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1.  Specific Membranous Structures Associated with the Replication of Group A Arboviruses 1 
Journal of Virology  1972;10(3):492-503.
Intracytoplasmic membranous structures of a unique type were associated with the replication of three group A arboviruses: Semliki Forest virus (SFV), Sindbis virus, or Western equine encephalomyelitis virus. The structures, referred to as type 1 cytopathic vacuoles (CPV-1), were membrane-limited and characteristically lined by regular membranous spherules measuring 50 nm in diameter. The membranous spherules typically contained a fine central density, but were neither virus cores nor virions. Detection of CPV-1 by electron microscopy at 3 to 6 hr postinfection coincided with the time of rapid virus growth and preceded the accumulation of virus nucleocapsids. A range of 20 to 100 CPV-1 profiles were counted per 100 ultrathin cell sections at 6 to 9 hr postinfection when viruses were grown in chick embryo, baby hamster kidney, or mouse L cells. Maximum counts remained in the same range even when the multiplicity of infection was varied over 100-fold. Inhibition of cellular ribonucleic acid (RNA) and protein synthesis by actinomycin D during SFV infection did not decrease the counts of CPV-1; however, biogenesis of CPV-1 was decreased when viral replication was limited by inhibitors of viral RNA synthesis (guanidine) or of viral protein synthesis (cycloheximide). On the basis of present and earlier findings, we concluded that formation of CPV-1 must result from a virus-specified modification of pre-existing host cell macromolecules.
PMCID: PMC356491  PMID: 4672392
2.  Biogenesis of the Semliki Forest Virus RNA Replication Complex 
Journal of Virology  2001;75(8):3873-3884.
The nonstructural (ns) proteins nsP1 to -4, the components of Semliki Forest virus (SFV) RNA polymerase, were localized in infected cells by confocal microscopy using double labeling with specific antisera against the individual ns proteins. All ns proteins were associated with large cytoplasmic vacuoles (CPV), the inner surfaces of which were covered by small invaginations, or spherules, typical of alphavirus infection. All ns proteins were localized by immuno-electron microscopy (EM) to the limiting membranes of CPV and to the spherules, together with newly labeled viral RNA. Along with earlier observations by EM-autoradiography (P. M. Grimley, I. K. Berezesky, and R. M. Friedman, J. Virol. 2:326–338, 1968), these results suggest that individual spherules represent template-associated RNA polymerase complexes. Immunoprecipitation of radiolabeled ns proteins showed that each antiserum precipitated the other three ns proteins, implying that they functioned as a complex. Double labeling with organelle-specific and anti-ns-protein antisera showed that CPV were derivatives of late endosomes and lysosomes. Indeed, CPV frequently contained endocytosed bovine serum albumin-coated gold particles, introduced into the medium at different times after infection. With time, increasing numbers of spherules were also observed on the cell surfaces; they were occasionally released into the medium, probably by secretory lysosomes. We suggest that the spherules arise by primary assembly of the RNA replication complexes at the plasma membrane, guided there by nsP1, which has affinity to lipids specific for the cytoplasmic leaflet of the plasma membrane. Endosomal recycling and fusion of CPV with the plasma membrane can circulate spherules between the plasma membrane and the endosomal-lysosomal compartment.
PMCID: PMC114878  PMID: 11264376
The Journal of Cell Biology  1972;53(1):73-91.
To determine the three-dimensional structure of the lumenal membrane of transitional epithelium, a study was made of sectioned, negatively stained, and freeze-etched specimens from intact epithelium and membrane fractions from rabbit urinary bladder. Particulate membrane components are confined to plaque regions within which the unit membrane is asymmetric, having a thicker outer leaflet. Transversely fractured freeze-etched plaques display a thick (∼80 A), particulate lumenal leaflet and a thin (∼40 A) cytoplasmic one. Four different faces of the two leaflets can be distinguished: two complementary, split, inner membrane faces exposed by freeze-cleaving the bilayer and two external (lumenal and cytoplasmic) membrane surfaces revealed by deep-etching. On the split, inner face of the lumenal leaflet appear polygonal plaques of hexagonally arranged particles. These fit into holes observed on the complementary, split, innerface of the cytoplasmic leaflet. The particles, which have a center-to-center spacing of ∼160 A, also seem to protrude from the external surface of the lumenal leaflet, where their subunits (∼50 A in diameter) are revealed by freeze-etching and negative staining. The plaques are separated from each other by smooth-surfaced regions, which cleave like simple lipid bilayers. Since the array of plaque particles covers only ∼73% of the membrane surface area, whereas 27% is taken up by particle-free interplaque regions, the presence of particles cannot in itself entirely account for the permeability barrier of the lumenal membrane. Although no particles are observed protruding from the cytoplasmic surface of the membrane, cytoplasmic filaments are attached to it by short, cross-bridge-like filaments that seem to contact the particles within the membrane. These long cytoplasmic filaments cross-link adjacent plaques. Therefore, we suggest that at least one function of the particles is to serve as anchoring sites for cytoplasmic filaments, which limit the expansion of the lumenal membrane during distention of the bladder, thereby preventing it from rupturing. The particle-free interplaque regions probably function as hinge areas between the stiff plaques, allowing the membrane to fold up when the bladder is contracted.
