. In the capillary wall several membrane-bridged fenestrae are seen. × 38,000.
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of venous origin, and since the walls of all the vessels having a single endothelial cell layer look structurally alike, all these vessels will be termed capillaries in the present study. The plasma membrane of the capillary endothelium appears in cross sections as a triple-layered structure with an overall thickness of about 80 A (Figs. 2-4). Two dense layers, each about 25 A thick, are separated by a light layer measuring about 30 A in thickness. The plasma membrane usually appears symmetric in material perfused with glutaraldehyde, postfixed with osmium tetroxide, and section-stained with uranyl acetate and lead citrate. Occasionally the cytoplasmic opaque layer appears slightly thicker and denser than the peripheral layer, giving the membrane an asymmetric structure. This is the usual picture of the membrane in glutaraldehydeperfused material embedded in Vestopal without postfixation with osmium tetroxide, as well as in material fixed by immersion in osmium tetroxide. In certain areas of the capillaries, the endothelial cytoplasm decreases in thickness. In such cases the plasma membranes facing the capillary lumen and the pericapillary connective tissue space are separated by a cytoplasmic sheath which measures only about 300-700 A across. In these thin cytoplasmic sheaths are found the structures that have been termed the fenestrae or fenestrations of capillary endothelium (8). The fenestrae can be identified in cross sections of the capillary wall as regions where no endothelial cytoplasm occurs and where the capillary lumen is separated from the extracapillary space only by an opaque single-layered membranous structure (Figs. 2--4). These regions appear spaced at random with more or less large portions of cytoplasmic sheaths in between. Usually they seem to be concentrated into small groups where the spacing of the fenestrae from center to center can be as little as about 0.15 #. These groups are separated from other similar fenestrae aggregates by larger cytoplasmic layers that usually also are thicker than the thin sheaths mentioned above. The opaque single-layered membrane of the fenestrae is 20-30 A thick. It is directly continuous with the peripheral opaque component of the triplelayered plasma membrane covering the capillary endothelium (Fig. 3). The total width of the fenestrae is very constant and averages about 500 A as measured from the outer surfaces of the surrounding triple-layered plasma membrane. A central structure is very often observed in the membrane of the fenestrae. In cross sections it appears to consist of two very dense patches in the membrane
FIG. 2. Longitudinal section of a capillary with a fenestrated area in the endothelium seen to the right. Several fenestrae with single-layered membranes separate the capillary lumen from the pericapillary space. Endothelial vesicles with single-layered membranes in their wails are also seen. In some of the single-layered membranes the central portion appears to contain two densely stained particles separated by a light region (arrows). The attenuated endothelial cytoplasm is bounded by a triple-layered plasma membrane, x 110,000.
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separated by a slightly tess opaque central area (Figs. 2 and 4). The thick membrane patches are about 50 A in diameter and seem to have a somewhat angular shape. The intermediate portion of the membrane is about 50-100 ~ wide and is filled with some material of a density similar to that of the rest of the membrane. This material very often extends for a short distance perpendicular to the plane of the fenestrae on both sides and particularly often is observed at the pericapillary side of the endothelium (Fig. 4). Occasionally no clear-cut differentiations of the individual central structures of the fenestrae can be demonstrated, and then only a central thickened area is observed. In other places the fenestrae appear to consist of a single-layered membrane without any indications of a central differentiated portion (Fig. 3). In areas where the capillary endothelium was oriented more or less parallel to the plane of the section, face-on views of the fenestrae could be demonstrated (Figs. 5-7). In sections which apparently cut through the center of the fenestrae and therefore could be assumed to contain the single-layered membrane of the fenestrae, the surrounding triple-layered plasma membrane of the endothelium could be observed (Figs. 6 and 7). However, the outline of the plasma membrane was often obscured by the adjacent superimposed cytoplasm of the endothelial cell, and the cytoplasmic component particularly often appeared thicker and denser than the peripheral component. The