The ultrastructural localization of antigen B10 of the hepatocyte plasma membrane in mouse hepatomas

The ultrastructure of the murine hepatocyte plasma membrane antigen (Ag B10) was studied by immunoelectron microscopy in 5 spontaneous and 3 chemical-induced hepatomas. Ag B10 was associated with plasmalemma of bile canaliculi and membrane of microvilli as in normal liver. Sometimes it was connected with plasmalemma of lateral domain of tumor cells. The availability of Ag B10 in the matrix of bile canaliculi and within microvilli was shown.     

Structural polarity and functional bile canaliculi in rat hepatocyte spheroids

Primary hepatocytes self-assemble into spheroids that possess tight junctions and microvilli-lined channels. We hypothesized that polarity develops gradually and that the channels structurally and functionally resemble bile canaliculi. Immunofluorescence labeling of apical and basolateral proteins demonstrated reorganization of the membrane proteins into a polarized distribution during spheroid culture. By means of fluorescent dextran diffusion and confocal microscopy, an extensive network of channels was revealed in the interior of the spheroids. These channels connected over several planes and opened to pores on the surface. To examine the content of apical proteins in the channel membranes, the bile canalicular enzyme dipeptidyl peptidase IV (DPPIV) was localized using a fluorogenic substrate, Ala-Pro-cresyl violet. The results show that DPPIV activity is heterogeneously distributed in spheroids and localized in part to channels. Bile acid excretion was then investigated to demonstrate functional polarity. A fluorescent bile acid analogue, fluorescein isothiocyanate-labeled glycocholate, was taken up into the spheroids and excreted into bile canalicular channels. Due to the structural polarity of spheroids and their ability to excrete bile into channels, they are a unique three-dimensional model of in vitro liver tissue self-assembly. (Videoanimations of some results are available at http://hugroup.cems.umn.edu/research_movies).     

Motility of bile canaliculi in the living animal: implications for bile flow

Modern fluorescence microscopic techniques were used to image the bile canalicular system in the intact rat liver, in vivo. By combining the use of sodium fluorescein secretion into bile, with digitally enhanced fluorescence microscopy and time-lapse video, it was possible to capture and record the canalicular motility events that accompany the secretion of bile in life. Active bile canalicular contractions were found predominantly in zone 1 (periportal) hepatocytes of the liver. The contractile movements were repetitive, forceful, and appeared unidirectional moving bile in a direction towards the portal bile ducts. Contractions were not seen in the network of canaliculi on the surface of the liver. Cytochalasin B administration resulted in reduced canalicular motility, progressive dilation of zone 1 canaliculi, and impairment of bile flow. Canalicular dilations invariably involved the branch points of the canalicular network. The findings add substantively to previous in vitro studies using couplets, and suggest that canalicular contractions contribute physiologically to bile flow in the liver.     

The ultrastructural localization and quantitation of cholinergic receptors at the mouse motor endplate

Summary The snake venom toxin, -bungarotoxin, is known to bind specifically to the acetylcholine receptor at skeletal muscle endplates. In this study, tritiated -bungarotoxin has been used in conjunction with electron-microscope autoradiography to visualize and enumerate acetylcholine receptor sites at the neuromuscular junctions of the mouse diaphragm. From an analysis of the grain distribution, the receptor sites appear to be located specifically on the postjunctional membrane. The density there is about 8,500/² of membrane surface. For comparison purposes, cholinesterases and related active centers were labeled using [³H] diisopropylfluorophosphate; they were shown to be at this same concentration over the synaptic membranes (or along the cleft). The 11 relationship of the receptors to the cholinesterase type of site, found previously to hold in studies on whole endplates, is also true at the ultrastructural level in this case. In fact, this 11 relationship is believed to be a characteristic of the postsynaptic membranes of endplates in other muscles and other vertebrates.Based on the constant density value thus arrived at, the total surface areas of postsynaptic and of presynaptic membranes are at once obtained from the known total numbers of these sites per endplate, available from previous studies in this laboratory. Examples of such synaptic surface area values are given. These values are only reliable for a given muscle type if the approximate fiber size is defined.     

Formation of bile canaliculi in long-term primary cultures of adult rat hepatocytes on permeable membrane: an ultrastructural study

Adult rat hepatocytes were cultured for 15 days on type I collagen-coated permeable membranes in a hormonally defined Waxman's modified medium supplemented with very low concentrations of insulin, glucagon and dexamethasone. Phase contrast examination showed that 15-day-old cultures still formed a regular monolayer of polygonal cells. In similarly aged cultures, intracellular glycogen was abundant and evenly distributed, while steatosis remained very limited. Scanning and transmission electron microscopy showed that well developed bile canaliculi could be observed on the lateral side of the hepatocyte membrane after 4 days of incubation and persisted for 2 weeks. These canalicular structures probably originated from coalescence of membrane invaginations observed in 1-day-old cultures. Transmission electron microscopy showed that the ultrastructure of the cells was very close to that of normal rat hepatocytes in the intact liver. These results suggest that rat hepatocytes cultured under these experimental conditions are able to develop and maintain tissue-specific cytochemical and morphological properties for at least 15 days.     

