The enteric surface coat on cat intestinal microvilli

Influence of the mechanical and geometrical parameters on the cellular uptake of nanoparticles: A stochastic approach

Nanoparticles (NPs) are used for drug delivery with enhanced selectivity and reduced side-effect toxicity in cancer treatments. Based on the literature, the influence of the NPs mechanical and geometrical properties on their cellular uptake has been studied through experimental investigations. However, due to the difficulty to vary the parameters independently in such a complex system, it remains hard to efficiently conclude on the influence of each one of them on the cellular internalization of a NP. In this context, different mechanical / mathematical models for the cellular uptake of NPs have been developed. In this paper, we numerically investigate the influence of the NP's aspect ratio, the membrane tension and the cell-NP adhesion on the uptake of the NP using the model introduced in¹ coupled with a numerical stochastic scheme to measure the weight of each one of the aforementioned parameters. The results reveal that the aspect ratio of the particle is the most influential parameter on the wrapping of the particle by the cell membrane. Then the adhesion contributes twice as much as the membrane tension. Our numerical results match the previous experimental observations.

Macro- and microstructures of the digestive tract in the Eurasian collared dove, Streptopelia decaocto (Frivaldszky 1838): Adaptive interplay between structure and dietary niche

We describe the functional morphology of the digestive tract of the Eurasian collared dove, Streptopelia decaocto using anatomical, morphometric, histological, histochemical, and ultrastructure techniques, and relate our findings to the species' dietary niche. Our results revealed that the esophagus is displaced on both sides of the neck and has highly folded tunica mucosa, which confer greater elasticity for efficient swallowing and passage of food to the crop. The proventriculus is delicate and its mucosal layer contains polymorphic glands with dense profound and superficial secretory units that open to the luminal surface by gastric pores. The ventriculus is biconvex and lined with a keratinized koilin membrane. The tubular glands within the mucosal lining include the isthmus, the neck, and the basal segment that comprise chief and basal cells with prominent nuclei. At the cuticle-mucosal interface, pyramidal vertical rodlets of the cuticle are secreted and superficially covered by a thin film of a horizontal matrix. The mucosa of the ileum form pyramidal villi that are oriented perpendicularly to the central lumen. Enterocytes infiltrated with goblet cells make up the epithelial lining of the villi. There are subtle differences in the thicknesses of corresponding tunics together with histochemical reactions of alcian blue (AB) and Masson-Goldner trichrome (MT) for their microstructures. Overall, our findings reveal remarkable convergence of both macro-and microstructures in S. decaocto to other granivorous species, and offer further evidence of the close association between functional morphology and feeding style relative to food swallowing, digestion, and absorption.

On Top of the Alveolar Epithelium: Surfactant and the Glycocalyx

Gas exchange in the lung takes place via the air-blood barrier in the septal walls of alveoli. The tissue elements that oxygen molecules have to cross are the alveolar epithelium, the interstitium and the capillary endothelium. The epithelium that lines the alveolar surface is covered by a thin and continuous liquid lining layer. Pulmonary surfactant acts at this air-liquid interface. By virtue of its biophysical and immunomodulatory functions, surfactant keeps alveoli open, dry and clean. What needs to be added to this picture is the glycocalyx of the alveolar epithelium. Here, we briefly review what is known about this glycocalyx and how it can be visualized using electron microscopy. The application of colloidal thorium dioxide as a staining agent reveals differences in the staining pattern between type I and type II alveolar epithelial cells and shows close associations of the glycocalyx with intraalveolar surfactant subtypes such as tubular myelin. These morphological findings indicate that specific spatial interactions between components of the surfactant system and those of the alveolar epithelial glycocalyx exist which may contribute to the maintenance of alveolar homeostasis, in particular to alveolar micromechanics, to the functional integrity of the air-blood barrier, to the regulation of the thickness and viscosity of the alveolar lining layer, and to the defence against inhaled pathogens. Exploring the alveolar epithelial glycocalyx in conjunction with the surfactant system opens novel physiological perspectives of potential clinical relevance for future research.

Membrane mucins of the intestine at a glance

Membrane mucins cover most mucosal surfaces throughout the human body. The intestine harbors complex population of microorganisms (the microbiota) and numerous exogenous molecules that can harm the epithelium. In the colon, where the microbial burden is high, a mucus barrier forms the first line of defense by keeping bacteria away from the epithelial cells. In the small intestine where the mucus layer is less organized, microbes are kept at bay by peristalsis and antimicrobial peptides. Additionally, a dense glycocalyx consisting of extended and heavily glycosylated membrane mucins covers the surface of enterocytes. Whereas many aspects of mucosal barriers are being discovered, the function of membrane mucins remains a largely overlooked topic, mainly because we lack the necessary reagents and experimental animal models to investigate these large glycoproteins. In this Cell Science at a Glance article and accompanying poster, we highlight central concepts of membrane mucin biology and the role of membrane mucins as integral components of intestinal mucosal barriers. We also present the current consensus concerning the role of membrane mucins in host-microbe interactions. Moreover, we discuss how regulatory circuits that govern membrane mucins in the healthy gut display strong overlap with pathways that are perturbed during chronic inflammation. Finally, we review how dysregulation of intestinal membrane mucins may contribute to human diseases, such as inflammation and cancer.

Nanoarchitecture and dynamics of the mouse enteric glycocalyx examined by freeze-etching electron tomography and intravital microscopy

The glycocalyx is a highly hydrated, glycoprotein-rich coat shrouding many eukaryotic and prokaryotic cells. The intestinal epithelial glycocalyx, comprising glycosylated transmembrane mucins, is part of the primary host-microbe interface and is essential for nutrient absorption. Its disruption has been implicated in numerous gastrointestinal diseases. Yet, due to challenges in preserving and visualizing its native organization, glycocalyx structure-function relationships remain unclear. Here, we characterize the nanoarchitecture of the murine enteric glycocalyx using freeze-etching and electron tomography. Micrometer-long mucin filaments emerge from microvillar-tips and, through zigzagged lateral interactions form a three-dimensional columnar network with a 30 nm mesh. Filament-termini converge into globular structures ~30 nm apart that are liquid-crystalline packed within a single plane. Finally, we assess glycocalyx deformability and porosity using intravital microscopy. We argue that the columnar network architecture and the liquid-crystalline packing of the filament termini allow the glycocalyx to function as a deformable size-exclusion filter of luminal contents.