shape of the fenestrae as seen in these sections was almost completely circular with a mean diameter of about 500 A as measured from the outer surfaces of the plasma membrane. This dimension is identical to the width of the fenestrae as measured in cross sections. In the center of the circular fenestrae a dense region can be observed surrounded by a homogeneous light area (Fig. 5). In high-resolution micrographs, the central dense part appears ring-shaped or as an irregularly outlined structure that sometimes is slightly less well stained in the middle than in the outer portion. I n the outer portion of the central structure, some densely stained patches occasionally are discernible (Fig. 7). The overall diameter of the central structure is about 150-200 A, the central light area being about 50-100 • wide. Occasionally the central ringstructure has been observed to connect at two opposite sides with the rim of the fenestrae by means of a dense bar-like structure about 20 A wide (Fig. 6). Another characteristic and frequently occurring feature of the endothelial cell are
FIG. 3. Cross section through a capillary wall containing several fenestrae. In the lower part of the picture are seen two endothelial vesicles facing the capillary lumen as well as the pericapillary space with a single-layered membrane. The relationship between the single-layered membrane of the fenestrae and the peripheral component of the triple-layered plasma membrane is clearly demonstrated in this micrograph. The cytoplasmic process to the left (1) is an extension of a chromaffin cell. x 190,000.
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round to oval vesicular structures that in part are closed off by the same type of single-layered membrane as that of the fenestrae (Fig. 2). The vesicles, which have a maximum diameter of about 800-900 A, are bounded in the major part of their circumference by a triple-layered membrane, which is continuous with the plasma membrane of the endothelial cell. The mergence of the vesicular membrane with the plasma membrane occurs at the free surfaces of the capillary wall. At these points the single-layered membrane is suspended from the peripheral surface of the triple-layered membrane. It appears to seal off the vesicular structure from either the capillary lumen or the pericapillary space, depending upon at which side of the endothelium the vesicle is located. In other cases vesicles are observed which are closed off both from the capillary lumen and the pericapillary space by single-layered membranes of the above-described type (Figs. 2 and 3). Sometimes two vesicles can be seen to border one another, the boundary being a single-layered membrane structure of the above-described type (Fig. 6). The vesicles usually appear completely empty. Very seldom plasma membrane enfoldings can be seen on either side of the endothelium. They have a diameter that is similar to that of the vesicles. Fig. 8a is a schematic representation of a capillary, depicting the various regions in which endothelial fenestrae can appear. Fig. 8b shows the dimensions and appearance of a single-layered membrane of a fenestra. The endothelial cytoplasm in these areas very often contains vesicles of the same size as those described above, which in single sections appear completely surrounded by a triple-layered membrane. It seems to be impossible in all these cases to determine whether the vesicular membrane at a level of the vesicle other than that covered by the section is single-layered and thus whether it faces directly onto the environment outside the endothelium. However, many of the vesicles when examined in serialsectioned material appear to be totally enclosed by the cytoplasm and to be surrounded entirely by a triple-layered membrane of the same structure as the plasma membrane. The pericapillary surface of the endothelium is covered by a basement membrane that consists of an opaque layer about 400 A thick. This layer is usually granular in structure, but sometimes also appears to contain some filamentous material (Fig. 4). it is separated from the plasma membrane of the endothelium by a light space
FIG. 4. Cross section through two fenestrae (arrows) in a capillary wall separating the capillary lumen to the right from the pericapillary space to the left. The single-layered membranes of the fenestrae contain a central portion in which two densely stained patches can be seen, separated by less dense material. This material, which is denser than the background material, extends for a short distance on both sides of the fenestrae, perpendicularly to the plane of the single-layered membrane. The basement membrane (BM) between the endothelium and the chromaffin cell (1) to the left consists of granular, and in some places filamentous, opaque material. ~ 290,000.
