Ultrastructure of the colon of Locusta migratoria

The ultrastructure of the colon of Locusta migratoria is described. The colon is lined by a thick cuticle that, for the most part, adheres to the underlying epithelium. The cuboid epithelial cells are characterized by moderate invaginations of the apical and, to a lesser extent, basal plasma membranes; the lateral plasma membranes are relatively flat. The bulk of the mitochondria are located in the apical region of the cell and are not particularly associated with any of the plasma membranes. The basal region of the cells contains much rough endoplasmic reticulum, glycogenlike granules, and a predominance of spherical, electron-dense bodies of various sizes. Where muscle fibers make contact with the epithelium, the cells are much reduced; the cytoplasm is usually less electron-dense, and, typically, the nucleus has a thick layer of granular material associated with the inner nuclear membrane. The apical and basal plasma membranes of the reduced epithelial cells contain numerous hemidesmosomes. The apical hemidesmosomes occur in pairs around an extracellular space that contains electron-opaque material. The latter forms tonofibrillae that extend into the endocuticle. Bundles of microtubules are associated with the hemidesmosomes. The tubules traverse the cell from the apical to the basal region. The possible significance of these findings is discussed.     

THE FINE STRUCTURE OF THE TRANSITIONAL EPITHELIUM OF RAT URETER

The fine structure of the transitional epithelium of rat ureter has been studied in thin sections with the electron microscope, including some stained cytochemically to show nucleoside triphosphatase activity. The epithelium is three to four cells deep with cuboidal or columnar basal cells, intermediate cells, and superficial squamous cells. The basal cells are attached by half desmosomes, or attachment plates, on their basal membranes to a basement membrane which separates the epithelium from the lamina propria. Fine extracellular fibres, ca. 100 A in diameter, are to be found in the connective tissue layer immediately below the basement membrane of this epithelium. The plasma membranes of the basal and intermediate cells and the lateral and basal membranes of the squamous cells are deeply interdigitated, and nucleoside triphosphatase activity is associated with them. All the cells have a dense feltwork of tonofilaments which ramify throughout the cytoplasm. The existence of junctional complexes, comprising a zonula occludens, zonula adhaerens, and macula adhaerens or desmosome, between the lateral borders of the squamous cells is reported. It is suggested that this complex is the major obstacle to the free flow of water from the extracellular spaces into the hypertonic urine. The free luminal surface of the squamous cells and many cytoplasmic vesicles in these cells are bounded by an unusually thick plasma membrane. The three leaflets of this unit membrane are asymmetric, with the outer one about twice as thick as the innermost one. The vesicles and the plasma membrane maintain angular conformations which suggest the membrane to be unusually rigid. No nucleoside triphosphatase activity is associated with this membrane. Arguments are presented to support a suggestion that this thick plasma membrane is the morphological site of a passive permeability barrier to water flow across the cells, and that keratin may be included in the membrane structure. The possible origin of the thick plasma membrane in the Golgi complex is discussed. Bodies with heterogeneous contents, including characteristic hexagonally packed stacks of thick membranes, are described. It is suggested that these are "disposal units" for old or surplus thick membrane. A cell type is described, which forms only 0.1 to 0.5 per cent of the total cell population and contains bundles of tubular fibres or crystallites. Their origin and function are not known.     

The fine structure of aleurone cells in the soybean seed coat

Summary The morphology and fine structure of aleurone cells of soybean [Glycine max (L.) Merr.] seed coats were analyzed with transmission electron microscopy for the period of rapid seed fill up to physiological maturity. Thin sections and freeze-fracture replicas were prepared for each stage. The aleurone is a tissue lining the embryo sac and consists of a single layer of cells attached to the aerenchyma of the seed coat proper. During seed fill, aleurone cells contained numerous Golgi-derived vesicles in the basal region of the cytoplasm that were either free or attached to the plasma membrane along the lateral and basal regions of the cell wall. Correspondingly, the Golgi apparatus were well developed with individual dictyosomes having 5 to 8, highly fenestrated stacked cisternae. The degree of fenestration along the periphery of each cisterna increased from the cis to trans region. Rough endoplasmic reticulum (RER) was also abundant, often consisting of up to 30, stacked swollen cisternae which occupied large regions of cytoplasm. Plasmodesmata which connected adjacent aleurone cells was not observed along the dorsal walls of aleurone cells that faced aerenchyma. At physiological maturity, dictyosome cisternae were less fenestrated and had fewer associated secretory vesicles. Stacked lamellae of RER were absent, being replaced by short tubular cisternae and small vesicles. At physiological maturity, the aleurone cells had thick walls, and contained numerous lipid bodies in apposition to the plasma membrane. The cytoplasm appeared densely stained in thin-sections and contained protein bodies and amyloplasts with large starch grains. We conclude that during the period of rapid seed fill aleurone cells produce, package, transport and secrete vesicular contents toward the embryo, that is followed at physiological maturity by the storage of lipid, protein and starch in the same cells. The embryo is the most likely destination for secretory products during the period of rapid seed fill. The fate of the stored food reserves in aleurone cells at physiological maturity may be analogous to that of aleurone tissue of grasses, being utilized during imbibition for processes important to germination.     