Sun, Krystofiak et al. show the nanoarchitecture of the murine enteric glycocalyx, glycoprotein-rich coat covering cells and assess its porosity and deformability in mice, providing a comprehensive structural framework. This study suggests that the glycocalyx may function as a deformable size-exclusion filter of luminal contents.

Absence of the Epithelial Glycocalyx As Potential Tumor Marker for the Early Detection of Colorectal Cancer

Detection of cancer at an early stage is pivotal for successful treatment and long term survival, yet early diagnosis requires sensitive and specific markers that can be easily detected by screening procedures. Differences in the surface structure of tumor and healthy cells, if sufficiently pronounced and discernible, may serve that purpose. We analyzed the luminal surface of healthy and neoplastic human colorectal tissues for the presence and architecture of the glycocalyx—a dense network of highly glycosylated proteins—using transmission electron microscopy. The ultrastructural analyses showed that 93% of healthy mucosae were covered by an intact glycocalyx. Contrarily, on over 90% of the surface of neoplastic cells the glycocalyx was absent. The sensitivity and specificity of our marker “absence of a glycocalyx” are excellent, being 91% (83–96%) and 96% (89–99%) for adenocarcinomas and 94% (73–100%) and 92% (85–97%) for precancerous polyps (means and 95% confidence intervals). Using a cell culture model we could demonstrate that a particulate probe targeting a cell surface receptor usually concealed beneath the glycocalyx can bind selectively to glycocalyx-free areas of a tumor cell layer. We propose that the absence of a glycocalyx may serve as novel type of tumor marker. If the absence of the glycocalyx can be detected e.g. via binding of imaging probes to non-shielded surface receptors of anomalously differentiated cells, this tumor marker could be used to enable early diagnosis of colorectal cancer.

Enterococcus faecalis readily colonizes the entire gastrointestinal tract and forms biofilms in a germ-free mouse model

The mammalian gastrointestinal (GI) tract is a complex organ system with a twist-a significant portion of its composition is a community of microbial symbionts. The microbiota plays an increasingly appreciated role in many clinically-relevant conditions. It is important to understand the details of biofilm development in the GI tract since bacteria in this state not only use biofilms to improve colonization, biofilm bacteria often exhibit high levels of resistance to common, clinically relevant antibacterial drugs. Here we examine the initial colonization of the germ-free murine GI tract by Enterococcus faecalis-one of the first bacterial colonizers of the naïve mammalian gut. We demonstrate strong morphological similarities to our previous in vitro E. faecalis biofilm microcolony architecture using 3 complementary imaging techniques: conventional tissue Gram stain, immunofluorescent imaging (IFM) of constitutive fluorescent protein reporter expression, and low-voltage scanning electron microscopy (LV-SEM). E. faecalis biofilm microcolonies were readily identifiable throughout the entire lower GI tract, from the duodenum to the colon. Notably, biofilm development appeared to occur as discrete microcolonies directly attached to the epithelial surface rather than confluent sheets of cells throughout the GI tract even in the presence of high (>10⁹) fecal bacterial loads. An in vivo competition experiment using a pool of 11 select E. faecalis mutant strains containing sequence-defined transposon insertions showed the potential of this model to identify genetic factors involved in E. faecalis colonization of the murine GI tract.

Morphological and immunohistochemical analyses of soluble proteins in mucous membranes of living mouse intestines by cryotechniques

We have performed immunohistochemical or ultrastructural analyses of living mouse small intestines using Epon blocks prepared by 'in vivo cryotechnique' (IVCT). By electron microscopy, intracellular ultrastructures of epithelial cells were well preserved in tissue areas 5-10 μm away from cryogen-contact surface tissues. Their microvilli contained dynamically waving actin filaments, and highly electron-dense organelles, such as mitochondria, were seen under the widely organized terminal web. By quick-freezing of fresh resected tissues (FT-QF), many erythrocytes were congested within blood vessels due to loss of blood pressure. By immersion-fixation (IM-DH) and perfusion-fixation (PF-DH), small vacuoles were often seen in the cytoplasm of epithelial cells, and their intercellular spaces were also dilated. Moreover, actin filament bundles were irregular in cross sections of microvilli, compared with those with IVCT. Epon-embedded thick sections were treated with sodium ethoxide, followed by antigen retrieval and immunostained for immunoglobulin A (IgA), Ig kappa light chain (Igκ), J-chain and albumin. By cryotechniques, IgA immunoreactivity was detected as tiny dot-like patterns in cytoplasm of some epithelial cells. Both J-chain and Igκ immunoreactivities were detected in the same local areas as those of IgA. By FT-QF, however, the IgA immunoreactivity was more weakly detected, compared with that with IVCT. In thick sections prepared by IM-DH and PF-DH, it was rarely observed in both plasma and epithelial cells. Another albumin was diffusely immunolocalized in extracellular matrices of mucous membranes and also within blood vessels. Thus, IVCT was useful for preservation of soluble proteins and ultrastructural analyses of dynamically changing epithelial cells of living mouse small intestines.