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about 300-400 A wide. In the areas where the fenestrae occur, the opaque basement membrane layer is continuous and the light space appears to be slightly wider than in other regions. In certain places, where the parenchymal cells of the adrenal medulla are close to the capillary wall, a fusion of the basement membranes covering the medullary cells and the endothelial cells is frequently found. The total distance between the fenestrae and the plasma membrane of the medullary cells is in these cases usually about 1200-1500 A. It seems important to emphasize that all the above-described features of the endothelial fenestrae have been observed also in well preserved areas of material fixed by direct immersion in an osmium tetroxide solution.
DISCUSSION Evidence for the bridging of the fenestrae of capillary endothelial cells by a thin membrane has been presented in a variety of organs and was recently extended to the glomerular capillary endothelium of the kidney in a study by Rhodin (15). The present study shows that the thin membrane of the fenestrae in capillaries of the adrenal medulla of the rat is single-layered and suspended from the peripheral opaque component of the triple-layered plasma membrane of the endothelium. The electron microscopical technique used in the present investigation has permitted a resolution that is sufficiently high to allow measurement of the thickness of the single-layered membrane with a high degree of precision and to exclude the possibility that the fenestral membrane represents thinner regions of a triple-layered membrane. The reliability of the observed membrane pattern, furthermore, is strengthened by the fact that the differences between the triple-layered plasma membrane and the fenestral membrane are observed and can be measured in the same electron micrographs. The highly organized structure of the single-layered membrane of the fenestrae makes it highly probable that these membranes are not artifacts due to precipitation of proteins across holes in the endothelial cells. The assumption that the fenestrae are true openings in the endothelium and that the observable thin diaphragm might represent a segment of the rim of the pore that is included in the section, as proposed by Wissig (23), seems highly improbable for the same reason. The thickness of the single-layered membrane is about 20-30 A, which is identical to that of the peripheral opaque layer of the plasma membrane. The triple-layered FI6. 5. Face-on view of fenestrae in a capillary wall. The fenestrae are circular and appear lightly stained as compared to the surrounding dense endothelial cytoplasm (E). A denser region can usually be observed in the middle of the fenestrae. This region appears in some places to be ringshaped with a denser periphery surrounding a less dense central area (arrows). A chromaffin cell type I to the left is closely associated with the capillary, x 62,000.
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structure of the plasma membrane as observed in osmium-fixed material has been assumed to indicate the location of a lipid double layer sandwiched between layers of proteins (21) as proposed by Robertson (16) for the triple-layered membrane structure observed in material fixed with potassium permanganate. The stained layers would then correspond to the location of the proteins or of the proteins and the polar ends of the lipid molecules. If this interpretation is applied to the plasma membrane of the endothelial cells and if the single-layered fenestral membrane can be considered as derived from and structurally related to the peripheral opaque component of the plasma membrane, we may have, in the case of the fenestral membrane, a single-layered membrane structure that consists entirely of protein. An alternative interpretation with respect to the molecular architecture of the plasma membrane has been that the peripheral opaque membrane component should consist of a layer of polysaccharides (17, 20, 21). Applying this alternative interpretation to the membrane of the fenestrae would indicate that this membrane consists of polysaccharide material or mucoprotein. At the present time such a possibility cannot be excluded. In any case, the single-layered membrane certainly must possess a certain strength as well as flexibility, considering the forces acting upon it in the form of the hydrostatic pressure of the blood. Regarding the central structure found in many of the fenestral membranes, cross sections show that it represents an area where the single-layered membrane is less well defined, and its outline usually appears to be interrupted. At the edge of the seemingly interrupted membrane, highly opaque particles are very often found. Sections that show face-on views of the fenestral membrane indicate the central portion as a circular structure with a middle light area being about 50-100 A in diameter. The interpretation of this structure as being structurally different from the rest of the membrane is supported by the observation of some stained material that fills the light middle region and extends for a short distance on both sides of the fenestrae perpendicular to the plane of the single-layered membrane. A discussion of the function of the fenestrae involves some basic problems with respect to capillary permeability and accordingly also to membrane permeability. According to Pappenheimer (13), physiological studies on capillary permeability show that at least two types of capillary structures appear to be involved in the Fie. 6. Section through an endothelial cell showing several fenestrae in a face-on view. The fenestrae possess a dense central area which sometimes appears as a ring-shaped structure. In one fenestra the ring-shaped region at two opposite sides seems to be connected with the rim of the fenestra by means of a dense bar-like structure (arrow). Two endothelial vesicles are seen which border each other with a single-layered membrane (*). × 110,000. Fro. 7. Face-on views of two endothelial fenestrae in which the central regions are seen to contain some very dense particles (arrows). The particles appear to be located in the periphery of the central regions. × 170,000.