Histological and ultrastructural localization of antigen B in the metacestode of Taenia solium

The morphological localization of antigen B (AgB) in the tissues of the Taenia solium metacestode was studied by immunological and biochemical methods. Indirect immunofluorescence carried out on vibratome sections showed that AgB is widely distributed throughout the tissue. A more intense fluorescence was observed in the tegumentary cytons of the bladder wall and in the lumen of the spiral canal of the invaginated scolex. Ultrastructural analysis of larvae washed in PBS after dissection from meat and then incubated with rabbit antibodies against AgB, followed by peroxidase-labeled goat anti-rabbit IgG, did not exhibit electron-dense material on the external surface. Larvae fixed in glutaraldehyde immediately after dissection and exposed to the immunoperoxidase reagents did exhibit electron-dense material on microtriches, indicating that AgB is only loosely bound to the external surface. Crude extracts of surface-radioiodinated cysticerci analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) contained no labeled proteins with the molecular weight of AgB. Autoradiography of the immunoelectrophoretograms in which the crude extract was confronted with antibodies to AgB demonstrated that this antigen was not labeled, and therefore is not exposed on the tegumentary surface. The results suggest that AgB is synthesized by the tegumentary cytons of the parasite and secreted through the tegumental membrane into the host tissues and the lumen of the spiral canal.     

A domain-specific marker for the hepatocyte plasma membrane: localization of leucine aminopeptidase to the bile canalicular domain

Indirect immunofluorescence was used to establish a domain-specific marker for hepatocyte plasma membranes. In frozen sections of fixed rat liver (0.5-4 microns), antibodies directed against rat intestinal leucine aminopeptidase (LAP) recognized an antigen that was restricted to the bile canalicular plasma membrane. Fluorescence was not observed on the sinusoidal or lateral membranes, and intracellular staining was not detected. The liver antigen was identified as LAP, based on its chemical similarity to intestinal LAP. First, immunoprecipitation experiments using trypsin-solubilized intestinal LAP (G-200 fraction, 91% pure) established a correlation between the loss of LAP enzyme activity from the soluble fraction and the appearance in the specific immunoprecipitates of polypeptides migrating on SDS PAGE between 110,000 and 130,000 daltons. The antigen precipitated from a detergent extract of liver plasma membranes had the same electrophoretic mobility. Second, the chymotryptic map of the major band in the liver immunoprecipitate was similar to that of purified intestinal LAP.     

The localization of the antibody response in milk or bile depends on the nature of the antigen

Immunization in the Peyer's patches of rats with horse spleen ferritin or Escherichia coli 06 carrying type 1 pili resulted in an IgA antibody response detected in milk and bile and an IgG and IgM antibody response in serum, milk, and bile. The IgA antibody response to type 1 pili was as a mean 5.0-fold higher in milk than in bile. In contrast IgA antibody activity to 06 LPS was as a mean 6.3-fold higher in bile than in milk. The IgA antibodies to ferritin were randomly distributed between milk and bile. The IgG and IgM antibody activity to all three antigens studied were higher in the milk than in the bile. The secretory antibody response could be transferred from immunized rats to unimmunized rats with mesenteric lymph node cells (MLN) taken from donor rats 4 days after immunization in the Peyer's patches. IgA antibodies to pili and ferritin appeared solely in the milk of the recipients, whereas IgA antibodies to the 06 LPS only appeared in the bile. The ratios serum:milk and serum:bile for the IgG and IgM antibodies indicated an antigen-specific direction of homing with local production of these two isotypes primarily in the mammary gland. Antibody-forming cells of the IgA class could not be detected in the MLN on the day the cells were transferred. It is concluded that the difference seen in antibody distribution between milk and bile is not due to dissemination of antigen, but instead a result of different homing or expansion at the mucosal-glandular site dependent on the antigen specificity of the migrating cells.     