PMCID: PMC2108692  PMID: 5013603
4.  Membrane-Associated Replication Complex in Arbovirus Infection 1 
Journal of Virology  1972;10(3):504-515.
Cytoplasmic extracts of chicken embryo fibroblast cells infected with Semliki Forest virus were subjected to isopycnic centrifugation in discontinuous sucrose gradients. Seven distinct bands were usually formed. The four upper bands contained predominantly smooth membranes and the lowest band was enriched in rough endoplasmic reticulum. One fraction (fraction 5), banding at a density of 1.16 g/cm3, was found to be heavily enriched in pulse-labeled ribonucleic acid (RNA), viral RNA polymerase, and viral RNA forms associated with RNA replication. Thus, fraction 5 evidently contained a membrane-associated viral replication complex of a type previously defined in picornavirus infections. Fraction 5 was also consistently enriched with unique membranous structures previously observed in intact cells as type 1 cytopathic vacuoles (CPV-1). When the CPV-1 in fraction 5 were isolated from cells briefly incubated with 3H-uridine and 3H-adenosine prior to cell disruption, a large proportion was found to be labeled by high-resolution autoradiography. Thus, ultrastructural, biochemical, and biological evidence were all consistent with the interpretation that the CPV-1 membranes represent a significant element of the viral replication complex.
PMCID: PMC356492  PMID: 4342056
5.  Phagocytosis of bacteria by polymorphonuclear leukocytes: a freeze-fracture, scanning electron microscope, and thin-section investigation of membrane structure 
The Journal of Cell Biology  1978;76(1):158-174.
The changes in membrane structure of rabbit polymorphonuclear (PMN) leukocytes during bacterial phagocytosis was investigated with scanning electron microscope (SEM), thin-section, and freeze-fracture techniques. SEM observations of bacterial attachment sites showed the involvement of limited areas of PMN membrane surface (0.01-0.25μm(2)). Frequently, these areas of attachment were located on membrane extensions. The membrane extensions were present before, during, and after the engulfment of bacteria, but were diminished in size after bacterial engulfment. In general, the results obtained with SEM and thin-section techniques aided in the interpretation of the three-dimensional freeze-fracture replicas. Freeze-fracture results revealed the PMN leukocytes had two fracture faces as determined by the relative density of intramembranous particles (IMP). Membranous extensions of the plasma membrane, lysosomes, and phagocytic vacuoles contained IMP's with a distribution and density similar to those of the plasma membrane. During phagocytosis, IMPs within the plasma membrane did not undergo a massive aggregation. In fact, structural changes within the membranes were infrequent and localized to regions such as the attachment sites of bacteria, the fusion sites on the plasma membrane, and small scale changes in the phagocytic vacuole membrane during membrane fusion. During the formation of the phagocytic vacuole, the IMPs of the plasma membrane appeared to move in with the lipid bilayer while maintaining a distribution and density of IMPs similar to those of the plasma membranes. Occasionally, IMPs were aligned to linear arrays within phagocytic vacuole membranes. This alignment might be due to an interaction with linearly arranged motile structures on the side of the phagocytic vacuole membranes. IMP-free regions were observed after fusion of lysosomes with the phagocytic vacuoles or plasma membrane. These IMP-free areas probably represent sites where membrane fusion occurred between lysosomal membrane and phagocytic vacuole membrane or plasma membrane. Highly symmetrical patterns of IMPs were not observed during lysosomal membrane fusion.
PMCID: PMC2109963  PMID: 338617
6.  Properly Folded Nonstructural Polyprotein Directs the Semliki Forest Virus Replication Complex to the Endosomal Compartment 
Journal of Virology  2003;77(3):1691-1702.
The late RNA synthesis in alphavirus-infected cells, generating plus-strand RNAs, takes place on cytoplasmic vacuoles (CPVs), which are modified endosomes and lysosomes. The cytosolic surface of CPVs consists of regular membrane invaginations or spherules, which are the sites of RNA synthesis (P. Kujala, A. Ikäheimonen, N. Ehsani, H. Vihinen, P. Auvinen, and L. Kääriäinen J. Virol. 75:3873-3884, 2001). To understand how CPVs arise, we have expressed the individual Semliki Forest virus (SFV) nonstructural proteins nsP1 to nsP4 in different combinations, as well as their precursor polyprotein P1234 and its cleavage intermediates. A complex of nsPs was obtained from P123 or P1234, indicating that the precursor stage is essential for the assembly of the polymerase complex. To prevent the processing of the polyprotein and its cleavage intermediates, constructs with the mutation C478A (designated with a superscript CA) in the active site of the protease domain of nsP2 were used. Uncleaved polyproteins containing nsP1 were membrane bound and palmitoylated, and those containing nsP3 were phosphorylated, reflecting properties of authentic nsP1 and nsP3, respectively. Similarly, polyproteins containing nsP1 or nsP2 had enzymatic activities specific for the individual proteins, indicating that they were correctly folded in the precursor state. Uncleaved P12CA was localized almost exclusively to the plasma membrane and filopodia, like nsP1 alone, whereas P12CA3 and P12CA34 were found on cytoplasmic vesicles, some of which contained late endosomal markers. In immunoelectron microscopy these vesicles resembled CPVs in SFV-infected cells. Our results indicate that the nsP1 domain alone is responsible for the membrane association of the nonstructural polyprotein, whereas the nsP1 domain together with the nsP3 domain targets it to the intracellular vesicles.