Localization of Bacillus thuringiensis Cry1A toxin-binding molecules in gypsy moth larval gut sections using fluorescence microscopy

The microbial insecticide Bacillus thuringiensis (Bt) produces Cry toxins, proteins that bind to the brush border membranes of gut epithelial cells of insects that ingest it, disrupting the integrity of the membranes, and leading to cell lysis and insect death. In gypsy moth, Lymantria dispar, two toxin-binding molecules for the Cry1A class of Bt toxins have been identified: an aminopeptidase N (APN-1) and a 270kDa anionic glycoconjugate (BTR-270). Studies have shown that APN-1 has a relatively weak affinity and a very narrow specificity to Cry1Ac, the only Cry1A toxin that it binds. In contrast, BTR-270 binds all toxins that are active against L. dispar larvae, and the affinities for these toxins to BTR-270 correlate positively with their respective toxicities. In this study, an immunohistochemical approach was coupled with fluorescence microscopy to localize APN-1 and BTR-270 in paraffin embedded midgut sections of L. dispar larvae. The distribution of cadherin and alkaline phosphatase in the gut tissue was also examined. A strong reaction indicative of polyanionic material was detected with alcian blue staining over the entire epithelial brush border, suggesting the presence of acidic glycoconjugates in the microvillar matrix. The Cry1A toxin-binding sites were confined to the apical surface of the gut epithelial cells with intense labeling of the apical tips of the microvilli. APN-1, BTR-270, and alkaline phosphatase were found to be present exclusively along the brush border microvilli along the entire gut epithelium. In contrast, cadherin, detected only in older gypsy moth larvae, was present both in the apical brush border and in the basement membrane anchoring the midgut epithelial cells. The topographical relationship between the Bt Cry toxin-binding molecules BTR-270 and APN-1 and the Cry1A toxin-binding sites that were confined to the apical brush border of the midgut cells is consistent with findings implicating their involvement in the mechanism of the action of Bt Cry toxins.

Biology of the rabbit

In recognition of Dr. Nathan Brewer's many years of dedicated service to AALAS and the community of research animal care specialists, the premier issue of JAALAS includes the following compilation of Dr. Brewer's essays on rabbit anatomy and physiology. These essays were originally published in the ASLAP newsletter (formerly called Synapse), and are reprinted here with the permission and endorsement of that organization. I would like to thank Nina Hahn, Jane Lacher, and Nancy Austin for assistance in compiling these essays. Publishing this information in JAALAS allows Dr. Brewer's work to become part of the searchable literature for laboratory animal science and medicine and also assures that the literature references and information he compiled will not be lost to posterity. However, readers should note that this material has undergone only minor editing for style, has not been edited for content, and, most importantly, has not undergone peer review. With the agreement of the associate editors and the AALAS leadership, I elected to forego peer review of this work, in contradiction to standard JAALAS policy, based on the status of this material as pre-published information from an affiliate organization that holds the copyright and on the esteem in which we hold for Dr. Brewer as a founding father of our organization.

Region-Dependent Role of the Mucous/Glycocalyx Layers in Insulin Permeation Across Rat Small Intestinal Membrane

The regional difference in the contribution of the mucous/glycocalyx layers in rat small intestine, as a diffusional or enzymatic barrier, to the absorption of insulin was investigated by in vitro studies. The mucous/glycocalyx layers from the duodenum, the jejunum, and the ileum in rat were successfully removed without damaging membrane integrity, by exposing them to a hyaluronidase solution in situ. In an in vitro transport experiment, the apparent permeability coefficient (P(app)) of insulin for the hyaluronidase-pretreated group was significantly increased compared to the PBS-pretreated (control) group in all small intestinal regions, and the P(app) of insulin in both PBS- and hyaluronidase-pretreated groups increased in the following order: duodenum < jejunum < ileum. On the other hand, irrespective of small intestinal regions, the P(app) of FD-4 and of antipyrine, respectively the passive para- and transcellular permeation marker, exhibited no significant differences between PBS- and hyaluronidase-pretreated group. In addition, a significant amount of insulin was degraded in the mucous/glycocalyx layers compartment removed by hyaluronidase pretreatment, and the degradation activity in the mucous/glycocalyx layers showed regional differences in the following order: duodenum > jejunum > ileum. These findings suggest that, irrespective of small intestinal regions, the mucous/glycocalyx layers contributed to insulin permeation predominantly as an enzymatic barrier, and not as a diffusional barrier. Furthermore, the variation of the enzymatic activities in the mucous/glycocalyx layers and in the brush-border membrane would be one factor that accounts for the regional differences in the transport of insulin.

Three-dimensional ultrastructure of the brush border glycocalyx in the mouse small intestine: a high resolution scanning electron microscopic study

The three-dimensional ultrastructure of the filamentous glycocalyx of the brush border in the mouse small intestine was successfully demonstrated by high resolution scanning electron microscopy (SEM). The specimens were fixed with 2% glutaraldehyde in a 0.1M phosphate buffer (pH 7.4), and rinsed with buffered solutions with differently adjusted pH values (pH 3.0, 7.0 or 11.0). They were then osmicated, dried, spatter-coated with gold (1.0-1.5 nm), and observed under a high resolution SEM. The glycocalyx on the luminal surface of the intestinal villi covered the top of the microvilli of the epithelial cells and were well preserved in the specimens treated with an alkaline buffer (pH 11.0). The glycocalyx was observed as filamentous structures, 7 to 15 nm thick in diameter. These filaments repeatedly branched and anastomosed with neighboring ones to form an actual network or plexus as a whole, in contrast with superimposed images in transmission electron microscopy (TEM) which suggested that such anastomoses were pseudo-networks. The filaments thickened globularly at the sites of the filament bifurcation or branching. On the other hand, specimens rinsed with an acid or neutral buffer showed no glycocalyx on their microvilli, whose naked top had knob-like structures. Thus, the pH values of the washing buffer solutions were considered to affect the preservation of the surface coat due to molecular characteristics.