Isolation of separate apical,lateral and basal plasma membrane from cells of an insect epithelium. A procedure based on tissue organization and ultrastructure

The tissue used in this study was the midgut of the tobacco hornworm larva, Manduca sexta. The midgut epithelium is a single layer of cells resting on a thin basal lamina and underlying discontinuous muscle layer. The epithelial cells are of two main types, goblet and columnar cells, joined together by the septate junctions characteristic of insect epithelia. From this tissue we were able to isolate four distinct plasma membrane fractions; the lateral membranes, the columnar cell apical membrane, the goblet cell apical membrane and a preparation of basal membranes from both cell types. The lateral membranes were isolated by density gradient centrifugation following gentle homogenization of the midgut hypotonic medium, which caused the cells to rupture at their apical and basal surfaces, releasing long segments of lateral membranes still joined by their septate junctions. For isolation of apical and basal membranes the tissue was disrupted by ultrasound, based on the light microscopic observation that carefully controlled ultrasound can be used to disrupt each cell in layers starting at the apical surface. The top layer contained the columnar cell apical membrane, which consists of microvilli forming a brush border covering the lumenal surface of the epithelium. The second layer contained the goblet cell apical membrane, which is invaginated to form a cavity occupying the apical half of the cell, and the third layer contained the basal membranes. As each layer was stripped off the epithelium it was collected and the plasma membrane purified by differential or density gradient centrifugation. For all four membrane fractions, the isolation procedure was designed to preserve the original structure of the membrane as far as possible. This allowed electron microscopy to be used to follow each step in the isolation procedure, and to identify the constituents of each subcellular preparation. Although developed specifically for M. sexta midgut, these techniques could readily be modified for use on other epithelia.     

The distribution of MUC1, an apical membrane glycoprotein, in mammary epithelial cells at the resolution of the electron microscope: implications for the mechanism of milk secretion

The distribution of the glycoprotein, mucin 1 (MUC1), was determined in lactating guinea-pig mammary tissue at the resolution of the electron microscope. MUC1 was detected on the apical plasma membrane of secretory epithelial cells, the surface of secreted milk-fat globules, the limiting membranes of secretory vesicles containing casein micelles and in small vesicles and tubules in the apical cytoplasm. Some of the small MUC1-containing vesicles were associated with the surfaces of secretory vesicles and fat droplets in the cytoplasm. MUC1 was detected in much lower amounts on basal and lateral plasma membranes. By quantitative immunocytochemistry, the ratio of MUC1 on apical membranes and milk-fat globules to that on secretory vesicle membranes was estimated to be 9.2:1 (density of colloidal gold particles/microm membrane length). The ratio of MUC1 on apical membranes compared with basal/lateral membranes was approximately 99:1. The data are consistent with a mechanism for milk-fat secretion in which lipid globules acquire an envelope of membrane from the apical surface and possibly from small vesicles containing MUC1 in the cytoplasm. During established lactation, secretory vesicle membrane does not appear to contribute substantially to the milk-fat globule membrane, or to give rise in toto to the apical plasma membrane.     