PMCID: PMC140886  PMID: 12525603
The Journal of Cell Biology  1969;43(1):16-31.
Spinach chloroplast lamellae were washed free of negatively staining surface particles (carboxydismutase and coupling factor protein) and the resulting smooth-surfaced lamellae still showed the usual large (175 A) and small (110 A) particles seen by freeze-etching. Therefore, the freeze-fracture plane probably occurs along an internal surface of the chloroplast membrane. Fractions obtained by differential centrifugation of digitonin-treated chloroplast membranes were studied by negative staining, thin sectioning, and freeze-etching techniques for electron microscopy. The material sedimenting between 1,000 g and 10,000 g, enriched in photosystem II activity, was shown to consist of membrane fragments. These freeze-etched membrane fragments were found to have large particles on most of the exposed fracture faces. The large particles had the same size and distribution pattern as the 175 A particles seen in intact chloroplast membranes. The material sedimenting between 50,000 g and 144,000 g, which had only photosystem I activity, was found to consist of particles in various degrees of aggregation. Freeze-etching of this fraction revealed only small particles corresponding to the 110 A particles seen in intact chloroplasts. A model is presented suggesting that chloroplast lamellar membranes have a binary structure, which digitonin splits into two components. The two membrane fragments have different structures, revealed by freeze-etching, and different photochemical and biochemical functions.
PMCID: PMC2107837  PMID: 4186409
8.  Cytoplasmic Structures Associated with an Arbovirus Infection: Loci of Viral Ribonucleic Acid Synthesis 
Journal of Virology  1968;2(11):1326-1338.
Unique cytoplasmic structures, herein designated as type I cytopathic vacuoles (CPV-I), are found in chick embryo cells early in the logarithmic phase of Semliki Forest virus replication. High resolution autoradiography demonstrated that the CPV-I are loci of 3H-uridine incorporation. This evidence correlates well with previous biochemical data and electron microscopy of the subcellular fractions active in Semliki Forest virus ribonucleic acid synthesis. Origin of the CPV-I within host cell cytoplasm is confirmed by the distribution of electron-dense tracer particles and sequential ultrastructural observations.
PMCID: PMC375472  PMID: 5750316
The Journal of Cell Biology  1972;53(2):494-509.
Cilia, primarily of the lamellibranch gill (Elliptio and Mytilus), have been examined in freeze-etch replicas. Without etching, cross fractures rarely reveal the 9 + 2 pattern, although suggestions of ninefold symmetry are present. In etched preparations, longitudinal fractures through the matrix show a triplet spoke alignment corresponding to the spoke periodicity seen in thin sections. Dynein rows can be visualized along the peripheral microtubules in some preparations. Fracture faces of the ciliary membrane are smooth with few membrane particles, except in the regions adjacent to the basal plate. In the transition region below the plate, a unique particle arrangement, the ciliary necklace, is found. In the Elliptio gill, on fracture face A the necklace is comprised of three well-defined rows or strands of membrane particles that encircle the ciliary shaft. The rows are scalloped and each scallop corresponds to a peripheral doublet microtubule. In thin sections at the level of these particles, a series of champagne-glass structures link the microtubular doublets to the ciliary membrane. The ciliary necklace and this "membrane-microtubule" complex may be involved in energy transduction or the timing of ciliary beat. Comparative studies show that these features are present in all somatic cilia examined including those of the ameboflagellate Tetramitus, sea urchin embryos, rat trachea, and nonmotile cilia of cultured chick embryo fibroblasts. The number of necklace strands differs with each species. The necklace has not been found in rat or sea urchin sperm.
PMCID: PMC2108734  PMID: 4554367
The Journal of Cell Biology  1972;53(1):92-104.
A technique has been devised for isolation of lumenal plasma membranes from transitional epithelial cells lining the urinary bladder in rabbits and for subsequent separation of particle-bearing plaque regions from particle-free areas of the membranes. The success of the procedures employed and their effects on the isolates were assessed by electron microscopy of conventional plastic sections, negatively stained preparations, and freeze-etch replicas. When bladders are distended with a solution of 0.01 M thioglycolic acid, which reduces sulfhydryl bridges, cytoplasmic filaments are disrupted, and large segments of the lumenal membranes rupture and float free into the lumen. A centrifugation procedure was developed for isolating a fraction enriched with the large fragments. A comparison of membranes isolated in the presence of thioglycolate with those isolated from epithelial cells homogenized in sucrose medium indicates that thioglycolate has little effect on their fine structure except for the removal of filaments which are normally associated with their cytoplasmic surface. The curved plaques of hexagonally arrayed particles and the particle-free interplaque regions, both characteristic of membranes before exposure to thioglycolate, are well preserved. Subsequent treatment of thioglycolate-isolated lumenal membranes with 1% sodium desoxycholate (DOC) severs many of the interplaque regions, releasing individual plaques in which the particles are more clearly visible than before exposure to desoxycholate. Presumably, DOC acts by disrupting the hydrophobic bonds within the membrane; therefore, this type of cohesive force probably is a major factor maintaining the structural integrity of interplaque regions. This conclusion is consistent with the observation that interplaque regions undergo freeze-cleaving like simple bilayers with a plane of hydrophobic bonding.