Acetylated sialic acid residues and blood group antigens localise within the epithelium in microvillous atrophy indicating internal accumulation of the glycocalyx

BACKGROUND

Microvillous atrophy, a disorder of intractable diarrhoea in infancy, is characterised by the intestinal epithelial cell abnormalities of abnormal accumulation of periodic acid-Schiff (PAS) positive secretory granules within the apical cytoplasm and the presence of microvillous inclusions. The identity of the PAS positive material is not known, and the aim of this paper was to further investigate its composition.

METHODS

Formaldehyde fixed sections were stained with alcian blue/PAS to identify the acidic or neutral nature of the material, phenylhydrazine blocking was employed to stain specifically for sialic acid, and saponification determined the presence of sialic acid acetylation. The specificity of sialic acid staining was tested by digestion with mild sulphuric acid. Expression of blood group related antigens was tested immunochemically.

RESULTS

Alcian blue/PAS staining identified a closely apposed layer of acidic material on the otherwise neutral (PAS positive) brush border in controls. In microvillous atrophy, a triple layer was seen with an outer acidic layer, an unstained brush border region, and accumulation within the epithelium of a neutral glycosubstance that contained acetylated sialic acid. Blood group antigens were detected on the brush border, in mucus, and within goblet cells in controls. In microvillous atrophy they were additionally expressed within the apical cytoplasm of epithelial cells mirroring the PAS abnormality. Immuno electron microscopy localised expression to secretory granules.

CONCLUSIONS

A neutral, blood group antigen positive, glycosubstance that contains acetylated sialic acid accumulates in the epithelium in microvillous atrophy. Previous studies have demonstrated that the direct and indirect constitutive pathways are intact in this disorder and it is speculated that the abnormal staining pattern reflects accumulation of glycocalyx related material.

Glycocalyx of lung epithelial cells

Due to their diversity and external location on cell membranes, glycans, as glycocalyx components, are key elements in eukaryotic cell, tissue, and organ homeostasis. Although information on the lung glycocalyx is scarce, this article aims to review, discuss, and summarize what is known about bronchoalveolar glycocalyx composition, mainly the sialic acids. It was deemed relevant, however, to make a brief introductory overview of the cell glycocalyx and its particular development in epithelial cells. After that, follows a summary of the evolution of the knowledge regarding the bronchoalveolar glycocalyx composition throughout the years, particularly its morphological features. Since sialic acids are located terminally on the bronchoalveolar lining cells' glycocalyx and play crucial roles, we focused mainly on the existing lung histochemical and biochemical data of these sugar residues, as well as their evolution throughout lung development. The functions of the lung glycocalyx sialic acids are discussed and interpretations of their roles analyzed, including those related to the negative overall superficial shield provided by these molecules. The increasing presence of these sugar residues throughout postnatal lung development should be regarded as pivotal in the development and maintenance of a dynamic bronchoalveolar architecture, supporting the normal histophysiology of the respiratory system. The case for a profound knowledge of lung glycocalyx--given its potential to provide answers to serious clinical problems--is made with particular reference to cystic fibrosis. Finally, concluding remarks and perspectives for future research in this field are put forth.

Epithelial cell polarity as reflected in enterocytes

Absorptive cells are the main cells present in the intestinal epithelium. The plasma membrane of these tall columnar cells reflects their high degree of polarization, by dividing into apical and basolateral domains with different compositions. The most characteristic structure of these cells consists of closely packed apical microvilli with the same height, looking like a brush, which is why they were named the brush border. The concentrated pattern of some apical markers observed in a restricted brush border domain shows that mature enterocytes are hyperpolarized epithelial cells: the filamentous brush border glycocalyx is anchored at the top of the microvilli and the annexin XIII is concentrated in the lower three fourths. Many studies have been carried out on the biosynthesis and intracellular pathway of domain markers. The results show clearly that the basolateral markers take a direct pathway from the trans-Golgi network to the basolateral membrane. However, the two apical pathways, one direct and one indirect pathway via the basolateral membrane, are used, depending on the apical protein involved. Efficient protein sorting and addressing are essential to the establishment and maintenance of cell polarity, on which the integrity of the epithelial barrier depends.

Microvesiculation of the microvilli provides a membranous medium within the lumen during digestion

A hypothesis is presented which states that there is a membranous medium within the intestinal lumen which occurs during digestion. The medium is thought to be generated by the microvesiculation of the microvilli.

Adherence of Salmonella typhimurium to Caco-2 cells: identification of a glycoconjugate receptor

The mechanism by which Salmonella species adhere to the epithelium of the intestine is not well understood. To identify components on intestinal epithelial cells that may be involved in the initial adherence of Salmonella typhimurium, we correlated patterns of adherence to well-differentiated Caco-2BBe cell monolayers with expression of brush border membrane components and lectin binding sites. This cloned cell line shows heterogeneous expression of sucrase-isomaltase and most lectin receptors. S. typhimurium adhered to a subpopulation of living or formaldehyde-fixed cells with a high multiplicity (up to 150 bacteria per cell). Bacterial binding to selected cells was not correlated with expression of the brush border hydrolases dipeptidyl-peptidase IV and sucrase-isomaltase or with binding of 10 of the 12 lectins tested. However, binding was correlated with the presence of binding sites for peanut agglutinin (PNA) [specific for Gal beta (1-3) GalNAc] and soybean agglutinin (specific for terminal GalNAc). Preincubation of live and fixed Caco-2BBe monolayers with PNA inhibited bacterial binding, while preincubation with soybean agglutinin did not. Electron microscopic analysis demonstrated that the initial adherence of S. typhimurium to Caco-2 cells in vitro involved peripheral components of the glycocalyx on apical microvilli. These results suggest that a Gal beta (1-3)GalNAc epitope recognized by PNA and located in the glycocalyx is involved in the early recognition events between S. typhimurium and Caco-2 cells and that differences in glycosylation patterns among individual epithelial cells may be a determinant in cell-selective adherence of S. typhimurium.