Localization of ryanodine receptor 3 in the sinus endothelial cells of the rat spleen

The ultrastructural localization of ryanodine receptors (RyR) in sinus endothelial cells of the rat spleen was examined by confocal laser scanning and electron microscopy by using isoform-specific antibodies to each of the RyR isoforms. Immunofluorescence microscopy of tissue cryosections revealed RyR3 to be localized, with a strand-like form, in the superficial layer and within the cytoplasm of endothelial cells. Antibodies to RyR1 and RyR2 did not react indicating RyR3 was the predominant isoform. RyR3 was observed over the cortical layer of actin filaments in the apical part and beneath stress fibers in the basal part of the endothelial cells. The distribution of Ca²⁺-storing tubulovesicular-structures within endothelial cells was established by tissue sections treated with osmium ferricyanide selectively to stain the sarcoplasmic reticulum and transverse tubules in muscle cells; electron microscopy revealed densely stained tubulovesicular structures located throughout the sinus endothelial cells and interconnected at various sites. These structures closely apposed the plasma membrane at the apical, lateral, and basal surfaces of the cells and occasionally ran closely parallel to the plasma membrane and near to the mitochondria. Immunogold electron microscopy revealed RyR in the membranes of the nucleus, tubulovesicular structures, and subplasmalemmal cisternae. In the subplasmalemmal cisternae at the apical, lateral, and basal surfaces, RyR was detected on the membranes near to the plasma membrane. Labeling was also present on the membranes of tubulovesicular structures near to caveolae and on the cristae of the mitochondria. Thus, RyR probably participates in Ca²⁺ signal transduction and/or mechanosignal transduction in sinus endothelial cells.This work was supported by Grant-in-Aid for Scientific Research (C), Japan.     

Polarity and reorganization of the endoplasmic reticulum during fertilization and ooplasmic segregation in the ascidian egg

During the first cell cycle of the ascidian egg, two phases of ooplasmic segregation create distinct cytoplasmic domains that are crucial for later development. We recently defined a domain enriched in ER in the vegetal region of Phallusia mammillata eggs. To explore the possible physiological and developmental function of this ER domain, we here investigate its organization and fate by labeling the ER network in vivo with DiIC16(3), and observing its distribution before and after fertilization in the living egg. In unfertilized eggs, the ER-rich vegetal cortex is overlaid by the ER-poor but mitochondria-rich subcortical myoplasm. Fertilization results in striking rearrangements of the ER network. First, ER accumulates at the vegetal-contraction pole as a thick layer between the plasma membrane and the myoplasm. This accompanies the relocation of the myoplasm toward that region during the first phase of ooplasmic segregation. In other parts of the cytoplasm, ER becomes progressively redistributed into ER-rich and ER- poor microdomains. As the sperm aster grows, ER accumulates in its centrosomal area and along its astral rays. During the second phase of ooplasmic segregation, which takes place once meiosis is completed, the concentrated ER domain at the vegetal-contraction pole moves with the sperm aster and the bulk of the myoplasm toward the future posterior side of the embryo. These results show that after fertilization, ER first accumulates in the vegetal area from which repetitive calcium waves are known to originate (Speksnijder, J. E. 1992. Dev. Biol. 153:259-271). This ER domain subsequently colocalizes with the myoplasm to the presumptive primary muscle cell region.     

Localization of axonally transported 125I-wheat germ agglutinin beneath the plasma membrane of chick retinal ganglion cells

The distribution of 125I-wheat germ agglutinin (WGA) transported by axons of chick retinal ganglion cells to layer d of the optic tectum was studied by electron microscopic autoradiography. We found that 52% of the radioactivity was located in axons and axon terminals in the contralateral optic tectum 22 h after intravitreal injection of affinity-purified 125I-WGA. Axons comprised 43% of the volume of layer d. Dendrites, glial cells, and neuron cell bodies contained 20%, 17%, and 3% of the label, whereas these structures comprised 24%, 21%, and 2% of the tissue volume, respectively. We also measured the distances between the autoradiographic silver grains and the plasma membranes of these profiles, and compared observed distributions of grains to theoretical distributions computed for band-shaped sources at various distances from the plasma membranes. This analysis revealed that the radioactive source within axons was distributed in a band of cytoplasm extending in from the plasma membrane a distance of 63 nm. Because WGA is known to bind to specific membrane glycoconjugates, we infer that at least some glycoconjugates may be concentrated within an annular region of cytoplasm just beneath the axonal plasma membrane after axoplasmic transport from the neuron cell body.     

A method to preserve extracellular surfactant-like phospholipids on the luminal surface of rodent gastric mucosa.