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exchange of materials through the capillary wall. " O n the one hand the permeability characteristics of the plagma membranes which envelop the endothelial cells have to be considered. This type of structure may exhibit a relatively low order of permeability to ions and lipid insoluble molecules and a high order of permeability to oxygen, carbon dioxide and other lipid soluble substances. On the other hand, we have to consider specialized regions through or between endothelial cells, with a relatively high degree of permeability to water, ions and large lipid insoluble molecules" (13). Pappenheimer (13) suggested that pores with a radius of 30-45 A may penetrate the capillary wall and be effective in allowing the passage of water- and lipid-insoluble molecules of various sizes. He stressed the fact, however, that his measurements of capillary permeability had been made on only three types of capillaries, i.e., frog mesenteric capillaries, capillaries in the hind leg of the cat, and glomerular capillaries of mammalian and frog kidney. His theory has been criticized by Bennett et aL (1) as applied to capillaries in general, since they were unable to detect any pore-like structures in electron micrographs of the vascular bed of the legs of mammals. Electron microscopic studies have so far not been able to reveal the true structural significance of the fenestrae. Results obtained on capillaries of the glomerulus (7) and of the ciliary processes and choroid plexus (12) after injection of colloidal tracer particles have given rise to conflicting views with respect to the mechanism for transfer of marker particles across the capillary walls. In the study of the glomerulus (7), where no fenestral diaphragm was observed, most of the particles, ferritin molecules, were retained in the capillary lumen and "within" the fenestrae for a long period of time and only gradually piled up against the basement membrane between the endothelium and the glomerular epithelium. These results were interpreted as indicating that the fenestrae are patent and allow free passage of particles and that the basement membrane is the main filtration barrier. In the ciliary processes and the choroid plexus, on the other hand, no evidence for passage through the numerous membrane-containing fenestrae was said to be found (12). The pathway for the markers--Thorotrast, gold sol, and saccharated iron oxide--was, therefore, assumed to be either via vesicles or vacuoles in the endothelial cytoplasm or through intercellular spaces of the vessel walls, in which regions marker particles could be observed. FtG. 8. (a) Schematic drawing of a cross-sectioned capillary, which illustrates the appearance of the fenestrae separating the capillary lumen from the pericapillary space in the rat adrenal medulla. The various locations in which a single-layered membrane has been observed in the walls of endothelial vesicles are also depicted. (b) Schematic drawing, which presents the dimensions and appearance of the fenestrae as seen in the present material. To the left is seen the single-layered membrane of a fenestra as it appears in cross section; to the right is depicted a face-on view of the fenestra. The appearance of the ring-like central structure is shown. In addition the face-on view also shows the bar-like element sometimes encountered.