Expression and glycosylation of the filamentous brush border glycocalyx (FBBG) during rabbit enterocyte differentiation along the crypt-villus axis

The filamentous brush border glycocalyx forming the ‘enteric surface coat’ of the intestinal epithelium is composed in rabbits of a 400 kDa mucin-type glycoprotein, which was purified using the 3A4 monoclonal antibody. This monoclonal antibody recognizes a filamentous brush border glycocalyx-specific glycosidic structure containing an O-acetylated sialic acid, which is absent from all the other glycoproteins in the epithelium, with the exception of certain goblet cell mucins. Here we establish that only 50% of the rabbits tested synthesized this glycosidic structure. Upon immunolabeling surface epithelia and sections of jejunum from these rabbits, the carbohydrate epitope recognized by the 3A4 mAb was found to be present on the filamentous brush border glycocalyx of a variable number of enterocytes, which were patchily distributed over all the villi. This heterogeneous expression of 3A4 antigenicity, which was also observed in the crypts, suggests the existence of differences between the patterns of differentiation of enterocytes, which results in the expression of different pools of glycosyltransferases and/or acetyl transferases. In mature enterocytes, the 3A4 determinants were present only on the filamentous brush border glycocalyx, which is anchored solely to the membrane microdomain at the tip of brush border microvilli. However, expression of 3A4 antigenicity begins in the median third of crypts, in enterocytes with a short, thin brush border devoid of apical filamentous brush border glycocalyx. Here the 3A4 epitopes were present over the whole brush border membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

The filamentous brush border glycocalyx, a mucin-like marker of enterocyte hyper-polarization

The probably sole constituent of the filamentous brush border glycocalyx, which has been defined on the basis of electron microscopic data as a set of filaments radiating from the tip of rabbit intestinal brush border microvilli, has been purified. It consists of a mucin-type glycoprotein that can be solubilized by either Triton extraction or papain treatment of the brush border membrane vesicles but is insensitive to phosphatidylinositol phospholipase C. The detergent- and papain-solubilized forms both have the same apparent molecular mass of 400 kDa (SDS/PAGE). This suggests that the filamentous brush border glycocalyx may be anchored to the membrane by a small hydrophobic peptidic tail. Ser, Thr, Pro and Ala amount to 65% of the protein core amino acid residues. The glycosidic moiety, which amounts to 73% of the molecular mass, has high O-acetylated sialic acid contents. A monoclonal antibody (3A4) raised against the purified material was produced which specifically recognized the 400-kDa band by immunoprecipitation and immunoblotting, and the filamentous brush border glycocalyx of villus enterocytes when jejunum sections were immunolabelled. The 3A4 determinant was identified with a filamentous brush border glycocalyx-specific carbohydrate structure containing an O-acetylated sialic acid. The fact that the labeled glycocalyx was anchored entirely in a membrane microdomain at the tip of the microvilli shows that mature enterocytes are hyper-polarized epithelial cells.

Thickening of the glycocalyx during methotrexate induced small intestinal mucosal atrophy in the rat

In this study, the morphology of the glycocalyx of small intestinal enterocytes in the rat was investigated after the induction of mucosal atrophy by methotrexate (MTX) in 18 Wistar rats. On the 2nd, 4th and 6th day after the intraperitoneal administration of MTX, a group of 6 rats was sacrificed and tissue specimens taken at the level of the Treitz' ligament were post fixed in a mixture of 2.0% watery osmiumtetroxide and 1.5% potassium-ferrocyanide to enhance visibility of glycoconjugates and processed for electron microscopy. In the control rats, the glycocalyx was hardly visible on the villous enterocytes, but rather conspicuous on the enterocytes in the crypts. On the second day after injection of MTX, the glycocalyx of the villous enterocytes was more pronounced but on the crypt enterocytes it resembled that of control animals. On the 4th day after MTX administration, a striking reduction and shortening of microvilli of villous enterocytes was seen, together with very pronounced glycocalyces, while crypt cells possessed well developed cell coats. On the 6th day after the injection of MTX, the glycocalyx of both villous and crypt cells appeared almost normal again.

In vitro studies of intestinal drug absorption. Determination of partition and distribution coefficients with brush border membrane vesicles

Brush border membrane vesicles (BBMV) were isolated from rat small intestine and characterized in terms of relative enrichment of specific organelle marker enzymes (20-fold enrichment; 20% yield), contamination by other subcellular organelles (less than 1%) and functional integrity (Na(+)-dependent glucose uptake). Using these vesicles, techniques were developed for the determination of partition and distribution coefficients for the model solutes, nitrobenzene, toluene and benzoic acid. No gender, age or regional variation along the small intestine in partition coefficient (log P) values was detected. There was no temperature (10-40 degrees) or pH (4.5-8.0) dependence in partition coefficients of nitrobenzene and toluene. Fair agreement was obtained for log P and log D values for these two solutes determined with BBMV and those reported with octanol and propylene glycol dipelargonate. Selective removal of proteins, both ecto-brush border and micro-villus core proteins, did not alter the partition coefficients of the three model solutes. In contrast, depletion of the BBMV of non-esterified fatty acids significantly decreased the partition coefficients. Liposomes prepared from BBMV lipid extracts were also used for partition coefficient determinations and gave similar values to intact BBMV; addition of increasing amounts of cholesterol to the lipid extract caused small increases in the partition coefficients of the model solutes in the liposomes. It was concluded that the partition coefficients of the BBMV were related to the lipid and not to the protein composition of the vesicles. The method offers a rapid and reliable means of measuring the partition coefficient of non-protein bound drugs and nutrients in isolated intestinal BBMV and should assist in the subsequent modelling and prediction of intestinal absorption in vivo.

