HDL-mediated cholesterol uptake and targeting to lipid droplets in adipocytes

Adipocytes express high levels of the HDL scavenger receptor class B type I in a differentiation-dependent manner. We thus have analyzed the routes of HDL cholesterol trafficking at different phases of adipocyte differentiation in the 3T3-L1 cell line. One novel and salient feature of this paper is the observation of a widespread distribution in the cell cytoplasm of Golgi markers, caveolin-2, and a fluorescent cholesterol analog NBD-cholesterol (NBD-chol), observed in the early phases of adipocyte formation, clearly distinct from that observed in mature fat cells (i.e., with fully formed lipid vesicles). Thus, in cells without visible lipid droplets, Golgi markers (Golgi 58K, Golgin 97, trans-Golgi network 38, Rab 6, and BODIPY-ceramide), caveolin-2, and NBD-chol all colocalize in a widespread distribution in the cell. In contrast, when lipid droplets are fully formed at latter stages, these markers clearly are distributed to distinct cell compartments: a compact juxtanuclear structure for the Golgi markers and caveolin-2, while NDB-chol concentrates in lipid droplets. In addition, disorganization of the Golgi using three different agents (Brefeldin, monensin, and N-ethyl-maleimide) drastically reduces NBD-chol uptake at different phases of adipocyte formation, strongly suggesting that the Golgi apparatus plays a critical role in HDL-mediated NBD uptake and routing to lipid droplets.     

Effects of brefeldin A on the Golgi apparatus, the nuclear envelope, and the endoplasmic reticulum in a green alga,Scenedesmus acutus

Summary The effects of brefeldin A (BFA) on the structure of the Golgi apparatus, the nuclear envelope, and the endoplasmic reticulum (ER), and on the thiamine pyrophosphatase (TPPase) activity in these organelles were examined in a green alga,Scenedesmus acutus, to obtain evidence for the existence of a retrograde transport from the Golgi apparatus to the ER via the nuclear envelope. InScenedesmus, Golgi bodies are situated close to the nuclear envelope throughout the cell cycle and receive the transition vesicles not directly from the ER, but from the nuclear envelope. BFA induced the disassembly of Golgi bodies and an increase in the ER cisternae at the trans-side of decomposed Golgi bodies in interphase cells and multinuclear cells before septum formation. The accumulated ER cisternae connected to the nuclear envelope at one part. TPPase activity was detected in all cisternae of Golgi bodies, but not in the nuclear envelope or the ER in nontreated cells. On the contrary, in BFA-treated cells, TPPase activity was detected in the nuclear envelope and the ER in addition to the decomposed Golgi bodies. When septum-forming cells were treated with BFA, the disassembly of Golgi bodies was less than that in interphase cells, and TPPase activity was detected in the Golgi cisternae but not in the nuclear envelope or the ER. These results suggest mat BFA blocks the anterograde transport from the nuclear envelope to the Golgi bodies but does not block the retrograde transport from the Golgi bodies to the nuclear envelope in interphase and multinuclear cells.Abbreviations BFA brefeldin A - ER endoplasmic reticulum - TPPase thiamine pyrophosphatase     

Retrograde traffic in the biosynthetic-secretory route

In the biosynthetic-secretory route from the rough endoplasmic reticulum, across the pre-Golgi intermediate compartments, the Golgi apparatus stacks, trans Golgi network, and post-Golgi organelles, anterograde transport is accompanied and counterbalanced by retrograde traffic of both membranes and contents. In the physiologic dynamics of cells, retrograde flow is necessary for retrieval of molecules that escaped from their compartments of function, for keeping the compartments’ balances, and maintenance of the functional integrities of organelles and compartments along the secretory route, for repeated use of molecules, and molecule repair. Internalized molecules may be transported in retrograde direction along certain sections of the secretory route, and compartments and machineries of the secretory pathway may be misused by toxins. An important example is the toxin of Shigella dysenteriae, which has been shown to travel from the cell surface across endosomes, and the Golgi apparatus en route to the endoplasmic reticulum, and the cytosol, where it exerts its deleterious effects. Most importantly in medical research, knowledge about the retrograde cellular pathways is increasingly being utilized for the development of strategies for targeted delivery of drugs to the interior of cells. Multiple details about the molecular transport machineries involved in retrograde traffic are known; a high number of the molecular constituents have been characterized, and the complicated fine structural architectures of the compartments involved become more and more visible. However, multiple contradictions exist, and already established traffic models again are in question by contradictory results obtained with diverse cell systems, and/or different techniques. Additional problems arise by the fact that the conditions used in the experimental protocols frequently do not reflect the physiologic situations of the cells. Regular and pathologic situations often are intermingled, and experimental treatments by themselves change cell organizations. This review addresses physiologic and pathologic situations, tries to correlate results obtained by different cell biologic techniques, and asks questions, which may be the basis and starting point for further investigations.     

