Tannic acid as an electron microscope tracer for permeable cell membranes

Isolation of extracellular vesicles from byproducts of cheesemaking by tangential flow filtration yields heterogeneous fractions of nanoparticles

Extracellular vesicles (EV) in milk, particularly exosomes, have attracted considerable attention as bioactive food compounds and for their use in drug delivery. The utility of small EV in milk (sMEV) as an animal feed additive and in drug delivery would be enhanced by cost-effective large-scale protocols for the enrichment of sMEV from byproducts in dairy plants. Here, we tested the hypothesis that sMEV may be enriched from byproducts of cheesemaking by tangential flow filtration (EV-FF) and that the sMEV have properties similar to sMEV prepared by ultracentrifugation (sMEV-UC). Three fractions of EV were purified from the whey fraction of cottage cheese making by using EV-FF that passed through a membrane with a 50-kDa cutoff (50 penetrate; 50P), and subfractions of 50P that were retained (100 retentate; 100R) or passed through (100 penetrate; 100P) a membrane with a 100-kDa cutoff; sMEV-UC controls were prepared by serial ultracentrifugation. The abundance of sMEV (<200 nm) was less than 0.3% in EV-FF compared with sMEV-UC (10¹²/mL of milk). Despite the low EV count, the protein content (mg/mL) of 100R (63 ± 0.02; ± standard deviation) was higher than that of 50P (0.75 ± 0.10), 100P (0.65 ± 0.40), and sMEV-UC (27 ± 0.02). There were 17, 14, 35, and 75 distinct proteins detected by nontargeted mass spectrometry analysis in 50P, 100R, 100P, and sMEV-UC, respectively. Exosome markers CD9, CD63, CD81, HSP-70, PDCD6IP, and TSG101 were detected in control sMEV-UC but not in EV-FF by using targeted mass spectrometry and immunoblot analyses. Negative exosome markers, APOB, β-integrin, and histone H3 were below the limit of detection in EV-FF and control sMEV-UC analyzed by immunoblotting. The abundance of the major milk fat globule protein butyrophilin showed the following pattern: 100R ≫ 100P = 50P > sMEV-UC. More than 100 mature microRNA were detected in sMEV-UC by using sequencing analysis, compared with 36 to 60 microRNA in EV-FF. Only 100R and sMEV-UC yielded mRNA in quantities and qualities sufficient for sequencing analysis; an average of 276,000 and 838,000 reads were mapped to approximately 14,600 and 18,500 genes in 100R and sMEV-UC, respectively. In principal component analysis, microRNA, mRNA, and protein in EV-FF preparations clustered separately from control sMEV-UC. We conclude that under the conditions used here, flow filtration yields a heterogeneous population of milk EV.

Mycobacterium tuberculosis Cells Surviving in the Continued Presence of Bactericidal Concentrations of Rifampicin in vitro Develop Negatively Charged Thickened Capsular Outer Layer That Restricts Permeability to the Antibiotic