N-terminal amino acid sequence, immunohistochemical localization and tissue distribution of a plasma membrane protein (Prot17) of rat enterocytes

Prot17, a protein of the basolateral membrane of rat small intestine with a mol.wt. of 17 kDa, can be isolated using a previously described method (Schiechl 1988). It occurs in the membrane as an oligomer with a mol.wt. of 90 kDa. In the present study a polyclonal antibody specific for Prot17 was used to explore by immunohistochemical techniques the tissue distribution of Prot17 and its ultrastructural localization within the cells. Furthermore the amino acid sequence of the N-terminal part of this molecule up to position 17 could be analyzed. The results are summarized as follows: Prot17 is a membrane anchored protein. Its partial amino acid sequence suggests that it is neither identical nor related to other known proteins. Immunofluorescence studies revealed, that it occurs only in epithelial cells. It is mainly found in the absorptive and goblet cells of the intestine and the acinar cells of the pancreas. Smaller quantities are found also in the bile duct epithelium of the liver, in the proximal tubule cells of the kidney and in the cells of the respiratory epithelium. Ultrastructural localization of Prot17 was possible in the intestinal epithelium and pancreas acinar cells. In both cell types it was found in the basolateral and microvillous membrane. In pancreas, Prot17 was also detected in the membrane of the zymogen granules. In the absorptive cells of the intestine Prot17 was found in both the membrane and the contents of subluminal vesicles. Furthermore, in apical granules of secretory cells of the respiratory epithelium binding of Prot17 specific antibody was found in the granular content, the membrane being negative.

Sensory hair fusion and glycocalyx changes following gentamicin exposure in the guinea pig vestibular organs

This study demonstrates the mechanism of sensory hair fusion and its relationship to glycocalyx on the hair cells in the vestibular epithelia of gentamicin-treated guinea pigs, using a ruthenium red staining technique. 0.1 ml of a solution containing 50 mg gentamicin sulphate per 1 ml was injected once into the middle ear. After 7 days, various stages of sensory hair degeneration were observed. The glycocalyx was clearly visualized by ruthenium red. The gentamicin-induced hair damage started with a local decrease in or loss of glycocalyx from neighbouring sensory hairs, followed by formation of local attachment areas of membrane sensory hairs. These findings suggest that the glycocalyx of the sensory cell may play an important role in separating the cilia as well as maintaining the organization of the bundle. Therefore, the decrease in or loss of glycocalyx from the hair bundle of sensory cells affected by gentamicin, either directly or secondarily, can be considered as closely related to the fusion of the sensory hairs.

Cell migration into neural tube lumen provides evidence for the "fixed cortex" theory of cell motility

We present a model of cell motility based on emigration of neural crest cells into the neural tube lumen under in vitro conditions (10% fetal calf serum or YIGSR) that inhibit their normal emigration from the base of the neuroepithelium into surrounding extracellular matrix (ECM). Ultrastructural observations reveal that cells lining the lumen are joined by zonulae adherentes (ZA), which are points of strong intercellular attachment, and thereby serve as markers for fixed regions of plasmalemma and cortical actin. Three major observations of the relationship of cells to the ZA support the "fixed cortex" model of mesenchymal cell migration. First, cells extend apical cell processes past the ZA into the lumen. To do this, they must make new apical plasmalemma and actin cortex that the endoplasm slides into. Second, elongated cells are observed in the lumen that are still attached via ZA to the neuroepithelium. This indicates that all of the endoplasm finally slides past the ZA. Third, numerous cytoplasmic pieces, often attached to each other and to the neuroepithelium via ZA, are found at the site where cells appear to have detached from the epithelium after entering the lumen. Since the ZA is fixed in location, the endoplasm must have slid past it into newly manufactured anterior cortex and plasmalemma, with the trailing end of the cell finally snapping off. The "fixed cortex" theory of cell migration agrees with existing data in that it predicts the polarized insertion of new plasmalemma and actin at the leading end of the cell, but it differs significantly from existing theories of mesenchymal cell migration in that it states that the cell surface remains firmly attached to the substratum while the myosin-rich endoplasm slides past it.

The ultrastructure of fiber cells in primate lenses: a model for studying membrane senescence

We have compared the surface morphology of the youngest (cortical) fiber cells with that of the most senescent (nuclear) fiber cells in monkey and baboon crystalline lenses by stereo scanning electron microscopy (SEM) and thick-section stereo transmission electron microscopy (TEM). Both the broad and the narrow faces of the most senescent fiber cells featured distinctive, polygonal areas (domains) of furrowed cell membrane. The domains ranged in size from 2.42 to 8.78 microns2. Stereopair SEM and TEM micrographs demonstrated precisely oriented microvilli measuring approximately 0.14 micron in diameter and ranging in length from 1.27 to 4.65 microns overlying each ridge in the domains. Formation of microvilli on senescent cells has been noted in other types of aging cells but they are imprecisely arranged and their function is unknown. Since every fiber cell remains in a fixed location (relative to other fiber cells) throughout life, the lens provides a unique model to study structure-function relationships of senescent microvilli in situ. The discovery of an age-related elaboration of numerous microvilli on senescent fiber cells of noncataractous lenses invalidates the currently accepted theory that close, parallel apposition of the broad faces of lens fiber cells is necessary for the lens to be transparent.

A newly identified surface coat on cochlear hair cells

Routine electron microscope methods do not well preserve or stain the surface coat or glycocalyx on cochlear hair cells. In other tissues, enhanced preservation and staining of these glycoconjugates was obtained following fixation with glutaraldehyde containing a cationic dye (e.g., Alcian blue and ruthenium red). When cochleas were fixed with glutaraldehyde containing Alcian blue, the endolymphatic surface of hair cells, but not the supporting cells, displayed an extensive (approximately 90 nm thick) surface coat. Alcian blue positive material was also observed in the tectorial and basilar membranes and in a portion of the spiral ligament. In addition, acellular bands of Alcian blue positive material were observed between the tectorial membrane and the reticular lamina or inner sulcus cells. Although the function of these cochlear glycoconjugates is not yet known, it is proposed that they serve to attach the tectorial membrane to the organ of Corti, and they are involved in stereocilia fusion following sound exposure and ototoxic drug administration.