Golgi apparatus and TGN during endocytosis

Wheat germ agglutinin labelled with horseradish peroxidase (WGA) was used for analyses of endosomal compartments and Golgi apparatus in HepG(2) hepatoma cells during early and late periods of endocytosis. WGA was rapidly transferred into the Golgi region. Transport of internalised WGA into the Golgi apparatus could be classified in three stages. A short stage I, characterised by predominance of vesicular endosomes, was followed by stage II showing new formations of extended endocytic trans Golgi networks (TGNs); the endocytic TGNs comprised reticular and globular parts, showed intimate associations with segments of the endoplasmic reticulum and budding of multiple coated vesicles. Parts of the endocytic TGNs associated with trans Golgi cisternae and became integrated into Golgi stacks. During stage III, concomitantly with integration into the stacks, the endocytic TGNs decreased in size and stacked Golgi cisternae became prominent endocytic compartments. Our results show that endocytosis of WGA is connected with extensive membrane dynamics at the trans Golgi side: an endocytic TGN is newly formed, increases in size and is consumed again. The findings suggest that incorporation of TGN elements into Golgi stacks provides a mechanism for uptake of internalised WGA into the Golgi apparatus.     

Simulated de novo assembly of golgi compartments by selective cargo capture during vesicle budding and targeted vesicle fusion

The Golgi apparatus is comprised of stacked cisternal membranes forming subcompartments specialized for posttranslational processing of newly synthesized secretory cargo. Recent experimental evidence indicates that the Golgi apparatus can undergo de novo biogenesis from the endoplasmic reticulum, but the mechanism by which the membranes self assemble into compartmentalized structures remains unknown. We developed a discrete-event computer simulation model to test whether two fundamental mechanisms—vesicle-coat-mediated selective concentration of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins during vesicle formation, and SNARE-mediated selective fusion of vesicles—suffice to generate and maintain compartments. Simulations verified that this minimal model is adequate for homeostasis of preestablished compartments, even in response to small perturbations, and for de novo formation of stable compartments. The model led to a novel prediction that Golgi size is in part dependent on target SNARE expression level. This prediction was supported by a demonstration that exogenous expression of the Golgi target SNARE syntaxin-5 alters Golgi size in living cells.     

The Endoplasmic Reticulum-Resident Chaperone Heat Shock Protein 47 Protects the Golgi Apparatus from the Effects of O-Glycosylation Inhibition

The Golgi apparatus is important for the transport of secretory cargo. Glycosylation is a major post-translational event. Recognition of O-glycans on proteins is necessary for glycoprotein trafficking. In this study, specific inhibition of O-glycosylation (Golgi stress) induced the expression of endoplasmic reticulum (ER)-resident heat shock protein (HSP) 47 in NIH3T3 cells, although cell death was not induced by Golgi stress alone. When HSP47 expression was downregulated by siRNA, inhibition of O-glycosylation caused cell death. Three days after the induction of Golgi stress, the Golgi apparatus was disassembled, many vacuoles appeared near the Golgi apparatus and extended into the cytoplasm, the nuclei had split, and cell death assay-positive cells appeared. Six hours after the induction of Golgi stress, HSP47-knockdown cells exhibited increased cleavage of Golgi-resident caspase-2. Furthermore, activation of mitochondrial caspase-9 and ER-resident unfolded protein response (UPR)-related molecules and efflux of cytochrome c from the mitochondria to the cytoplasm was observed in HSP47-knockdown cells 24 h after the induction of Golgi stress. These findings indicate that (i) the ER-resident chaperon HSP47 protected cells from Golgi stress, and (ii) Golgi stress-induced cell death caused by the inhibition of HSP47 expression resulted from caspase-2 activation in the Golgi apparatus, extending to the ER and mitochondria.     