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The localization of marker particles, however, does not necessarily indicate passage. Free passage of the particles through the fenestrae, for example, seems unlikely to be associated with a concentration of particles, since such a concentration could indicate an affinity of the markers to the membrane with a binding of the markers to the site of the membrane of the fenestrae. Differences with respect to affinity to the triple-layered plasma membrane and the single-layered membrane of the fenestrae can either be due to chemical differences or to differences in electric charge of the membrane surface. The finding of marker particles in membrane-bounded cytoplasmic vesicles might be an indication of phagocytotic activity of the endothelial ceils and might not necessarily mean that a transport of the markers through the capillary wall takes place by means of these vesicles. It seems justifiable to assume that fenestrae with a membrane consisting entirely of proteins or polysaccharides would facilitate the passage of water and certain water-soluble substances through the capillary wall. The fenestral membranes therefore may very well account for part of the specialized areas anticipated by Pappenheimer (13). Regarding the functional significance of the middle part of the singlelayered membrane, a tentative interpretation is that it represents a region which is less rigid than the rest of the membrane and perhaps more permeable to large lipidinsoluble molecules. The diameter, 50-100 A, of the innermost area corresponds surprisingly well to the pore diameter as suggested by Pappenheimer, i.e., 60-90 A. Vesicles facing the capillary lumen and/or the pericapillary space with a singlelayered membrane are frequently observed in the endothelial cells. These vesicles might represent the structural manifestations of different phases in a dynamic process that constantly takes place in the endothelium in connection with passage of material. It seems reasonable to assume that the single-layered membrane of a vesicle can be formed under certain circumstances if the triple-layered membrane of the vesicle fuses with the plasma membrane of the endothelium. If this occurs on opposing sides of the endothelial wall simultaneously, a vesicular structure with two singlelayered membranes will appear. Similarly the limiting membranes of two adjacent endothelial vesicles might possess the capability of changing structure and forming a single-layered membrane in the event of a fusion. If this is correct, the vesicles demonstrated in the present study would presumably act as a sort of transport vehicle for some of the material entering and passing the endothelial cells, the singlelayered membrane functioning as the main barrier with respect to this material. An alternative interpretation with respect to the function of the vesicles is that they are of the true pinocytotic type. This would indicate that the single-layered membrane of the vesicles represents a transitory structure which might be formed when the plasma membrane closes and opens around material contained in a plasma membrane enfolding. However, the rare appearance of plasma membrane enfoldings
CAPILLARY FENESTRAE IN THE ADRENAL MEDULLA
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in the endothelium, along with the observation of the highly organized structure of the single-layered membrane, are factors that speak against this as being the main function of the vesicles in the adrenal medullary capillaries. In this connection it is interesting that Karrer (9) in a study of capillaries in striated muscles of thoracic and lung veins noticed the rare appearance of fenestrae in this tissue and therefore suggested that they were not permanent but transitory structures formed when a membrane enfolding "breaks through" the whole width of the endothelium. There are no findings at the present time, however, that would support such a hypothesis with respect to the fenestrae in the rat adrenal medulla. Regarding the formation of the single fenestrae in the endothelium, it may be tentatively suggested that the vesicles might form transitory stages in their development. This would mean that one of the single-layered membranes of a vesicle facing both the capillary lumen and the pericapillary space might eventually disappear, leaving only one single-layered membrane in the capillary