Majority of the cells in the bacterial populations exposed to lethal concentrations of antibiotics for prolonged duration succumbs to the antibiotics' sterilizing activity. The remaining cells survive by diverse mechanisms that include reduced permeability of the antibiotics. However, in the cells surviving in the continued presence of lethal concentrations of antibiotics, it is not known whether any cell surface alterations occur that in turn may reduce permeability of the antibiotics. Here we report the presence of a highly negatively charged, hydrophilic, thickened capsular outer layer (TCOL) on a small proportion of the rifampicin surviving population (RSP) of Mycobacterium tuberculosis (Mtb) cells upon prolonged continuous exposure to bactericidal concentrations of rifampicin in vitro. The TCOL reduced the intracellular entry of 5-carboxyfluorescein-rifampicin (5-FAM-rifampicin), a fluorochrome-conjugated rifampicin permeability probe of negligible bacteriocidal activity but comparable properties. Gentle mechanical removal of the TCOL enabled significant increase in the 5-FAM-rifampicin permeability. Zeta potential measurements of the cells' surface charge and hexadecane assay for cell surface hydrophobicity showed that the TCOL imparted high negative charge and polar nature to the cells' surface. Flow cytometry using the MLP and RSP cells, stained with calcofluor white, which specifically binds glucose/mannose units in β (1 → 4) or β (1 → 3) linkages, revealed the presence of lower content of polysaccharides containing such residues in the TCOL. GC-MS analyses of the TCOL and the normal capsular outer layer (NCOL) of MLP cells showed elevated levels of α-D-glucopyranoside, mannose, arabinose, galactose, and their derivatives in the TCOL, indicating the presence of high content of polysaccharides with these residues. We hypothesize that the significantly high thickness and the elevated negative charge of the TCOL might have functioned as a physical barrier restricting the permeability of the relatively non-polar rifampicin. This might have reduced intracellular rifampicin concentration enabling the cells' survival in the continued presence of high doses of rifampicin. In the context of our earlier report on the de novo emergence of rifampicin-resistant genetic mutants of Mtb from the population surviving under lethal doses of the antibiotic, the present findings attain clinical significance if a subpopulation of the tubercle bacilli in tuberculosis patients possesses TCOL.

Tannic acid, a novel histone acetyltransferase inhibitor, prevents non-alcoholic fatty liver disease both in vivo and in vitro model

Objective

We examined the potential of tannic acid (TA) as a novel histone acetyltransferase inhibitor (HATi) and demonstrated that TA prevents non-alcoholic fatty liver disease (NAFLD) by inhibiting HAT activity.

Methods

The anti-HAT activity of TA was examined using HAT activity assays. An in vitro NAFLD model was generated by treating HepG2 cells with oleic and palmitic acids. Male C57BL/6J mice were fed a control diet (CD) or Western diet (WD) with or without supplementation with either 1% or 3% TA (w/w) for 12 weeks. Finally, the possibility of interacting p300 and TA was simulated.

Results

TA suppressed HAT activity both in vitro and in vivo. Interestingly, TA abrogated occupancy of p300 on the sterol regulatory element in the fatty acid synthase and ATP-citrate lyase promoters, eventually inducing hypoacetylation of H3K9 and H3K36. Furthermore, TA decreased acetylation at lysine residues 9 and 36 of histone H3 protein and that of total proteins. Consequently, TA decreased the mRNA expression of lipogenesis-related genes and attenuated lipid accumulation in vivo. We observed that NAFLD features, including body weight, liver mass, fat mass, and lipid profile in serum, were improved by TA supplementation in vivo. Finally, we demonstrated the possibility that TA directly binds to p300 through docking simulation between ligand and protein.

Conclusions

Our findings demonstrate that TA, a novel HATi, has potential application for the prevention of NAFLD.

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Tannic acid is a general inhibitor of histone acetyltransferase.

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Tannic acid decreases transcriptional activity of the lipogenesis-related genes through its HATi activity.

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Tannic acid ameliorates non-alcoholic fatty liver disease in the western diet-fed mice through its HATi activity.

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Tannic acid binds to EP300, possibly reducing its activity through inducing conformational change of EP300.

Discs of mammalian rod photoreceptors form through the membrane evagination mechanism

Ultrastructural analysis of membrane organization in mouse rod cells demonstrates that new photoreceptor discs are contiguous evaginations of the outer segment plasma membrane, thereby resolving the debate of whether discs are formed through membrane evagination or intracellular vesicular fusion.

Photoreceptor discs are membrane organelles harboring components of the visual signal transduction pathway. The mechanism by which discs form remains enigmatic and is the subject of a major controversy. Classical studies suggest that discs are formed as serial plasma membrane evaginations, whereas a recent alternative postulates that discs, at least in mammalian rods, are formed through intracellular vesicular fusion. We evaluated these models in mouse rods using methods that distinguish between the intracellular vesicular structures and plasma membrane folds independently of their appearance in electron micrographs. The first differentiated membranes exposed to the extracellular space from intracellular membranes; the second interrogated the orientation of protein molecules in new discs. Both approaches revealed that new discs are plasma membrane evaginations. We further demonstrated that vesiculation and plasma membrane enclosure at the site of new disc formation are artifacts of tissue fixation. These data indicate that all vertebrate photoreceptors use the evolutionary conserved membrane evagination mechanism to build their discs.