PMCID: PMC2108712  PMID: 4111147
11.  Thymic nurse cells. Lymphoepithelial cell complexes in murine thymuses: morphological and serological characterization 
We describe a new cellular component of normal mouse thymuses, which is isolated by fractionated trypsin dissociation of minced thymus tissue followed by repeated unit gravity sedimentation. These cells are of unusually large size, with diameters of 30 μm and more. They represent cellular complexes of single large cells filled with high numbers of lymphoid cells. The majority of the engulfed lymphoid cells is not only fully intact, as judged by morphological criteria, but, moreover, includes a high proportion of mitotic figures. Electron microscopic investigations reveal the epithelial character of the large thymic nurse cells (TNC). The peripherally situated cytoplasmic tonofilament streams, and characteristic vacuoles filled with coarse, unidentified material, closely resemble cytoplasmic organelles found in the cortical reticuloepithelial cells described in situ. The internalized lymphocytes are located within caveolae lined by plasma membranes. These TNC caveolae are completely sequestered, and have lost any communication with the extracellular space, as demonstrated by the inability of an electrondense marker, cationized ferritin, to diffuse into the perilymphocytic clefts. The structural interactions between the membranes of the engulfed thymocytes with the surrounding TNC caveolar membranes were investigated both in ultrathin sections and in freeze-etch preparates. Two distinct contact types between both membranes were discerned: (a) complete, close contact along the entire lymphocyte circumference, and (b) more frequently, contact restricted to discrete, localized areas. Judging from their size and distribution, the localized contacts could correspond particle aggregates of freeze-etch preparates, which morphologically resemble certain stages of gap junction. Furthermore, we regularly found square arrays of particles of uniform size, which so far have been thought to be typical for cell membranes actively engaged in ion exchange. Tight junction-like particle arrays, which were present on TNC outer membranes, and probably represented disrupted contacts between adjacent TNC in the intact tissue, could not be found on caveolar or lymphocyte membranes. Finally, one of the most conspicuous specializations of the TNC caveolar membrane were membrane invaginations, which were arranged mainly in groups, and which probably reflect endo- or exocytotoxic events. We investigated the surface antigen phenotype of TNC by indirect immunofluorescence, with monoclonal antibodies against determinants of H-2- complex subregions as well as against lymphocyte differentiation markers. Semiquantification was reached with flow cytofluorimetry, followed by morphological control by fluorescence microscopy. The surface antigen formula of TNC is: Ig(-), Thy-l(-), H-2K(++), I-A (++), I-E/C(+), H-D(++), Ly-1(-), Ly-2(-), Qat-4(-), Qat-5(-), and peanut agglutinin (PNA)(-). Thymic macrophages, which were identified by double fluorescence, with rhodamine- coupled zymosan as a phagocytosis marker, were serologically identical with TNC. Free thymocytes, in contrast, had the following antigen formula: Ig(-), Thy-1(++), H-2K(+/-), I-A(-), I-E/C(-), H-2D(+/-), Ly-1(+/-), Ly-2(+), Qat- 4(-), Qat-5(-), and PNA(+). The unprecedented finding of high numbers of dividing thymocytes sojourning within thymic epithelial cells, and the particular specializations of the TNC caveolar membranes surrounding these engulfed thymocytes is the basis of a hypothesis that postulates that an intraepithelial differentiation cycle is one essential step in, intrathymic T lymphocyte generation.
PMCID: PMC2185829  PMID: 6966312
The Journal of Cell Biology  1963;17(3):609-628.
The freeze-etching technique, which is a special kind of freeze-drying, allows electron microscopic investigation of cells and tissues in the frozen state. In regard to yeast cells (Saccharomyces cerevisiae) a freeze-fixation technique has been developed which does not kill the object. The electron micrographs therefore are considered to impart an image of high fidelity. The cutting of the frozen object, which actually consists of a fine splintering, produces not only cross-sectional views (cross-fractures) of the structures but also surface views of the membranes and organelles. Many surface structures are described which have not been shown by the usual sectioning techniques. The cytoplasmic membrane contains hexagonal arrangements of particles which are apparently involved in the production of the glucan fibrils of the cell wall. Alterations of the distribution of nuclear pores are shown in cells of different ages. Freeze-etching enables a clear distinction of endoplasmic reticulum and vacuoles in yeast cells. The membranes of the vesicular systems are covered by ribosomes arranged in circular patterns. The mitochondrial envelope shows small perforations which could allow the exchange of macromolecules. The storage granules consist of concentric layers of lipid, presumably phosphatide. A Golgi apparatus has been detected which may be involved in the storage of lipid. The structure of the unit membrane and the membrane structures of all organelles as revealed by chemical fixation are confirmed in principle. Glycogen agglomerations are identified in the ground plasm of older cells. Insight into artifacts introduced by common chemical fixation and embedding techniques is obtained and discussed.
PMCID: PMC2106217  PMID: 19866628
13.  A thin-section and freeze-fracture study of microfilament-membrane attachments in choroid plexus and intestinal microvilli 
The Journal of Cell Biology  1978;79(3):774-787.