Previous results from our laboratory employing the phospholipid-selective cytochemical stain iodoplatinate (IP) suggest that surfactant-like phospholipids (SLPL) are intracellularly contained within rodent gastric mucous cells and are occasionally seen extracellularly within the mucous gel layer. This hydrophobic lipid coating may provide the stomach with a protective water-repellent lining against the corrosive gastric juice in the lumen. Extracellular SLPL are frequently removed during tissue processing for electron microscopy. In this study, we developed a simple method employing an agarose-embedding technique to retain these extracellular SLPL in gastric mucosa excised from rats pre-treated with prostaglandin (to stimulate gastric surfactant/mucus secretion). With the help of polypropylene supporting screens and cassette carriers, thin slices of agarose-embedded gastric mucosa were well preserved and uniformly stained with IP. Extracellular myelin figures were well retained over the interfoveolar surface as well as in the pit region. The IP-reactive substances were seen within or coating the surface of the mucous gel. Our results also indicate that agarose is useful not only for supporting soft tissue while preparing sections with a microslicer but also for preservation of extracellular lipoidal material for electron microscopic observation.     

Function of the cytoskeleton in cells with microridges from the oral epithelium of the carp Cyprinus carpio

Superficial cells of the oral mucosal epithelium in the carp and the cytoskeleton of the epithelial cells are examined by scanning and transmission electron microscopy. Microridges are formed on the surface of the epithelium. Epithelial cells contain two types of vesicles: mucous secretory vesicles and coated vesicles. Most of the mucous vesicles are situated in the center of the cell near the Golgi apparatus. In freeze-fracture replicas, intramembranous particles are abundant in the membranes of the secretory vesicles but rare in the apical plasma membrane. Coated vesicles are situated in the apical and subapical cytoplasm. A great number of thick filaments, considered to be keratin filaments, run randomly throughout the cell to form a meshwork. Thick filaments, which are sparse in the central cytoplasm, are connected to the membranes of the secretory vesicles and other membranous organelles. A layer of closely packed thin filaments, considered to be actin filaments, is found just beneath the apical plasma membrane. Microtubules also occur in the apical cytoplasm and run almost parallel to the cell surface. Both kinds of vesicles are connected to the thin and thick filaments. Their functional significance in the regulation of membrane at the free surface is discussed.     

Immunolocalization of a spectrin-like protein (fodrin) in pancreatic acinar cells

A spectrin-like protein (fodrin) was localized in porcine pancreas using an immunoperoxidase procedure with antibodies raised against erythrocyte spectrin. Fodrin was primarily associated with the cell plasma membrane although some was also detectable in the cytoplasm of the acinar cells. The membrane labelling of the acinar cells was uneven such that the lateral and basal membranes were strongly labelled by anti-spectrin antibodies whereas the apical membranes were poorly labelled. The implications of the results to secretion and to the occurrence of specific membrane domains are discussed.     

Fine structure of the rectal pads in the desert locust Schistocerca gregaria with reference to the mechanism of water uptake

The rectal pads of Schistocerca gregaria are composed of three different cell types: epithelial, secondary and junctional cells. The rectal pads are interconnected by simple rectal cells and both are lined internally by a articular intima. The epithelial cells exhibit extensive infoldings of the apical plasma membranes that are closely associated with mitochondria. Their lateral plasma membranes are highly folded around large mitochondria and enclose intercellular channels and spaces. They are united by belt and spot desmosomes, septate junctions, gap junctions and scalariform junctions, but terminate in a basal syncytium without contacting the basal plasma membranes. The apical and basal cytoplasm contain coated vesicles, dense tubular elements, multivesicular bodies and lysosomes, suggesting receptor-mediated endocytosis of small peptide molecules into the epithelial cells. The apical membrane infoldings of the secondary cells are also associated with large mitochondria. Their basal plasma membranes are covered by connective cell processes and connected with them by spot desmosomes which may be involved in solute recycling. The presence of neurosecretory-like axons near the secondary cells suggests that they exert local control on the function of these cells. The ultrastructural details are examined in relation to their role in solute and water transport.     

Involvement of syntaxin 4 in the transport of membrane-type 1 matrix metalloproteinase to the plasma membrane in human gastric epithelial cells

Membrane-type 1 matrix metalloproteinase (MT1-MMP) localized on the plasma membrane plays a central role in various normal biological responses including tissue remodeling, wound heeling, and angiogenesis and in cancer cell invasion and metastasis, by functioning as a collagenase and activating other matrix metalloproteinases. In order to elucidate the molecular mechanism of the MT1-MMP targeted localization on the plasma membrane, we examined the participation of syntaxin proteins in MT1-MMP intracellular transport to the plasma membrane in human gastric epithelial AGS cells. Western blotting showed that syntaxin 3 and 4 proteins, which are known to function in intracellular transport towards the plasma membrane, were expressed in AGS cells. Immunocytochemistry revealed that transient transfection of AGS cells with dominant-negative mutant syntaxin 4 decreased plasma membrane MT1-MMP expression. In contrast, transient transfection with either dominant-negative mutant syntaxin 3 or 7 did not affect MT1-MMP localization on the plasma membrane. Cell surface biotinylation assay and Matrigel chamber assay demonstrated that stable transfection with dominant-negative mutant syntaxin 4 decreased the amount of MT1-MMP on the plasma membranes and inhibited the cell invasiveness. We suggest that syntaxin 4 is involved in the intracellular transport of MT1-MMP toward the plasma membrane.     