Mapping of the rat liver endothelial membrane with lectins and glycosylated ferritins

We explored the luminal surface of liver sinus endothelium for the presence of lectin receptors and lectinlike substances capable of interacting with specific sugars. We used ferritin-conjugated lectins and glycosylated ferritins as probes. Incubation of small blocks of rat liver with these probes led to the binding of concanavalin A (on A), Ricinus communis (RCA), wheat germ agglutinin (WGA), phytohemagglutinin (PHA) and mannosyl ferritins to the luminal surface of endothelium. Ulex europaeus agglutinin I (UEA), fucosyl, galactosyl, and chitobiosyl-ferritins did not bind. The binding was patchy and sparse in the case of Con A and mannosyl-ferritins but uniform for others. Binding density did not correlate with hemagglutinability of lectins, suggesting that the difference in the hemagglutinability of these lectins did not account for the difference in their binding densities. Bindings were all completely inhibited in the presence of excess specific sugar inhibitors, indicating the specificity of binding. The distribution of binding was segregated on the endothelial membrane, being heaviest on luminal pits. To define the functional significance of this segregated distribution, sinus endothelium was compared to portal-vein endothelium in which endothelial fenestrations are also seen; and these fenestrations as well as pits may be covered by a thin diaphragm. Of interest was the total absence of binding to the diaphragm. The significance of these findings is discussed.

Ultrastructure of the epithelial cells of the small intestinal villi, the surface epithelium of the large intestinal mucosa and some elements of the intestinal mucosal lamina propria in rats with acute uraemia

Electron microscopic studies on the dynamics of changes in the epithelial cells and some elements of the intestinal mucosal lamina propria were performed on rats after bilateral high ureter ligation. Simultaneously, they were referred to blood serum biochemical and electrolyte disturbances. The ultrastructural changes indicate that as early as in the initial period of acute uraemia water-electrolyte disturbances become evident, which are still increasing. In the later period cell organelles alter and bacterial infection sets in. Ultrastructural abnormalities of the intestinal mucous membrane are increasing proportionally to biochemical and electrolyte disturbances of the blood serum.

Light microscopic histochemistry on plastic sections

As compared with conventional paraffin, celloidin, and frozen sections, semithin plastic sections offer a superior quality of the light microscopic image in terms of better resolution, absence of distortion and shrinkage artifacts, and suitability for calcified tissues. Application of histochemical methods to such sections often encounters, however, serious difficulties resulting from a considerably reduced reactivity of plastic-embedded biological material. Factors involved include a poor penetration of reagents into plastic embedding media due to a steric or hydrophobic hindrance, as well as a blockade of the reactive chemical groups in the sample due to interactions with fixatives and plastics. Embedding in polar (hydrophilic) plastics, such as glycol methacrylate, permits carrying out a large number of histochemical reactions, including the demonstration of enzymatic activities, directly on sections, but is less suitable for combined light/electron microscopic studies because of an imperfect ultrastructural preservation of tissues. Embedding in nonpolar epoxy resins, particularly if combined with a double aldehyde-osmium fixation, results in a high quality ultrastructure but almost fully inhibits the histochemical reactivity of the embedded material. In order to restore this reactivity, i.e. to unmask chemical groups bound by the polymerized resin, semithin epoxy sections require the removal of the embedding matrix by alkoxides prior to the histochemical procedure. Additional steps are also often necessary: treatment of osmium-fixed sections with oxidative agents, e.g., hydrogen peroxide or periodate which reoxidize the bound osmium and remove it from tissue, and a controlled proteolytic digestion, especially useful in immunocytochemical studies, which probably cleaves the bonds between the primary aldehyde fixative, and the reactive sites. This article reviews histochemical methods which have been successfully applied to plastic-embedded material. Using polar methacrylates and/or nonpolar epoxy resins as embedding media, it has been possible to demonstrate proteins and aminoacid residues, carbohydrates, lipids, nucleic acids, biogenic amines, inorganic ions, and some enzymes, although the spectrum of methods found as suitable for plastic-embedded material is far narrower than that available for paraffin or frozen sections.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructure of the daughter sporocyst and developing cercaria of Schistosoma japonicum in experimentally infected snails, Oncomelania hupensis hupensis

Ultrastructure of daughter sporocysts and cercariae of Schistosoma japonicum were studied 2 and 4 months after infection of Oncomelania hupensis hupensis. The body walls of daughter sporocysts are similar at all infectious stages. They consist of an external syncytial tegument on a basement membrane, and an internal cellular subtegument surrounding a body cavity containing developing cercariae. The cercariae embryos develop 2 months after infection from germinal balls in the brood chamber of the daughter sporocyst. They are at first enveloped by a primitive epithelium rising from the daughter sporocyst. Four months after infection, the cercariae were almost fully developed and the primitive epithelium had degenerated. The body wall of the cercaria consists of a thin tegument covered by a surface coat of fibrous material and connected to the subtegumental cells by cytoplasmic processes. The matrix of the tegument contains numerous dense bodies, vacuoles, and spines. Two types of sensory structures - uniciliated and multiciliated - are found at the anterior tip of the cercaria. There are five pairs of penetration gland cells of two distinct types differentiated by the morphology of secretory granules. Flame cells are found in both daughter sporocysts and in cercariae. The cilia of the flame cells are characterized by the typical 9 and 2 cilium pattern.

Effects of tunicamycin on thin-section and freeze-fracture images of microvilli of the duodenal epithelial cells of the mouse

The effect of tunicamycin, which is known to inhibit the synthesis of N-linked glycoprotein, on the duodenal absorptive epithelial cell of the mouse was studied in thin-section as well as freeze-fracture images. In tunicamycin-treated animals, the apical part of the epithelial cell was almost negative to the PAS reaction. Moreover, microvilli of the epithelial cell became shorter, larger in diameter, and fewer in number in tunicamycin-treated mice. In addition, freeze-fracture images revealed that the population density of membrane particles of the microvillus membrane was lowered by tunicamycin treatment. These results may indicate that the inhibition of synthesis of N-linked glycoprotein causes a decrease of membrane supply from the Golgi apparatus to the apical plasma membrane.