[Functional morphology of ischemic cardiomyopathy]

OBJECTIVE

To show that ischemic cardiomyopathy (ICM) that is generally believed to be a variant of chronic heart disease (CHD) and an analogue of chronic heart failure is an initial stage and the background of all further forms of CHD.

SUBJECTS AND METHODS

Sixty cardiac biopsies obtained from patients during aortocoronary bypass surgery for CHD and myocardia from 55 rabbits in which the investigators simulated atherosclerosis over time, as well as acute, transient, and recurrent coronary insufficiency after vasopressin administration were examined. A number of adequate histological, histochemical, and immunohistological procedures, electron microscopic and electron histochemical studies, and morphometry were employed. The investigators made biochemical monitoring of blood cholesterol and triglyceride levels and ECG monitoring in the animals had received intravenous vasopressin.

RESULTS

The authors have indicated that CHD begins with endothelial changes in the trophic component of the microhemocirculatory bed, which gives rise to its impaired blood flow and to hypoxia at the level of the myocardial structural and functional unit--fascicules. Over time, the disease develops a set of processes that cause damage to microvessels, cardiomyocytes, stroma, lymph capillaries, and nerve endings and changes in secretion. Compensatory and adaptive responses that long mask the symptoms of damage to the vascular bed and myocardial tissue take place simultaneously. The occurring lesions result in irregular diffuse cardiosclerosis, micro- and lymphocirculatory, innervation, and conduction disturbances, and irregular cardiomyocyte damage. The authors consider this set of myocardial changes to be an initial phase of ICM that shows no clinical manifestation at all. As coronary atherosclerosis progresses, the already damaged myocardium is superimposed by the morphological changes caused by higher hypoxia at the whole organ level. Depending upon a combination of these changes, one or another form of CHD and progression of chronic CHD may prevail at a later time.

CONCLUSION

ICM is a broader concept than chronic heart failure and it is a basis for the pathogenesis and morphogenesis of CHD.

Postovulatory follicle: a model for experimental studies of programmed cell death or apoptosis in teleosts

This is the first evidence of programmed cell death, or apoptosis, occurring in the postovulatory follicle (POF) of teleost fish. Females of Astyanax bimaculatus lacustris were submitted to induced ovulation through injecting pituitary extract. Ultrastructural analyses of POFs at time intervals varying from zero to four days postspawning showed several characteristic events of the apoptosis. Typical apoptotic figures, such as nucleus with chromatin condensation underlying the nuclear envelope in a crescent pattern and apoptotic bodies at different stages of formation and reabsorption, were observed in the follicular cells a few days after the onset of the postovulatory period. The results indicated that apoptosis is the major mechanism responsible for the elimination of the follicular cells in the POFs of A. bimaculatus lacustris during ovarian recovery postspawning. It is suggested that POFs might be used as an experimental model in dynamic studies involving cell death in teleosts.

Tannic acid supplemented fixation improves ultrastructural evaluation of respiratory epithelium in children with recurrent respiratory tract infections