Fine structure of the larval midgut of the fly Rhynchosciara and its physiological implications

The midgut of Rhynchosciara americana larvae consists of a cylindrical ventriculus from which protrudes two gastric caeca formed by polyhedral cells with microvilli covering their apical faces. The basal plasma membrane of these cells is infolded and displays associated mitochondria which are, nevertheless, more conspicuous in the apical cytoplasm. The anterior ventricular cells possess elaborate mitochondria-associated basal plasma membrane infoldings extending almost to the tips of the cells, and small microvilli disposed in the cell apexes. Distal posterior ventricular cells with long apical microvilli are grouped into major epithelial foldings forming multicellular crypts. In these cells the majority of the mitochondria are dispersed in the apical cytoplasm, minor amounts being associated with moderately-developed basal plasma membrane infoldings. The proximal posterior ventriculus represents a transition region between the anterior ventriculus and the distal posterior ventriculus. The resemblance between the gastric caeca and distal posterior ventricular cells is stressed by the finding that their microvilli preparations display similar alkaline phosphatase-specific activities. The results lend support to the proposal, based mainly on previous data on enzyme excretion rates, that the endo-ectoperitrophic circulation of digestive enzymes is a consequence of fluid fluxes caused by the transport of water into the first two thirds of midgut lumen, and its transference back to the haemolymph in the gastric caeca and in the distal posterior ventriculus.     

Ultrastructure of the Rectum Epithelial Cells in the Mosquito Larvae, Anopheles sinensis Wiedemann

ABSTRACT The Ultrastructure of rectum epithelial cells in the mosquito larvae, Anopheles sinensis Wiedemann, was studied using electron microscope. The rectal epithelium forms rectal papillae composed of the absorptive cells and the surrounding basal cells. Moreover, rectal epithelium was covered with thin cuticular intima. Apical plasma membrane of the epithelial cells had infoldings and in between them, mitochondria developed into elongated shape were attached. In addition, the membrane infoldings reach down into the cell cytoplasm to form several layers of leaflet-like prolongations. On both sides of these prolongations were also large, well-developed mitochondria. Their formation was that mitochondria were attached to 3 μm length and 4–13 layers of membrane wrinkle lump. Many spherites, which are lamelated crystals that form an illusory structure in concentric circles inside of the cytoplasm of epithelial cell were observed. Basal plasma membrane in the epithelial cells was also wrinkled to promulgate into the cytoplasm to become basal infoldings producing canaliculi in basal labyrinth formation. There were many mitochondria scattered in these formations as well. On the bottom of the epithelial cell, basal lamina was attached and between basal lamina and muscle bundle was subepithelial space, which is connective tissue. Inside the space, tracheal and nerve cells were observed.     

The effect of salinity acclimation on the ultrastructure of the gills of Gammarus oceanicus (Segerstråle, 1947) (Crustacea: Amphipoda)

Summary Acclimation to low salinity induces changes in the ultrastructure of the gill cells of the marine euryhaline amphipod, Gammarus oceanicus. The gills are composed of a single cell type. In 100% artificial sea water, these cells contain moderate numbers of mitochondria which are randomly distributed in the cytoplasm. The plasma membrane is extensively invaginated at the apical, lateral, and basal surfaces. Acclimation to 20% artificial sea water induces a further invagination at the apical cell membrane to form an elaborate apical labyrinth. The extracellular spaces between the folds in the basal cell membrane dilate to 1500 Å or more. Mitochondria are more abundant and in many cells they undergo a change in conformation. The mitochondria are crowded into thin leaflets of cytoplasm between the dilated basal invaginations or into the narrow space between apical and basal cell membranes. Consequently, they lie in close contact with the plasma membrane over much of their surface.Supported in part by grants from the United States Public Health Service, 5 RO1 AM13455-03 and PHS FR-07085-04, and by a grant from the National Research Council of Canada administered by Dr. G. P. Morris.     