Choroid plexus and intestinal microvilli in thin sections have microfilaments in the cytoplasm adjacent to the membranes, and in replicas have broken strands of filaments in both cytoplasm and on E faces of plasm membranes. The microfilaments contain actin as indicated by their binding of heavy meromyosin (HMM). In sections of choroid plexus, the microfilaments are 7-8 nm in diameter and form a loose meshwork which lies parallel to the membrane and which is connected to the membranes both by short, connecting filaments (8 times 30 nm) and dense globules (approximately 15-20 nm). The filamentous strands seen in replicas are approximately 8 nm in diameter. Because they are similar in diameter and are connected to the membrane, these filamentous strands seen in replicas apparently represent the connecting structures, portions of the microfilaments, or both. The filamentous strands attached to the membrane are usually associated with the E face and appear to be pulled through the P half-membrane. In replicas of intestinal brush border microvilli, the connecting strands attaching core microfilaments to the membrane are readily visualized. In contrast, regions of attachment of core microfilaments to dense material at the tips of microvilli are associated with few particles on P faces and with few filamentous strands on the E faces of the membranes. Freeze-fracture replicas suggest a morphologically similar type of connecting strand attachment for microfilament-membrane binding in both choroid plexus and intestinal microvilli, despite the lack of a prominent core bundle of microfilaments in choroid plexus microvilli.
PMCID: PMC2110279  PMID: 569660
The Journal of Cell Biology  1974;62(3):679-694.
An "apical endocytic complex" in the ileal lining cells of suckling rats is described. The complex consists of a continuous network of membrane-limited tubules which originate as invaginations of the apical plasma membrane at the base of the microvilli, some associated vesicles, and a giant vacuole. The lumenal surface of this tubular network of membranes and associated vesicles is covered with a regular repeating particulate structure. The repeating unit is an ∼7.5-nm diameter particle which has a distinct subunit structure composed of possibly nine smaller particles each ∼3 nm in diameter. The ∼7.5-nm diameter particles are joined together with a center-to-center separation of ∼15 nm to form long rows. These linear aggregates, when arranged laterally, give rise to several square and oblique two-dimensional lattice arrangements of the particles which cover the surface of the membrane. Whether a square or oblique lattice is generated depends on the center-to-center separation of the rows and on the relative displacement of the particles in adjacent rows. Four membrane faces are revealed by fracturing frozen membranes of the apical tubules and vesicles: two complementary inner membrane faces exposed by the fracturing process and the lumenal and cytoplasmic membrane surfaces revealed by etching. The outer membrane face reveals a distinct array of membrane particles. This array also sometimes can be seen on the outer (B) fracture face and is sometimes faintly visible on the inner (A) fracture face. Combined data from sectioned, negatively stained, and freeze-etched preparations indicate that this regular particulate structure is a specialization that is primarily localized in the outer half of the membrane mainly in the outer leaflet.
PMCID: PMC2109218  PMID: 4854072
15.  Characterization of intracytoplasmic hydrocarbon inclusions from the hydrocarbon-oxidizing Acinetobacter species HO1-N. 
Journal of Bacteriology  1976;127(1):481-489.
The ultrastructure of Acinetobacter sp. strain HO1-N grown on hydrocarbon and nonhydrocarbon substrates was compared using thin sections and freeze-etching. Hydrocarbon-grown cells were characterized by the presence of intracytoplasmic membrane-bound hexadecane inclusions. This membrane did not exhibit a typical unit membrane structure but appeared as a monolayer. The freeze-etch technique revealed the internal structure of the hexadecane inclusions and provided evidence for the presence of a smooth-surfaced limiting membrane. Freeze-etching also revealed intracytoplasmic membranes in the hexadecane-grown cells. These ultrastructural modifications were not present in nonhydrocarbon-grown cells. The hexadecane inclusions were isolated from Acinetobacter. Negative-staining of the inclusions revealed electron-transparent vesicles approximating the size of the inclusions seen in whole cells. Freeze-etching of the purified inclusions revealed membrane-bound vesicles. The purified inclusions exhibited a relatively high value of lipid phosphorus to protein. The lipid composition and the electrophoretic banding pattern of the inclusions on sodium dodecyl sulfate-polyacrylamide gels were determined and compared with other membrane fractions (outer membrane and cytoplasmic membrane) previously isolated from this organism.
PMCID: PMC233081  PMID: 179978
16.  Thrombocytopoiesis--analysis by membrane tracer and freeze-fracture studies on fresh human and cultured mouse megakaryocytes 
The Journal of Cell Biology  1984;99(2):390-402.