Samples of the respiratory mucosa of children with recurrent respiratory infections suspected of having primary ciliary dyskinesia are routinely fixed with glutaraldehyde before ultrastructural examination. This standard technique, however, may not be optimal for visualizing ciliary components or for preserving several cellular and extracellular structures during dehydration and embedding procedures. In this study, brushes of nasal (28 samples) and/or tracheal (9 samples) mucosa from 32 children with recurrent respiratory tract infections were examined. Twenty-nine samples were fixed with glutaraldehyde supplemented with tannic acid to determine if the ultrastructural analysis of respiratory epithelium and bronchial secretions could be improved. Eight samples were conventionally fixed with glutaraldehyde alone. Lesions of the cellular membrane and damaged cells were easily visualized using tannic acid supplemented fixation. Internal ciliary structures including individual microtubules and dynein arms were also more clearly observed. In addition, the internal structure of microvilli of the respiratory epithelium could be studied and the presence of phospholipid-rich surfactant-like material within nasal and tracheal secretions were visualized after tannic acid supplemented fixation. We suggest that addition of tannic acid during fixation is useful for accurate ultrastructural evaluation of respiratory mucosa in both clinical and experimental situations.

Ciliated cell damage in the bronchial epithelium of asthmatics and non-asthmatics

The importance of bronchial epithelial shedding in the pathogenesis of asthma has been highlighted by many investigators as a potential mechanism for bronchial hyperresponsiveness. It has been suggested that this disruption is the result of cytotoxic injury leading to shedding of damaged cells. To investigate whether damaged ciliated epithelial cells can be detected within the bronchial mucosa, we used tannic acid which only permeates disrupted cellular membranes, as a marker of cell damage. Bronchial biopsies from seven asthmatic and six normal subjects, were processed in tannic acid prior to preparation and sectioning for electronmicroscopic examination. Ciliated epithelial cells staining darkly with tannic acid were seen to comprise a similar proportion of the intact portion of bronchial epithelium in both normals and asthmatics (medians 31% vs 40%). We suggest that ciliated epithelial cells are not shed from the bronchial mucosa immediately after damage and that mechanisms other than granulocyte-mediated cytotoxicity may account for epithelial disruption in asthma, possibly involving the selective damage or reduced expression of intraepithelial intercellular adhesion molecules.

Degeneration of osteoblasts involved in intramembranous ossification of fetal rat calvaria

Ossification of calvariae from day-21 rat fetuses was reinvestigated by electron microscopy using different fixation techniques (glutaraldehyde/OsO4, tannic acid, ruthenium red, K-pyroantimonate). An osteoid layer with scattered mineral deposits was found at the mineralization front. Directly beyond this layer, a sheet of one to two layers of necrotic and degenerating osteoblasts was present. Above this sheet, normal and healthy cells were seen, formed by six to eight layers of flattened cells, embedded in a collagenous matrix. The osteoblasts on the less mineralizing opposite side of the calcified calvariae and the osteocytes embedded in the calcified calvariae appeared healthy. Closer inspection of the necrotic zone revealed apatite crystals in vesicles which most probably originated from mitochondria of the degenerated cells. Large K-pyroantimonate deposits were found throughout the osteoid and the necrotic zone, whereas only small granules were scattered in the cytoplasm and at the plasma membrane of the healthy cells directly adjacent to the necrotic zone. A concept of intramembranous mineralization is outlined, according to which osteoblasts store enormous amounts of calcium, which are liberated by physiological cell death in the vicinity of the mineralizing front.

Leakiness of gastric superficial and foveolar cells. A quantitative electron microscopic study using tannic acid

Two adjacent antral biopsies were taken endoscopically from 41 patients attending a clinic for the investigation of upper gastrointestinal tract symptoms. One was processed for histopathological grading and assessment of the Helicobacter pylori by light microscopy. The other was fixed in tannic acid, which demonstrates cell leakiness, and processed for electron microscopy. On each grid, the light (electron lucent) and dark (electron dense-leaky) gastric superficial and foveolar cells away from mechanical trauma of biopsy were counted, along with the number of H. pylori which were in contact with the epithelial cells. There was a significant relationship between histological grade of gastritis and the extent of electron microscopic damage, i.e., the degradation of normal to leaky cells and the number of H. pylori per leaky cell. If the patients were divided into two groups with minimal and maximal gastritis and compared against each other, then there was a significant difference between the two groups with respect to surface cell damage, polymorphs per unit area, the number of glands invaded by polymorphs, light/dark cells, Helicobacter grades assessed light microscopically or totals counted in contact with epithelial cells by electron microscopy, the number of Helicobacter per cell, and the age of the patients. There was no difference between the two groups on their endoscopic appearances or treatment with H2 blockers or NSAIDS.