Structure and Histochemistry of the Skin of a Torrent Catfish, Liobagrus mediadiposalis

The epidermis of the torrent catfish, Liobagrus mediadiposalis, consists of three layers: the outermost layer, middle layer and stratum germinativum. The epidermis consists of two types of skin glands, small mucus cell and voluminous club cell. The unicellular mucus cell contains acid sulfomucins (some sialomucins) and the club cell, sometimes binucleate, is proteinaceous. Well-developed vascularization is one of the characteristics of epidermis of L. mediadiposalis. Well-developed lymphatic spaces contain lymphocytes in the epidermis. The dermis lacks scales and consists mostly of a thick, dense connective tissue; its superficial region just below the basal membrane is supplied with fine blood capillaries. These histological features of the skin in L. mediadiposalis are consistent with that required for cutaneous respiration.     

Bone morphology of weanling rats from dams subjected to fluoride

Epithelial cells and surrounding free cells in the choroid plexus were examined cytochemically using filipin to clarify the distribution pattern of cholesterol within plasma membranes. The apical and basal membranes of the choroid epithelial cell are less susceptible to filipin than the lateral epithelial membrane and plasma membranes of adjacent mesenchymal cells such as macrophages and fibroblasts. Apical and basal domains of the epithelial membranes, which are relatively resistant to action of filipin, appear to have a slightly lower cholesterol content. We suggest that the apical and basal membranes may possess a unique membrane fluidity or stability that differs from that of the lateral epithelial, macrophage or fibroblast membranes.     

Evidence for a transcellular cisternal route across the caecal epithelium of an insect

Summary The cells of the mesenteric caeca in the midgut of certain insects possess a labyrinth of transepithelial cisternae. Their existence can be seen in thin sections of lanthanum-incubated tissue, where the tracer enters not only the intercellular clefts but also membranous cisternae which are inpocketings from, and, in continuity with, both the lateral clefts and basal membrane. These infoldings, which are numerous, run from the basal or lateral surfaces into the perinuclear region of the cells, where they are found, laden with lanthanum, as smooth cisternae or vesicles in the peripheral cytoplasm near the plasma membrane. These can be followed in serial sections and are quite distinct from other sub-surface cisternae of the lateral borders which are studded with ribosomes on the cytoplasmic surface. Near the luminal surface, tracer-laden structures in the form of vesicles and granules become increasingly predominant over those in the form of cisternae. Freeze-fracture replicas confirm the above observations, in that the plasma membrane of the intercellular cleft can be characterized as such unequivocally, since it exhibits smooth septate junctional E face grooves and P face ridges. Lateral infoldings, cisternae and vesicles can be seen arising directly from these junction-bearing membranes. The transepithelial cisternae and vesicles may be the morphological basis of an insect transcellular transport system, comparable to the tubulocisternal endoplasmic reticulum present in the transporting secretory and absorptive epithelia of vertebrate tissues. However, in insect midgut caecal epithelia, the cisternae appear to be, albeit presumably transiently, in direct continuity with the extracellular space, forming a plasma membrane reticular system which seems not to be the case with the tubulo-cisternal endoplasmic reticulum which terminates in subsurface cisternae.     

Localization of caveolin-3 in the sinus endothelial cells of the rat spleen

The localization of caveolins in the sinus endothelial cells of the rat spleen has been demonstrated by confocal laser scanning and electron microscopy. Caveolin-3, a muscle-specific caveolin, was detected by Western blot analysis and immunofluorescence microscopy of isolated sinus endothelial cells and tissue cryosections of the spleen. During the immunofluorescence microscopy of isolated endothelial cells, both caveolin-3 and caveolin-1 were found. In tissue cryosections of the spleen, caveolin-3, as well as caveolin-1 and -2, was present in the contours and cytoplasm of the cells. Immunogold electron microscopy of tissue cryosections revealed caveolin-3, -1, and -2 to be present in caveolae in the apical, lateral, and basal plasma membranes and some vesicular profiles in the cytoplasm of sinus endothelial cells. Furthermore, caveolin-3 was colocalized with caveolin-1 in the same caveolae in the apical, lateral, and basal plasma membranes. Stress fibers and tubulovesicular structures were situated in the vicinity of caveolae labeled with anti-caveolin-3, anti-caveolin-1, and anti-caveolin-2 antibodies. It is speculated that caveolae in sinus endothelial cells play an important role in the constriction of stress fibers.