Structure and maintenance of the epidermis in Friedmaniella sp. (Prolecithophora)

The epidermis of Friedmaniella sp. has been studied using light and electron microscopy. Three main morphological features characterize its cells, namely (1) DNA bodies in the nuclei, (2) an extensive Golgi apparatus with a well-developed system of transport vesicles, (3) clusters of centrioles mainly in the basal cytoplasm and axonemes and rootlets in the middle and apical cell parts. These peculiarities may indicate continuous physiological regeneration within the cell at the level of cell organelles. DNA bodies may prove to be a taxonomic feature distinguishing the Prolecithophora from other turbellarians.     

Immunocytochemical evidence for the involvement of golgi apparatus in the deposition of seed lectin ofBauhinia purpurea (Leguminosae)

Summary The seed lectin of the tree legume,Bauhinia purpurea alba, was localized by electron microscopic immunocytochemistry. The pattern of lectin deposition and site of intracellular localization was examined in mid- to late-maturation seeds. The seed tissue was embedded in Lowicryl K4M, the use of which with seed tissues is discussed. Immunocytochemical labeling was accomplished with colloidal gold coupled to a second antibody. The immunocytochemical reaction was specific and sensitive. Protein bodies, Golgi apparatus and Golgi secretion vesicles were densely labeled. Golgi apparatus was oriented such that Golgi secretion vesicles were in close proximity to the protein bodies. The entire Golgi apparatus was labeled with no concentration gradient across the Golgi stack. These observations indicate that the final site of lectin deposition is the protein body, and that the Golgi apparatus plays an essential role in the deposition process.     

Immunochemical studies on the role of the Golgi complex in protein-body formation in rice seeds

Antibodies raised against purified glutelins and prolamines were employed as probes to study the cellular routes by which these proteins are deposited into protein bodies of rice (Oryza sativa L.) endosperm. Three morphologically distinct protein bodies, large spherical, small spherical, and irregularly-shaped, were observed, in agreement with existing reports. Immunocytochemical studies showed the presence of glutelins in the irregularly-shaped protein bodies while the prolamines were found in both the large and small spherical protein bodies. Both the large and small spherical protein bodies, distinguishable by electron density and gold-labeling patterns, appear to be formed by direct deposition of the newly formed proteins into the lumen of the rough endoplasmic reticulum (ER). In contrast, glutelin protein bodies are formed via the Golgi apparatus. Small electron-lucent vesicles are often found at one side of the Golgi. Electron-dense vesicles, whose contents are labeled by glutelin antibody-gold particles, are commonly observed at the distal side of the Golgi apparatus and fuse to form the irregularly shaped protein bodies in endosperm cells. These observations indicate that the transport of rice glutelins from their site of synthesis, the ER, to the site of deposition, the protein bodies, is mediated by the Golgi apparatus.Abbreviations BSA bovine serum albumin - Da dalton - DAF days after flowering - ER endoplasmic reticulum - GL irregularly shaped - L large spherical - S small spherical (protein bodies) - PBS phosphate-buffered saline - PTA phosphotungstic acid     

An electron microscopical study of absorbing cells in the posterior caput epididymidis of rabbits

Summary The columnar cells in regions 3 and 4 of the ductus epididymidis in rabbits display ultrastructural features characteristic of absorbing cells. The stereocilia show basal anastomoses and often a fibrillar core continuous with a fibrillar web in the apical cytoplasm. Numerous invaginations of the slightly downy apical cell membrane and many thick-walled apical vesicles and vacuoles contain an opaque substance similar to that seen in the lumen. The vacuoles often contain small vesicles or bodies, probably formed from the vacuolar wall by budding. Numerous bodies or vacuoles with moderately dense contents are seen in the Golgi area and in the supranuclear and intranuclear cytoplasm in region 3. In region 4 they are denser and mainly seen above the nucleus. A high acid phosphatase activity was demonstrated in most dense and some light bodies. India ink introduced by way of the rete testis was taken up from the lumen into apical invaginations, vesicles and vacuoles and slowly transferred to denser bodies below the Golgi apparatus.These observations are interpreted as evidence for a resorption of substances from the lumen by a pinocytotic process, and for their storage and perhaps digestion in the dense bodies, which appear to have a lysosomal character. The Golgi apparatus is large with many vesicles of two types and empty cisternae but few typical Golgi vacuoles. The partly granular endoplasmic reticulum is very well developed and has opaque contents. Microtubules run from the terminal bar region into the Golgi area. Thick-walled vesicles occur throughout the cytoplasm, sometimes in continuity with the cell membrane. The basal parts of the cell borders often interdigitate.Supported by a grant from the Swedish State Medical Research Council.     