Biological and pathobiological aspects of the glycocalyx of the small intestinal epithelium. A review

The literature on the glycocalyx of small intestinal epithelium is reviewed. The structure, general and barrier functions, synthesis, and degradation of the glycocalyx, and pathobiological aspects of the glycocalyx in relation to its barrier function are mentioned. Topics for future research are indicated.

Surface ultrastructure of the small intestine mucosa in healthy children and adults: a scanning electron microscopic study with some methodological aspects

Biopsy specimens of light microscopically (LM) normal small intestine mucosa from eight healthy, constitutionally short-statured children without signs of gastrointestinal disease and six healthy adults were studied by scanning electron microscopy (SEM) supplemented by transmission electron microscopy (TEM). The effects on surface morphology of various preparative procedures were also investigated, using small intestine mucosa from cats and rats. Fixation with OsO4--either alone, or following glutaraldehyde fixation--markedly changed the surface ultrastructure compared to that after glutaraldehyde fixation only. By low power SEM, some differences were observed in the appearance of the small gut mucosa between adults and young children. In adults and in children above 3 years of age, the villi were usually shaped like fingers or leaves, but in infants, ridge-shaped villi predominated. The villi showed, however, a smooth surface in both infants and adults, and medium and high power SEM displayed similar pictures, irrespective of age; here the typical structural features of the normal small gut mucosa in humans were (1) distinct extrusion zones at the crests of the villi and almost no signs of enterocyte extrusion along the sides of the villi, and (2) regular enterocytes with polygonal, flat, apical surfaces covered by a thick glycocalyx that obscured the underlying microvilli.

Mapping of the bone marrow sinus endothelium with lectins and glycosylated ferritins: identification of differentiated microdomains and their functional significance

The distribution of lectin binding sites and sugar-recognizing systems (lectin-like substances) was studied on the luminal side of the bone marrow sinus endothelium in rats. Ferritin-conjugated lectins (Con A, PHA, RCA, WGA, UEA) and glycosylated ferritins (mannosyl, fucosyl, chitobiosyl) were used as probes. With the exception of UEA, all lectins bound to the endothelial surface. The binding was heavier on the plasmalemma proper of the nuclear region (PP-N) compared to that of the tapered region (PP-T). Lectin mapping also identified differentiated microdomains with less or no bindings. These domains related to the transport organelles in the endothelium: luminal vesicles (LV) and diaphragmmed fenestrae (DF). The relative density of binding for these four domains demonstrated the following spectrum PP-N greater than PP-T greater than LV greater than DF. The relation of this binding pattern to the transport function of marrow sinus endothelium for various cells and molecules has been discussed. Lectin mapping also identified heavily labeled microvilli and microprojections from the membrane. These organelles may have a function in recognizing cells and molecules for the transport. The absence of binding for UEA, also absent in liver endothelium, may differentiate sinus endothelia from other endothelia. This lectin is thought to be endothelium-specific in other systems. No binding was observed with glycosylated ferritins suggesting the absence of lectin-like substances on marrow sinus endothelium.

Structural and functional relationship between the membrane and the cytoskeleton in brush border microvilli

Electron microscopic and biochemical studies have described the organization and composition of microvilli from chicken intestinal brush borders. An actin-based cytoskeleton, composed of a paracrystalline core of bundled microfilaments, maintains the finger-like shape of the membrane through a helical array of membrane-microfilament linkages. Two proteins, fimbrin and villin, are components of the core bundle in situ and can independently bundle the actin filaments in vitro. Structural studies comparing microvillar core bundles with villin bundles and fimbrin bundles suggest that fimbrin, and not villin, is the major actin-filament-bundling protein in the microvillus core. These points, together with the capability of villin to sever actin filaments when activated by Ca2+, raise questions about villin's function in the microvillus. One possible explanation is that villin induces vesiculation of the membrane by disassembling the underlying cytoskeleton.

The spectrin-related molecule, TW-260/240, cross-links the actin bundles of the microvillus rootlets in the brush borders of intestinal epithelial cells

Previous studies have shown that molecules related to erythrocyte spectrin are present in the cortical cytoplasm of nonerythroid cells. We report here the localization by immunoelectron microscopy of one such molecule, TW-260/240, in the brush border of intestinal epithelial cells. Using highly specific antibodies against TW-260 and TW-240 as well as antibodies against fodrin, another spectrinlike molecule, we have found that the TW-260/240 molecules are displayed between rootlets at all levels of the terminal web. Occasionally, extended structures appear labeled suggestive of the fine filaments known to cross-link actin bundles. These results are in line with previous in vitro studies showing that TW-260/240 binds to, and cross-links, actin filaments. The results are discussed in terms of a model in which rootlets are immobilized in the terminal web in a matrix of TW-260/240.

Choroid plexus papilloma. Light and electron microscopic study

Choroid plexus papilloma (CPP) was observed by light and electron microscopy, using surgically excised tissues in 7 cases and cultivated cells. CPP cells had numerous microvillous processes showing balloon-like features, lysosomes containing haemosiderin, and highly electron-dense irregular granules about 150-300 nm in diameter. Interstitial cells with highly electron-dense cytoplasm inserted their long and thin processes into the invagination of basal plasmalemma of CPP cells, occasionally breaking down the basal lamina. Many of them were located in the intercellular space among CPP cells, sometimes adhering to the ventricular surface of CPP cells. Ruthenium red stain was positive on the surface of CPP cells and was especially intense on the surface of microvilli and cilia. In culture, CPP cells and interstitial cells migrating from the CPP cell mass showed a phagocytic activity after treatment with Latex.