Spontaneous formation of intercellular bile canaliculi and hybrid biliary-pancreatic canaliculi in co-culture of hepatocytes and exocrine pancreas cells from carp

When cultured together in a primary serum-free hormone-free system, hepatocytes and exocrine pancreas cells from the carp, Cyprinus carpio, spontaneously establish unique morphological structures that do not occur in vivo. These structures include intercellular bile canaliculi between neighbouring hepatocytes and hybrid canaliculi between hepatocytes and pancreas cells. In vivo, carp hepatocytes form only unicellular bile canaliculi; hybrid canaliculi between hepatocytes and exocrine pancreas cells do not exist at all in nature. This study shows that, in an artificial environment, cells are able spontaneously to establish novel morphological structures that are absent in the animal from which the cells have been obtained. Received: 3 January 1996 / Accepted: 17 March 1997     

Immunohistochemical localization of type IV collagen in mouse tooth germ: an ultrastructural study

Localization of type IV collagen was analyzed at the ultrastructural level in mouse embryonic molars by using a preembedding technique. Cryostat sections were incubated with type IV collagen antibody and then treated with the peroxidase-antiperoxidase complex. This antibody was visualized at the epithelio-mesenchymal interface. Labeling was intense and uniformly distributed throughout the basement membrane. However, it was mainly restricted to the lamina densa. No immunostaining was detectable in the lamina lucida but it was crossed by fine filaments that appeared as projections from the lamina densa to the epithelial cell plasma membrane. At the mesenchymal aspect of the basement membrane, projections of labeled material extended from the lamina densa in the underlying dental mesenchyme. At the presecretory stage of odontoblasts, these projections were in close connection with mesenchymal cell processes.     

Effects of total parenteral nutrition on rat bile canaliculi: a stereologic analysis

OBJECTIVE: To quantitate, in a stereologic manner, changes in bile canalicular morphology before and after choleretic infusion of total parenteral nutrition (TPN) and to determine whether TPN produces changes in localized regions within the hepatic lobule. STUDY DESIGN: Livers were obtained from sham-operated on normal adult male rats (control) and from rats that received intravenous TPN solution containing 20% glucose and 3.5% Molipron F. The tissues, obtained by a rigorous sampling procedure, were systematically subjected to stereologic analysis. Measurements were made on electron micrographs at two levels of magnification by point, intersection and profile counts, and then volume, surface area and length were estimated per unit parenchymal volume. RESULTS: The surface area of the canalicular wall per parenchymal volume increased significantly (from 5.33 x 10(-2) to 6.73 x 10(-2) microns 2/micron 3) after TPN treatment, as did the length of microvilli (from 0.241 to 0.267 microns/micron 3). However, the volume of bile canalicular lumina per parenchymal volume (0.306% and 0.320%), total length of bile canaliculi (1.05 x 10(-2) and 1.06 x 10(-2) microns/micron 3) and diameter of microvilli (8.73 x 10(-2) and 8.94 x 10(-2) microns) remained constant. CONCLUSION: These results indicate that changes in canalicular shape and microvillus hypertrophy may cause lowering efficiency of the bile flow rate.     

How to induce non-polarized cells of hepatic origin to express typical hepatocyte polarity: generation of new highly polarized cell models with developed and functional bile canaliculi

Few in vitro models expressing complex hepatocyte polarity are available. We used the unpolarized rat Fao cell line to isolate the polarized WIF-B line. These complex rat-human hybrid cells form functional simple bile canaliculi. To obtain Fao-derived polarized models with a simpler chromosome content and developed bile canaliculi, we employed two approaches. Partial success was achieved with monochromosomal hybrids. As shown by the immunolocalization of apical, basolateral, and tight-junctional proteins, monochromosomal hybrid 11-3 cells were polarized. They formed simple functional bile canaliculi and transiently expressed the typical polarity of simple epithelial cells. One subclone blocked in this polarity state was isolated. A more robust approach was provided by spheroid culture, a three-dimensional system that strengthens cell-cell contacts. Transient spheroid culture induced irreversible polarization of Fao cells. This induction occurred in most spheroids (approximately 1% of the cells). From populations enriched in stably polarized cells, we generated new polarized cell models, designated Can. Can 3-1 cells formed simple functional bile canaliculi when plated at high density. Regardless of plating density, Can 9 and Can 10 cells formed long tubular branched canaliculi competent for vectorial transport of organic anions and bile acids, and involving several dozen adjacent cells. Thus, we have generated new cell models stably expressing typical hepatocyte polarity. Among these models, Can 9 and Can 10 are the first capable of forming functional, highly developed bile canaliculi similar to those formed in vivo. This work was supported by grants from the Association pour la Recherche sur le Cancer (no.6551), the Institut Curie (PIC Signalisation Cellulaire, no. 914) and the Institut National de la Santé et de la Recherche Médicale (contract PRISME 98-09).     

Adhesive stability of the ultrastructural elements of hepatocyte contacts

A new method is suggested for the estimation of cell junction adhesion in the liver. It was shown that simple (spacing) junctions of hepatocytes consisted of low and high adhesive parts (sites). The hepatocyte junctions were positioned in the following sequence according to the increase of their adhesive durability; 1) low adhesive sites of simple junction and stude like junctions; 2) high adhesive sites of simple junction and desmosomes; 3) zonulae adherentes; 4) gap and tight junctions.