The plant Golgi apparatus: Last 10 years of answered and open questions

Plant Golgi bodies possess unique morphological and functional characteristics that are key to several biological and biotechnological processes, such as transport of the cell’s building blocks to energy-rich compartments, including chloroplasts, storage vacuoles and a cellulosic cell wall. During the last decade it has become apparent that the plant Golgi apparatus has features that are remarkably different from other systems. Here we summarize the most recent findings on this organelle and we highlight pressing questions that are likely to drive the next 10 years of research on the plant Golgi apparatus.     

Alteration of intracellular traffic by monensin; mechanism,specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion comples is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself.One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions.Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2–5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H⁺ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs.In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus.In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented. It is becoming clear that endocytosis involves both acidic and non-acidic compartments and that monensin inhibits those processes that normally occur in acidic compartments.Thus, monensin, which is capable of collapsing Na⁺ and H⁺ gradients, has gained wide-spread acceptance as a tool for studying Golgi apparatus function and for localizing and identifying the molecular pathways of subcellular vesicular traffic involving acid compartments. Among its advantages are the low concentrations at which inhibitions are produced (0.01–1.0 μM), a minimum of troublesome side effects (e.g., little or no change of protein synthesis or ATP levels) and a reversible action. Because the affinity of monensin for Na⁺ is ten times that for K⁺, its nearest competitor, monensin mediates primarily a Na⁺-H⁺ exchange. Monensin has little tendency to bind calcium.Not only is monensin of importance as an experimental tool, it is of great commercial value as a coccidiostat for poultry and to promote more efficient utilization of feed in cattle. The mechanisms by which monensin interact with coccidia and rumen microflora to achieved these benefits are reasonably well documented. However, the interactions between monensin and the tissues of the host animal are not well understood although the severe toxicological manifestations of monensin poisoning are well known. Equine species are particularly susceptible to monensin poisoning, and a common effect of monensin poisoning is vacuolization and/or swelling of mitochondria in striated muscle. Other pathological injuries to striated muscle, spleen, lung, liver and kidney also have been noted. A consistent observation is cardiac myocyte degeneration as well as vacuolization. Differences in cellular response resulting from exposure to monensin (i.e., Golgi apparatus swelling in cultured cells, isolated tissues, and plants vs.mitochondrial swelling in animals fed monensin) suggest that myocardial damage is due either to a monensin metabolite or is a secondary response to some other derivation. However, as pointed out by Bergen and Bates [26], the underlying mode of action of ionophores is on transmembrane ion fluxes which dissipate cation and proton gradients. Consequently, some or all of the observed monensin effects in vivo in animals could be secondary phenomena caused by disruption of normal membrane physiology resulting from altered ion fluxes.     

Selective staining of dictyosome-like structures (DLS) from spermatocytes of the guinea pig using phosphotungstic acid at low pH

Dictyosome-like structures (DLS) occur abundantly in primary spermatocytes of the guinea pig. DLS superficially resemble dictyosomes of Golgi apparatus in that they consist of stacked cisternae and react similarly to some cytochemical markers. DLS saccules are also present in residual bodies and in the cytoplasmic droplet of the sperm, but the stacked configuration (or dictyosome form) is seldom present at these stages of development. A mixture of 1% phosphotungstic acid in 10% chromic acid selectively stains the DLS and DLS saccules of guinea pig germ cells. The thick cisternae of spermatid Golgi apparatus and the sperm plasma membrane also stain, but endoplasmic reticulum and the parts of the Golgi apparatus other than the thick cisternae do not stain. The specificity of the stain is retained in crude homogenates as well as in purified cell fractions and may be helpful in identification of DLS in cell fractionation studies. Additionally, the information obtained provides clues to the origin and fate of DLS in the developing mammalian germ cells.     