Two different secretion mechanisms in the inner ear's interdental cells

Fixation with a mixture of tannic acid and glutaraldehyde reveals a darkly stained substance in the intercellular clefts between juxtapposed interdental cells, where it is included by pynocytotic vesicles. Over the interdental cells, similar-sized vesicles (35-45 nm) are present in clear spaces of the amorphous layer of the limbal portion of the tectorial membrane. Some images suggest that they may be secreted through small disruptions of the membrane of the interdental cells' microvilli. In addition, a TA-unstained, amorphous material is present inside both basal ducts and apical cavities of the interdental cells, thus suggesting another secretory route from the basal region of the interdental cells towards either the endolymph, or the tectorial membrane. These two secretion processes coexist in a single interdental complex, and the two secretion products may be involved in the turnover of the adult tectorial membrane and/or the secretion of some component of the endolymph.

An improved method for preserving the cellular membrane ultrastructure of in vitro cultivated cells using tannic acid

Treatment of cultured cells with tannic acid after fixation with glutaraldehyde and osmification prevents the ultrastructural deterioration of cellular structures during dehydration and impregnation. Membranes of tannic acid-treated cells showed an increased contrast and sharp delineation of the polar zone. These results are discussed on the basis of simultaneous interactions of tannic acid with saturated phospholipids and OSO4. No ultrastructural differences are found between cells treated with tannic acids of different molecular weights.

Tannic acid enhances staining of microtubule associated proteins, but impairs neuronal physiology

Exposure of the buccal ganglion of the mollusc, Helisoma trivolvis, to tannic acid ringer (TARI), pH 6.8, permits the ultrastructural detection of exocytotically released material within the neuropil. At pH 4.0, tannic acid penetrated cell membranes and resulted in a marked enhancement in the electron density of regularly speed microtubule associated proteins. Electrophysiological analysis of neurons 19, 4 and 5 exposed to TARI, pH 6.8 and 4.0, demonstrated that the TARI procedure altered the neuronal firing patterns in an irreversible manner. We conclude that the neurotoxic effects of TARI preclude its usefulness in quantitative ultrastructural analysis of exocytotic events, but that at pH 4.0 it is useful for ultrastructural investigations of microtubules and their associated proteins.

Subsurface material in outer hair cells

Tannic acid stains a homogenous material inside the outer hair cells of the organ of Corti of the guinea pig. This material is always placed between the plasma membrane and the first layer of subsurface cisterns, but only in those areas along the lateral surface of the outer hair cell lining the spaces of Nuel. The possibility that this material is related to some particular function of outer hair cell lateral face is discussed.

Some limitations of the use of tannic acid as a marker of damaged skeletal muscle fibres

Mouse biceps brachii muscles containing single transverse incisions were treated with low molecular weight tannic acid. In the absence of fibre degeneration, intracellular spread of tannic acid was confined to a short distance along the fibres from the damaged portions of the cell membranes. Beyond this, damaged and undamaged fibres could not be distinguished from one another. Tannic acid is not a reliable marker of localized trauma to skeletal muscle fibres.

Hyperthermia-induced vascular injury in normal and neoplastic tissue

The sequential morphologic alterations in normal skeletal muscle in rats, Walker 256 tumors in rats, and transmissible venereal tumors (TVT) in dogs following microwave-induced hyperthermia (43 degrees C and 45 degrees C for 20 minutes), were studied by histologic and ultrastructural examination. Normal muscle and Walker 256 tumors showed edema, congestion, and hemorrhage at 5 minutes post-heating (PH), followed by suppuration, macrophage infiltration, and thrombosis at 6 and 48 hours PH, and finally by regeneration and repair by 7 days PH. Vascular endothelial damage and parenchymal degeneration were present 5 minutes PH. Progressive injury occurred for at least 48 hours PH. Two hyperthermia treatments separated by a 30- or 60-min cooling interval, were applied to Walker 256 tumors in a subsequent study. Increased selective heating of tumor tissue versus surrounding normal tissue, and increased intratumoral steady state temperatures were found during the second hyperthermia treatment. Canine TVTs were resistant to hyperthermia damage. These results suggest that vascular damage contributes to the immediate and latent cytotoxic effects of hyperthermia in normal tissue and some types of neoplastic tissue, and that selective heating of neoplastic tissue occurs in tumor tissue with disrupted microvasculature.