wall. The development of the fenestrae might presumably also occur after fusion of opposing areas of the endothelial plasma membrane. Bennett et al. (1) have suggested that the capillary endothelial cells may be labile and may change their structural characteristics under the influence of circumstances such as anoxia or under the influence of anesthetic agents or perfusion. It seems highly probable that the fenestrae and the structures related to the fenestrae are not static or permanent structures, as discussed above. However, previous results (2, 3, 6, 15, 24) and the present findings in material fixed by perfusion as well as immergion techniques justify the conclusion that these endothelial structures do exist under normal circumstances. Although several studies have shown capillary fenestrae to be bridged by a membrane, the exact relationship between this membrane and the endothelial plasma membrane has not been established. Ekholm (2) and Ekholm and SjSstrand (3) described the endothelial fenestrae of capillaries in the mouse thyroid gland as being bridged by a membrane having a thickness of about 50/~, whereas the endothelial plasma membrane was 70 ~ thick. Similarly, in the anterior pituitary capillaries, Farquhar (6) found the capillary fenestrae to be closed by a membrane that appeared thinner and more tenuous than the cell membrane, as did Pappas and Tennyson (12) in the ciliary processes and choroid plexus of the rabbit. The diaphragm of the fenestrae in glomerular capillary walls, on the other hand, was reported to have a thickness of about 60/~, which was identical to that of the endothelial plasma membrane, according to Rhodin (15). Rhodin, therefore, concluded that the basic structure of the plasma membrane probably has a tendency to cover without interruption the endothelial cells in the glomerular capillaries. The present study shows that this is not the case in the adrenal medulla of the rat, but that a clear-cut structural modifi4 6 - 651823 J . Ultrastructure Research
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cation of the plasma m e m b r a n e occurs at the site of the fenestrae. Recent studies on capillaries of the thyroid gland of rats and mice have shown similar structural features of the fenestral m e m b r a n e as those described in the present investigation (5). The possibility must be considered that the m e m b r a n e picture observed here might be representative for the structure of capillary fenestrae in general. The author is indebted to Professor Fritiof S. Sj6strand for his interest in this work and for placing the facilities of his laboratory at the author's disposal. Thanks are also due to Miss Solweig Gustafsson for technical assistance, to Mr. Herman Kabe for help with the prints, to Miss Kaye Torvik for help with the drawings, and to Mrs. Marilyn Oreck for linguistic and typing help.
REFERENCES 1. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
BENNETT,H. S., LUFT, J. H. and HAMPTON, J. C., Am. J. Physiol. 196, 38t (1959). EKnOLM, R., Z. Zellforsch. Mikroskop. Anat. 46, 139 (1957). EKHOLM, R. and SJOSTRAND, F. S., or. Ultrastruet. Res. 1, 178 (1957). ELFVIN, L.-G., J. Ultrastruct. Res., 12, 263 (1965). -unpublished observation. FARQUHAR,M. G., Angiology 12, 270 (1961). FARQUHAR,M. G., WISSIG, S. L. and PALAOE, G. E., J. Exptl. Med. 113, 47 (1961). HALL, V. B., Proc. Ann. Conf. Nephr. Syndrome, 5th, p. 1 (1953). K•RRER, H. E., J. Biophys. Biochem. Cytol. 8, 135 (1960). KELLENBERGER,E., RYTER, A. and S~CHAUD, J., J. Biophys. Biochem. Cytol. 4, 671 (1958). PALAY, S. L., McGEE-RUSSELL, S. M., GORDON, S., JR, and GRmLO, M. A., J. Cell Biol. 12, 385 (1962). PAPPAS, G. D. and TENNYSON, V. M., J. Cell Biol. 15, 227 (1962). PAPPENHEIMER,J. R., Physiol. Rev. 33, 387 (1953). REYNOLDS,E. S., Y. Cell Biol. 17, 208 (1963). RnODIN, J. A. G., ,1. Ultrastruct. Res. 6, 171 (1962). ROBERTSON,J. D., ar. Biophys. Biochem. Cytol. 3, 1043 (1957). -Progr. Biophys. Biophys. Chem. 10, 343 (1960). RYTER, A. and KELLENRERGER,E., J. Ultrastruct. Res. 2, 200 (1958). SABATIN~,D. D., BENSCH, K. and BARRNETT,R. J., or. Cell Biol. 17, 19 (1963). SJOSTRAND,'F. S., Radiation Res., Suppl. 2, 349 (1960). SJOSTRAND,F. S. and ELFVIN, L.-G., J. Ultrastruct. Res. 7, 504 (1962). WATSON, M. L., Y. Biophys. Biochem. Cytol. 4, 475 (1958). W~ssm, S. L., J. Biophys. Biochem. Cytol. 7, 419 (1960). ZELANDER,T., J. Ultrastruct. Res., Suppl. 2 (1959).