Acid phosphatase activity during spore differentiation of the red algaeGigartina teedii andChondria tenuissima

The acid phosphatase activity during carposporogenesis inGigartina and tetrasporogenesis inChondria was studied using the Gomori technique. During the first steps of gonimoblast maturation ofGigartina, portions of cytoplasm are ensheathed by ER cisternae with acid phosphatase activity, giving rise to autolysosomal concentric membrane bodies. In a similar way large mucilage sacs are severed. They extrude their contents in a kind of exocytosis. Multivesicular bodies, concentrically arranged cisternae and extracytoplasmic compartments, each with acid phosphatase activity, remain in young carpospores for some time, probably as remnants of the autophagocytotic and exocytotic events. The Golgi apparatus is poorly developed in gonimoblast cells and young carpospores. It becomes a prominent cell component in maturing carpospores and then participates in cell wall formation. Only some of the dictyosomal cisternae contain acid phosphatase; these are irregularly distributed in the dictyosome. — In pre- and postmeiotic tetraspore mother cells ofChondria massive lead deposits are found in the dictyosomes and in adjacent Golgi vesicles. Finer lead precipitates occur in ER cisternae, especially in those which are sequestering starch-grain-containing portions of the cytoplasm to give rise to autolysosomes. During cell cleavage, the dictyosomes aggregate. They become devoid of acid phosphatase activity with the exception of vesicles at the trans face. Later, Golgi stacks associate and have common, Gomori positively reacting, narrow cisternae at the cis face. The Golgi apparatus derived cored vesicles do not contain lead precipitates whereas the Golgi cisternae in the final stage of tetrasporogenesis show acid phosphatase activity. Variations in acid phosphatase distribution are explained in the light of current models of membrane flow.Dedicated to Univ.-Prof. DrO. Härtel on the occasion of his 80th birthday.     

Golgi apparatus, GERL, and secretory granule formation within neurons of the hypothalamo-neurohypophysial system of control and hyperosmotically stressed mice

The vasopressin-producing neurons of the hypothalamo-neurohypophysial system are a particularly good model with which to consider the relationship between the Golgi apparatus nd GERL and their roles in secretory granule production because these neurons increase their synthesis and secretion of vasopressin in response to hyperosmotic stress. Enzyme cytochemical techniques for acid phosphatase (AcPase) and thiamine pyrophosphatase (TPPase) activities were used to distinguish GERL from the Golgi apparatus in cell bodies of the supraoptic nucleus from normal mice, mice hyperosmotically stressed by drinking 2% salt water, and mice allowed to recover for 5-10 d from hyperosmotic stress. In nonincubated preparations of control supraoptic perikarya, immature secretory granules at the trans face of the Golgi apparatus were frequently attached to a narrow, smooth membrane cisterna identified as GERL. Secretory granules were occasionally seen attached to Golgi saccules. TPPase activity was present in one or two of the trans Golgi saccules; AcPase activity appeared in GERL and attached immature secretory granules, rarely in the trans Golgi saccules, and in secondary lysosomes. As a result of hyperosmotic stress, the Golgi apparatus hypertrophied, and secretory granules formed from all Golgi saccules and GERL. Little or no AcPase activity could be demonstrated in GERL, whereas all Golgi saccules and GERL-like cisternae were TPPase positive. During recovery, AcPase activity in GERL returned to normal; however, the elevated TPPase activity and secretory granule formation seen in GERL-like cisternae and all Golgi saccules during hyperosmotic stress persisted. These results suggest that under normal conditions GERL is the predominant site for the secretory granule formation, but during hyperosmotic stress, the Golgi saccules assume increased importance in this function. The observed cytochemical modulations in Golgi saccules and GERL suggest that GERL is structurally and functionally related to the Golgi saccules.     

Golgi-membrane dynamics are cytoskeleton dependent: A study on Golgi stack movement induced by brefeldin A

Summary Brefeldin A (BFA) induces a major aggregation of the Golgi apparatus (GA) in root cells followed by a complete, but reversible, vesiculation of the Golgi stacks which form two or more BFA compartments in the cytosol. This effect is monitored by the immunofluorescence of a Golgi antigen stained with a monoclonal antibody JIM 84, and by transmission electron microscopy. Depolymerisation of the microtubule cytoskeleton after oryzalin or colchicine treatment does not disturb the three dimensional organisation of the GA in control cells and does not affect any BFA induced Golgi stack movements. After disruption of the actin cytoskeleton with cytochalasin D many aggregates of Golgi stacks can be observed in root cells and these are sensitive to BFA treatment, forming multiple BFA compartments. N-ethyl-maleimide has no effect on the organisation of the GA in control roots but totally inhibits the action of BFA. Thus, it appears that spatial organisation of the GA and the BFA induced Golgi stack movements are actin dependent whilst BFA induced vesiculation of the GA is a structurally separate event.     