Hyperthermia-induced vascular injury in normal and neoplastic tissue

The sequential morphologic alterations in normal skeletal muscle in rats, Walker 256 tumors in rats, and transmissible venereal tumors (TVT) in dogs following microwave-induced hyperthermia (43 degrees C and 45 degrees C for 20 minutes), were studied by histologic and ultrastructural examination. Normal muscle and Walker 256 tumors showed edema, congestion, and hemorrhage at 5 minutes post-heating (PH), followed by suppuration, macrophage infiltration, and thrombosis at 6 and 48 hours PH, and finally by regeneration and repair by 7 days PH. Vascular endothelial damage and parenchymal degeneration were present 5 minutes PH. Progressive injury occurred for at least 48 hours PH. Two hyperthermia treatments separated by a 30- or 60-min cooling interval, were applied to Walker 256 tumors in a subsequent study. Increased selective heating of tumor tissue versus surrounding normal tissue, and increased intratumoral steady state temperatures were found during the second hyperthermia treatment. Canine TVTs were resistant to hyperthermia damage. These results suggest that vascular damage contributes to the immediate and latent cytotoxic effects of hyperthermia in normal tissue and some types of neoplastic tissue, and that selective heating of neoplastic tissue occurs in tumor tissue with disrupted microvasculature.

Lamellar bodies in differentiating insect tissues during basal lamina formation as revealed by tannic acid

Tannic acid penetrates differentiating tissues differentially resulting in variable contrast, extraction and dense bodies with a lamellar substructure. The penetrability appears to correlate with the existence and/or robustness of a basal lamina. In the male genital tract, probably of mesodermal origin, tannic acid penetrates the epithelium until there is a basal lamina, but in the ectodermal bursa copulatrix it does not penetrate since there is always a basal lamina. The lamellae of the dense bodies have a center-to-center spacing of 4.65 +/- 0.025 nm, dimensions which resemble those of phospholipids.

Development of mouse embryos grown in human cord serum (HCS) in vitro

Mouse embryos of the blastocyst stage (day 0 of culture) were cultured in human cord serum (HCS) for 8 days. Until day 7 in vitro, the embryos had developed to the 4-5 somite stage. Apart from the occurrence of some cell necroses in the embryonic ectoderm and vacuoles in the yolk sac entoderm and to a lesser degree even in the embryonic entoderm, the in vitro development of the embryos was indistinguishable from the in vivo development. However, between days 7 and 8 in vitro the embryonic development was heavily disturbed. The most pronounced disturbances were visible in the central nervous system (CNS) of the embryos. A great number of the neuroepithelial cells of the entire CNS were necrotic. Even mesodermal structures, mainly the axial structures in the posterior part of the embryos, were affected, leading to disorganization of somites and notochord in this region. Since in the presence of rat serum embryos develop normally between days 7 and 8 in vitro (Hsu 1981; Chen and Hsu 1982), it was stated that HCS lacks an embryonic growth and differentiation factor (EGDF-4) present in rat serum which regulates the development of the CNS and the organization of the axial structures between days 7 and 8 in vitro, i.e., during the 4-5 somite stage and the time of the turning of the embryos (8-12 somite stage).