The origin of platelets (Pt) from megakaryocytes (MK) is beyond question, but the mechanism whereby Pts are released from the precursor cell is still debated. A widely-held theory claims that the MK plasma membrane invaginates to form demarcation membranes (DMS), which delineate Pt territories. Accordingly, Pts would be derived mostly from the periphery of the MK, and the MK and Pt plasma membranes would have to be virtually identical. Since, on morphologic grounds, this theory is untenable, several aspects of thrombocytopoiesis were reexamined with the help of membrane tracer and freeze-fracture analyses of freshly-collected human and cultured mouse MK. To our surprise, freeze- cleavage of the MK plasma membrane revealed that the vast majority of intramembranous particles (IMP) remained associated with the protoplasmic leaflet (P face), whereas the partition coefficient of IMPs of the platelet membrane was the reverse. This is the first time that any difference between MK and Pt membranes has been determined. Replicas of freeze-fractured MK that were in the process of thrombocytopoiesis revealed an additional novel phenomenon, i.e., numerous areas of membrane discontinuity that appeared to be related to Pt discharge. When such areas were small, the IMP were lined up along the margin of the crevice. At a later phase, a labyrinth of fenestrations was observed. Thin sections of MK at various stages of differentiation showed that Pt territories were fully demarcated before connections of the DMS with the surface could be found. Therefore, the Pt envelope is probably not derived from invaginations of the MK plasma membrane. When living, MK were incubated with cationic ferritin or peroxidase at 37 degrees C, the tracers entered into the DMS but did not delineate all membranes with which the DMS was in continuity, suggesting the existence of distinctive membrane domains. Interiorization of tracer was not energy-dependent, but arrested at low temperatures. At 4 degrees C the DMS remained empty, unless there was evidence that Pts had been released. In such instances, the tracers outlined infoldings of peripheral cytoplasm that was devoid of organelles. Thus, the majority of Pts seem to originate from the interior of the MK, and the surface membranes of the two cells differ in origin and structure. The observations do not only throw new light on the process of thrombocytopoiesis, but also strengthen the possibility that MKs and Pts may be subject to different stimuli.
PMCID: PMC2113260  PMID: 6204991
17.  Structure of Escherichia coli After Freeze-Etching 
Journal of Bacteriology  1970;101(1):304-313.
Survival of Escherichia coli, quick-frozen under conditions similar to those employed for freeze-etching, is close to 100%. For determination of cell shrinkage, the diameters of freeze-etched E. coli cells (average, 0.99 μm) were compared with those of preparations after negative staining and after ultrathin sectioning. Negatively stained cells measured from 0.65 to 1.0 μm in diameter, and ultrathin sections showed average cell diameters of 0.70 μm. Freeze-etched replicas of logarithmically growing, as well as stationary, E. coli B cells revealed a smooth, finely pitted cell surface in contrast to cell surfaces seen with other preparative methods. The frozen cell wall may cleave in two planes, exposing (i) a smooth fracture face within the lipid layer and (ii) in rare instances an ill-defined particulate layer. Most frequently, however, cleavage of the envelope occurred between wall and protoplasmic membrane; large areas of the membrane were then exposed and showed a surface studded with predominantly spherical particles, an appearance which did not significantly change when the cells were fixed in formaldehyde and osmium tetroxide before freeze-etching. The distribution of these particles differed between logarithmically growing cells and stationary cells.
PMCID: PMC250482  PMID: 4189229
18.  Fine Structures of Cell Envelopes of Chlamydia Organisms as Revealed by Freeze-Etching and Negative Staining Techniques 
Journal of Bacteriology  1973;116(3):1355-1363.
The cell walls of Chlamydia psittaci (meningopneumonitis strain) were examined by the freeze-etching and negative staining techniques. It was observed that the cleaved convex surface of the developmental, reticulate body was covered with numerous non-etchable particles 9 to 10 nm in diameter, these particles being rarely seen on the concave surface. Similarly, the convex surface of the mature, elementary body (EB) was covered with many particles but the concavity lacked these particles. After etching, the smooth concave surface of the EB appeared to have a hexagonally arrayed subunit structure, on which the button structure (B structure) was observed. Each B structure had a diameter of 27 nm and several B structures were grouped together in a hexagonal arrangement with a center-to-center spacing of 45 nm. In a limited area of the negatively stained EB cell wall, hexagonally arrayed rosette structures were present, with a center-to-center spacing similar to the B structures seen in the freeze-etched preparation. Each rosette, about 19 to 20 nm in diameter, appeared to be composed of a radial arrangement of nine subunits. The freeze-fractured cell wall-cytoplasmic membrane complexes indicated that the outer surface of the cytoplasmic membrane which appeared as the convex surface was covered with the fine particles, and thus it was likely that frozen EB was cleaved at the gap between the cell wall and ctyoplasmic membrane. On the cleaved inclusion, several groups of fine particles were observed. In each group, the particles were arranged hexagonally with the spacing ranging from 20 to 50 nm.
PMCID: PMC246495  PMID: 4127629
19.  Microtrabecular structure of the axoplasmic matrix: visualization of cross-linking structures and their distribution 
The Journal of Cell Biology  1980;87(2):464-479.
Axoplasmic transport is a dramatic example of cytoplasmic motility. Constituents of axoplasm migrate as far as 400 mm/d or at approximately 5 micron/s. Thin-section studies have identified the major morphological elements within the axoplasm as being microtubules, neurofilaments (100-A filaments), an interconnected and elongated varicose component of smooth endoplasmic reticulum (SER), more dilated and vesicular organelles resembling portions of SER, multivesicular bodies, mitochondria, and, finally, a matrix of ground substance in which the tubules, filaments, and vesicles are suspended. In the ordinary thin-section image, the ground substance is comprised of wispy fragments which, in not being noticeably tied together, do not give the impression of representing more than a condensation of what might be a homogeneous solution of proteins. With the high-voltage microscope on thick (0.5-micron) sections, we have noticed, however, that the so- called wispy fragments are part of a three-dimensional lattice. We contend that this lattice is not an artifact of aldehyde fixation, and our contention is supported by its visability after rapid-freezing and freeze-substitution. This lattice or microtrabecular matrix of axoplasm was found to consist of an organized system of cross-bridges between microtubules, neurofilaments, cisternae of the SER, and the plasma membrane. We propose that formation and deformation of this system are involved in rapid axonal transport. To facilitate electron microscope visualization of the trabecular connections between elements of axoplasm, the following three techniques were used: first, the addition of tannic acid to the primary fixative, OsO4 postfixation, then en bloc staining in uranyl acetate for conventional transmission electron microscope (TEM); second, embedding tissue in polyethylene glycol for thin sectioning, dissolving out the embedding medium from the sections and blocks, critical-point-drying (J. J. Wolosewick, 1980, J. Cell Biol., 86:675-681.), and then observing the matrix-free sections with TEM or the blocks with a scanning electron microscope; and third, rapid freezing of fixed tissue followed by freeze-etching and rotary- shadowing with replicas observed by TEM. All of these procedures yielded images of cross-linking elements between neurofilaments and organelles of the axoplasm. These improvements in visualization should enable us to examine the distribution of trabecular links on motile axonal organelles.