Distribution of xylosylation and fucosylation in the plant Golgi apparatus

Antibodies have been immunopurified which are specific for carbohydrate epitopes containing the β1→2 xylose or α1→3 fucose residues found on complex N-linked glycans in plants. The antibody specificity was determined by taking advantage of an Arabidopsis thaliana N-glycosylation mutant which lacks N-acetyl-glucosaminyltransferase I and is unable to synthesize complex glycans. These antibodies were used to immunolocalize xylose- and fucose-containing glycoproteins in suspension-cultured sycamore cells (Acer pseudoplatanus). By mapping the enzymatic reaction products within the Golgi apparatus, the fucosyl- and xylosyltransferase subcellular localization was made possible using immunocytochemistry on thin sections of high-pressure frozen and freeze-substituted sycamore cells. This procedure allows a much better preservation of organelles, and particularly of the Golgi stack morphology, than that obtained with conventionally fixed samples. Glycoproteins containing β→2 xylose and α1→3 fucose residues were immunodetected in the cell wall, the vacuole, and the Golgi cisternae. The extent of immunolabeling over the different cisternae of 50 Golgi stacks was quantified after treatment with anti-xylose or anti-fucose antibodies. Labeling for xylose-containing glycoproteins was predominent in the medial cisternae, while fucose-containing glycoproteins were mainly detected in the trans compartment. Therefore, in plants, complex N-linked glycan xylosylation probably occurs mostly at the medial Golgi level and α1→3 fucose is mainly incorporated in the trans cisternae. Finally, fucose- and xylose-containing glycoproteins were also immunolocalized, albeit to a lesser extent, in earlier Golgi compartments. This indicates that the glycosylation events are a continuous process with some maxima in given compartments, rather than a succession of discrete and compartment-dependent steps.     

Elektronenmikroskopische Untersuchungen von Differenzierungsvorgängen bei Moosen

Summary In meristematic cells of the gemma of Riella helicophylla and in young bud cells from the protonema of Funaria hygrometrica the cell plate is formed by fusion of small vesicles originating from the Golgi apparatus. These spherical vesicles of about 0.1 m diameter have an electron dense centre, probably consisting of pectic substances or their precursors. The endoplasmic reticulum producing multivesicular bodies participate in cell plate formation too. Another cytoplasmic component forming the cell plate are coated vesicles, the origin of which is the Golgi apparatus and perhaps also the endoplasmic reticulum. In view of these observations the question of whether the endoplasmic reticulum or the Golgi apparatus forms the cell plate must be answered in this way: both endoplasmic reticulum and Golgi apparatus supply material for growth of the cell plate. Multivesicular bodies, coated vesicles and other small vesicles of unknown nature participate in the formation of the primary wall.

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Zum Teil finanziert mit Sondermitteln des Landes Niedersachsen an Prof. Dr. M. Bopp.     

Swelling response of Golgi apparatus cisternae in cells treated with monensin is reduced by cell injury.

The effect of mechanical stress on Golgi apparatus was examined in thin slices of rat liver. The findings should be of relevance both to electron microscopists who routinely mince tissue, and to biochemists who homogenize tissues to isolate membranous components. The swelling response of Golgi apparatus to monensin was used as an assay because the swelling response is distinct and is thought to result from a well-characterized metabolic process, namely the acidification of vesicles. The results showed that the swelling response was compromised by monensin as far away as 6-7 cells from a cut surface even though other aspects of cell ultrastructure were not altered from normal. The monensin-induced swelling response was also evaluated in isolated Golgi apparatus and found to be similar to that with tissue. Thus, mechanical stress such as commonly used to mince tissue or isolate tissue components, appears to markedly alter Golgi apparatus function compared to the situation in vivo. In this example, the altered response of Golgi apparatus to monensin indicated that some aspects associated with the ATP-dependent proton-pumping machinery of the trans-most cisternae and trans Golgi network were compromised.