Plasma membrane specializations in the cells of the kidney distal segment of the lamprey, Lampetra japonica (von Martens)

The unique and highly specialized structural features of the plasma membrane in the cells of the kidney distal segment of the lamprey, Lampetra japonica, were studied by electron microscopy. The cells of the distal segment are largely filled by a continuous network of cytoplasmic tubules which are derived from the basolateral plasma membrane. Thin sections and freeze-fracture replicas of the membrane of the cytoplasmic tubules show spirally wound parallel rows of particles. The rows are approximately 17 nm apart and are wound at a pitch of approximately 45 degrees with respect to the major axis of the tubules. Another type of membrane specialization was found in the freeze-cleaved surface of the basolateral plasma membrane. It consists of large square aggregations of membrane particles containing 100-400 cuboidal subunits. The distribution of these particles in this cell, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.

Analysis of visual system development in Drosophila melanogaster: mutations at the Glued locus

We have analyzed several aspects of the development of flies carrying mutations at the Glued locus. Optic lobe abnormalities in individuals heterozygous for the original Glued allele were previously shown to result from an action of this mutation in the retinula cells. We have estimated when the functioning of this gene or its product is required for normal visual system development by using genetic mosaicism induced by somatic recombination and temperature shifts of a temperature-sensitive mutation at this locus. Both methods point to a period in the mid-third instar, suggesting that early events in the formation of ommatidia and/or late events in the program of retinal cells are affected. Application of a new histological stain for developing axons indicates that individuals heterozygous for Glued exhibit abnormalities in the retinula fiber projection by the late third instar. Thus, the adult phenotype is not solely the result of later cellular degeneration or rearrangement. Beneath M+ Gl+ clones which encompass the entire eye were found optic lobe abnormalities with features not seen in either other mosaics or Gl heterozygotes. The possibility that these abnormalities result from temporal asynchrony in the development of eye and and optic lobe in these individuals is discussed and the results of attempts to test this hypothesis are presented.

Continuity of intracellular channels with extracellular space in adipose tissue and liver: demonstrated with tannic acid and lanthanum

Tannic acid was used to demonstrate continuity of intracellular channels with extracellular space in white adipose tissue of adult rats, brown adipose tissue of suckling rats, and liver of diabetic rats. Electron-opaque material resulting from treatment of glutaraldehyde-fixed tissue with tannic acid was found in extracellular space, invaginations of cell surfaces, vesicles, and intracellular channels. Electron-opaque material was present in channels that surrounded lipid droplets in both white and brown adipocytes and in hepatocytes. The small distance between the lumen of marked channels and lipid droplets in adipocytes indicates that a monolayered structure, perhaps a leaflet of membrane lining the channel. Similar findings were obtained in brown adipose tissue using lanthanum instead of tannic acid to mark intracellular channels continuous with extracellular space. Since endoplasmic reticulum is the primary site of triacylglycerol synthesis in adipocytes, marked channels near lipid droplets may be elements of endoplasmic reticulum. Some of the channels marked with tannic acid in hepatocytes contained lipoprotein particles, whereas others were located, in relation to mitochondria and lipid droplets, in the same sites as endoplasmic reticulum in untreated tissue. This indicates that some of the channels marked with tannic acid in hepatocytes are endoplasmic reticulum. Presence of electron-opaque material in intracellular channels and vesicles, but not in cytoplasm, of treated tissue indicates the channels and vesicles were open to extracellular space during treatment with tannic acid or lanthanum and, furthermore, that their membranes were continuous with plasma membrane.

The use of glutaraldehyde and tannic acid to preserve reconstituted collagen for electron microscopy

The effects of glutaraldehyde and tannic acid on the axial periodicity of collagen have measured. Both fixatives produce axial shrinkage of the collagen but whereas glutaraldehyde produces 7% shrinkage, tannic acid produces only 2% shrinkage. The technique of carbon/platinum shadowing was used to estimate the extent to which the collagen fibrils flatten down when they are dried onto grids for electron microscopy without prior embedding and sectioning. The influence of fixation was studied and it was found that minimum distortion occurred when both tannic acid and glutaraldehyde were used to preserve the protein structure.