PMCID: PMC2110738  PMID: 6159361
The Journal of Cell Biology  1974;62(2):473-490.
We have examined thin sections and replicas of freeze-fractured cilia of Tetrahymena pyriformis. The ciliary necklace located at the base of all freeze-fractured oral and somatic cilia has been studied in thin sections. Since electron-dense linkers have been found to connect both microtubule doublets and triplets to the ciliary membrane at the level of the necklace, the linkers and the associated necklace seem to be related to the transition region between the doublets and triplets of a cilium. Plaque structures, consisting of small rectangular patches of particles located distal to the ciliary necklace, are found in strain GL, but are absent in other strains examined in this study. In freeze-cleaved material, additional structural differentiations are observed in the distal region of the ciliary membranes of somatic and oral cilia. Somatic cilia contain many randomly distributed particles within their membrane. Oral cilia can be divided into three categories on the basis of the morphology of their freeze-fractured membranes: (a) undifferentiated cilia with very few randomly distributed particles: (b) cilia with particles arranged in parallel longitudinal rows spaced at intervals of 810–1080 Å that are located on one side of the cilium; and (c) cilia with patches of particles arranged in short rows oriented obliquely to the main axis of the cilium. The latter particles, found on one side of the cilium, seem to serve as attachment sites for bristles 375–750 Å long and 100 Å wide which extend into the surrounding medium. The particles with bristles are located at the tips of cilia in the outermost membranelle and may be used to detect food particles and/or to modify currents in the oral region so that food particles are propelled more efficiently into the buccal cavity. Examination of thin-sectioned material indicates that the particles in oral cilia which form the longitudinal rows could be linked to microtubule doublets. Linkage between microtubule doublets and adjacent membrane areas on one side of the cilium could modify the form of ciliary beat by restricting the sliding of the microtubules. It is suggested that membrane-microtubule interactions may form the basis for the various forms of ciliary beat observed in different organisms.
PMCID: PMC2109394  PMID: 4214824
21.  Periplasmic Structure of Frozen-Etched and Negatively Stained Cells of Bacillus licheniformis as Correlated with Penicillinase Formation 
Journal of Bacteriology  1969;100(2):1002-1009.
Bacillus licheniformis strain 749/C (constitutive for penicillinase formation) and uninduced cells of strain 749 (penicillinase-inducible) were examined after freezeetching. In the early stationary phase, strain 749/C organisms had clusters of vesicles (30 to 40 nm in diameter) on the outer surface of the plasma membrane. These are randomly distributed on the membrane, including the region of septum formation. The vesicles are not intimately associated with the plasma membrane, and their inner and outer surfaces are devoid of particles. Periplasmic vesicles were not detected by freeze-etching in strain 749 (uninduced) or in young cells of 749/C; however, the membrane of mid-logarithmic phase 749/C cells had a corrugated appearance. Negatively stained 749/C cells (logarithmic phase) also showed many vesicular and tubular bodies in the periplasm as well as septal and cytoplasmic mesosomes of typical morphology. The periplasmic structures appear to be formed either by evagination of plasma membrane or by migration of vesicular bodies from the membranous pockets of the cytoplasm. Stationary phase cells of 749/C still have many periplasmic vesicular bodies; however, the mesosomes are greatly reduced both in number and size. In sharp contrast, strain 749 organisms have very few structures similar to the periplasmic bodies of strain 749/C. These findings support our previous view that penicillinase-producing cells of 749/C have periplasmic membranous structures that are rare in the uninduced strain 749, though there is some lack of correspondence between freeze-etching, negative staining, and thin section data. These structures may be important for the retention or storage of penicillinase in the cell.
PMCID: PMC250187  PMID: 4187700
The Journal of Cell Biology  1973;56(2):492-499.
Pulmonary endothelial cells are capable of metabolizing a variety of circulating hormonal substances. Indirect evidence indicates that some of the relevant enzymes are located on the plasma membrane. The associated caveolae are of special interest as globular subunits, possibly enzyme clusters, are evident in their membranes. In the present study, freeze-etch techniques were used to improve understanding of the fine structure of endothelial cells and to extend our investigations of possible sites of enzymes capable of metabolizing circulating vasoactive agents. As in other cells studied by freeze-etching, intramembranous particles are found on both inner aspects of the plasma membrane. In undifferentiated areas of plasma membrane, the particles appear to have a random distribution. These areas fracture such that approximately equal proportions of the particles adhere to the cytoplasmic aspect of the outer leaflet and the extracellular aspect of the inner leaflet. However, the particles organize into rosettes and plaques at the base of caveolae, and, after fracture, the rosettes and plaques adhere predominantly to the cytoplasmic aspect of the outer leaflet. The peculiar organization of particles in association with caveolae supports the concept that caveolae have a stomal skeletal structure and raises the possibility that the organization may be in some way related to pinocytosis.
PMCID: PMC2108905  PMID: 4566524
23.  Prefracture and cold-fracture images of yeast plasma membranes 
The Journal of Cell Biology  1980;86(1):113-122.
Fracture-temperature related differences in the ultrastructure of plasmalemma P faces of freeze-fractured baker's yeast (Saccharomyces cerevisiae) have been observed in high-resolution replicas prepared in freeze-etch systems pumped to 2 X 10(-7) torr in which the specimens were protected from contamination by use of liquid nitrogen-cooled shrouds. Two major P-face images were observed regardless of the source of the yeast, the age of the culture, the growth temperature, the physiological condition, or the suspending medium used: (a) a "cold- fracture image" with many strands closely associuated with tubelike particles (essentially the same image as those previously published for yeast freeze-fractured at 77 degrees K), and (b) a "prefracture image" characterized by the presence of more distinct tubelike particles with few or no associated strands (for aging cultures, the image recently referred to as "paracrystalline arrays" of "craterlike particles"). Both types of P-face image can be found in separate areas of single replicas and occasionally even within a single plasma membrane. Whereas portions of replicas known to be fractured at any temperature colder than 218 degrees K reveal only the cold-fracture image, prefracture images are found in cells intentionally fractured at 243 degrees K and in cracks or fissures which develop during the freezing of other specimens. These findings demonstrate that the prefracture image results from the fracturing of specimens at some temperature above 230 degrees K, no t from fracturing specimens at some temperature between 173 degrees and 77 degrees K, and not from the use of "starved" yeast cells.
PMCID: PMC2110657  PMID: 6998983
24.  Location of the Fracture Faces Within the Cell Envelope of Acinetobacter Species Strain MJT/F5/5 
Journal of Bacteriology  1974;118(2):693-707.
The cell wall of the gram-negative bacterium Acinetobacter species strain MJT/F5/5 shows in thin section an external “additional” layer, an outer membrane, an intermediate layer, and a dense layer. Negatively stained preparations showed that the additional layer is composed of hexagonally arranged subunits. In glycerol-treated preparations, freeze-etching revealed that the cell walls consist of four layers, with the main plane of fracture between layers cw 2 and cw 3. The surface of [Formula: see text] 2 consisted of densely packed particles, whereas [Formula: see text] 3 appeared to be fibrillar. In cell envelopes treated with lysozyme by various methods, the removal of the dense layer has detached the outer membrane and additional layer from the underlying layers, as shown in thin sections. When freeze-etched in the absence of glycerol, these detached outer membranes with additional layers fractured to reveal both the faces [Formula: see text] 2 and [Formula: see text] 3 with their characteristic surface structures, and, in addition, both the external and internal etched surfaces were revealed. This experiment provided conclusive evidence that the main fracture plane in the cell wall lies within the interior of the outer membrane. This and other evidence showed that the corresponding layers in thin sections and freeze-etched preparations are: the additional layer, cw 1; the outer membrane, cw (2 + 3); and the intermediate and dense layers together from cw 4. Because of similarities in structure between this Acinetobacter and other gram-negative bacteria, it seemed probable that the interior of the outer membrane is the plane most liable to fracture in the cell walls of most gram-negative bacteria.
PMCID: PMC246804  PMID: 4133353
25.  Surfaces of rod photoreceptor disk membranes: integral membrane components 
The Journal of Cell Biology  1982;95(2):487-500.
The membrane surfaces within the rod outer segment of the toad, Bufo marinus, were exposed by rapid-freezing followed by freeze-fracture and deep-etching. Platinum-carbon replicas of disk membranes prepared in this way demonstrate a distinct sidedness. The membrane surface that faces the lumen of the disk shows a fine granularity; particles of approximately 6 nm are packed at a density of approximately 30,000/micron 2. These dimensions suggest that the particles represent protrusions of the integral membrane protein, rhodopsin, into the intradisk space. In addition, when rhodopsin packing is intentionally perturbed by exhaustive digestion with phospholipase C, a concomitant change is observed in the appearance of the luminal surface granularity. The cytoplasmic surface of the disk rarely displays this rough texture; instead it exhibits a collection of much larger particles (8-12 nm) present at approximately 10% of the concentration of rhodopsin. This is about the size and concentration expected for certain light-regulated enzymes, cGMP phosphodiesterase and GTP-binding protein, which are currently thought to localize on or near the cytoplasmic surface of the disk. The molecular identity of the 8-12-nm particles will be identified in the following companion paper. A further differentiation of the cytoplasmic surface can be seen around the very edge, or rim, of each disk. This rim has relatively few 8-12- nm particles and instead displays short filamentlike structures connecting it to other membranes. These filaments extend between adjacent disks, across disk incisures, and from disk rims to the nearby plasma membrane.
PMCID: PMC2112956  PMID: 6815210

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