The correlation between the saturation of membrane fatty acids and the presence of membrane fracture faces after osmium fixation

Complex dynamic development of poliovirus membranous replication complexes

Replication of all positive-strand RNA viruses is intimately associated with membranes. Here we utilize electron tomography and other methods to investigate the remodeling of membranes in poliovirus-infected cells. We found that the viral replication structures previously described as "vesicles" are in fact convoluted, branching chambers with complex and dynamic morphology. They are likely to originate from cis-Golgi membranes and are represented during the early stages of infection by single-walled connecting and branching tubular compartments. These early viral organelles gradually transform into double-membrane structures by extension of membranous walls and/or collapsing of the luminal cavity of the single-membrane structures. As the double-membrane regions develop, they enclose cytoplasmic material. At this stage, a continuous membranous structure may have double- and single-walled membrane morphology at adjacent cross-sections. In the late stages of the replication cycle, the structures are represented mostly by double-membrane vesicles. Viral replication proteins, double-stranded RNA species, and actively replicating RNA are associated with both double- and single-membrane structures. However, the exponential phase of viral RNA synthesis occurs when single-membrane formations are predominant in the cell. It has been shown previously that replication complexes of some other positive-strand RNA viruses form on membrane invaginations, which result from negative membrane curvature. Our data show that the remodeling of cellular membranes in poliovirus-infected cells produces structures with positive curvature of membranes. Thus, it is likely that there is a fundamental divergence in the requirements for the supporting cellular membrane-shaping machinery among different groups of positive-strand RNA viruses.

The mechanism of diatom locomotion. I. An ultrastructural study of the motility apparatus

Observations of raphe-associated cell structure of the diatomNavicula cuspidatasuggest the involvement in cell locomotion of secretory vesicles, the locally specialized plasmalemma opposite the raphe, microfilamentous bundles and strands of mucilage in the raphe. It is proposed th at diatoms are propelled by the flow of adhesive strands of mucilage that project from the raphe, powered and controlled by a membrane-associated microfilamentous system.

Epidermal permeability barrier: transformation of lamellar granule-disks into intercellular sheets by a membrane-fusion process, a freeze-fracture study

Freeze-fracture replication of lamellar granules and intercellular sheets of the horny layer in mouse, chicken, and snake epidermis reveals a pattern of serial fracture faces which is highly suggestive of polar lipids in a bilayer configuration. The occurrence of alternating wide and narrow fracture faces separated by intervening steps supports the view that epidermal barrier bilayers display lipid asymmetry similar to membranes. Within the lamellar granules, bilayers arrange to form disks which in fact are equivalent to flattened unilamellar liposomes. Stacking of the disks in turn gives rise to the lamellar pattern. After exocytosis into the intercellular space, the disks are arranged parallel to the cell membranes. In tangentially fractured specimens, the cleavage plane jumps back and forth from the plasma membrane to a disk-bilayer, thereby giving rise to the known phenomenon of EF-ridges (on the extracellular fracture face) and PF-grooves (in the plasmatic fracture face) which both represent the level of the plasma membrane sur- or subjacent to the aisles between disks. Concomitantly with the upward movement of the keratinocytes, the ridges and grooves become narrower until they fade away by the second or third cell layer of the stratum corneum. This phenomenon is explained by the fusion of adjacent disks at their highly curved brims due to a mechanism similar to the process of membrane fusion which causes the formation of wide, uninterrupted sheets.

Understanding the artefact problem in freeze-fracture replication: a review

Freeze-fracture and freeze-etching techniques do not provide artefact-free images of native in vivo or in vitro cells and tissues. Each preparation stage can produce specific artefacts which must be recognized and understood if these methods are to contribute meaningful information to cell biology, This paper reviews the latest information available on artefacts in freeze-fracture replication (and etching) methods and points to possibilities for avoiding some of them. Different specimens show different sensitivity to artefactual changes and the final images must be interpreted carefully with regard to the multi-event process that has led to their production.

Intramembranous particle distribution in human erythrocytes: effects of lysis, glutaraldehyde, and poly-L-lysine

Freeze-fracture combined with quantitative electron microscopy of the intact human erythrocyte (RBC) and ghost revealed significant differences in their intramembranous particle coefficients. External (E) fracture-faces of unfixed ghost membranes were found to contain 40% fewer particles than those of intact unfixed RBC. The particle distribution of the intact RBC membrane depended on the use of glutaraldehyde fixation and glycerol cryoprotection. Whereas glutaraldehyde- and glycerol-treated cells disclosed 70% fewer E-face particles than did intact unfixed cells, poly-L-lysine-treated, intact, unfixed RBC showed no such differences. Treatment with a combination of poly-L-lysine and glutaraldehyde, however, increased the amount of E-face particles while reducing those of the protoplasmic (P) face. The poly-L-lysine effect varied with its concentration and was unaffected by previous application of neuraminidase. Nor did the lectin phytohemagglutinin induce particle rearrangement in intact cells. Our data demonstrate that the processes of glutaraldehyde fixation and glycerol cryoprotection modify the RBC membrane by decreasing the number of E-face particles present. In addition, the combination of poly-L-lysine and glutaraldehyde alters the affinity of some particles for one half of the membrane, suggesting that in freeze-fractured RBC, chemical bonds formed at the extracellular surface of the membrane can influence particle partitioning.

Changes in the structure of neuromuscular junctions caused by variations in osmotic pressure

Neuromuscular junctions of the frog, Rana pipiens, were examined for structural modifications produced by exposure to increased and reduced osmotic pressure (pi). Preparations exposed to increased pi for varying lengths of time were fixed with either OSO4-Veronal with and without calcium, glutaraldehyde-phosphate, or glutaraldehyde-formaldehyde-phosphate as primary fixatives. The greatest difference between the fixatives was seen in preparations exposed to increased pi for 5 min, corresponding to the time when miniature endplate potential frequency is highest. The 5-min OSO4 calcium-free preparations appeared comparatively normal, while those fixed with OSO4 and 2 mM CaCl2 or aldehyde-phosphate had wide infoldings of the presynaptic membrane and a reduced number of synaptic vesicles. Aldehyde-phosphate had the same effect on mouse diaphragm. Another series of frog preparations were conditioned to elevated pi and then returned to normal Ringer's for varying times before fixation in OSO4-phosphate. Preparations fixed 2 min after their return to normal Ringer's showed marked disruption of the presynaptic membrane as well as apparently rupturing vesicles. If fixed after 10 min, terminals were depleted of vesicles although the presynaptic membrane had returned to its normal position and appearance.

The cryopreservation of Chlorella. 2. Effect of growth temperature on freezing tolerance

The temperature at which Chlorella 211/8h was grown determined the response to a subsequent stress of freezing to and thawing from-196 degrees C. Cells cultured at 20 degrees C were the most sensitive to freezing injury; at both higher and lower growth temperatures resistance to damage induced by freezing developed. At all culture temperatures examined the freezing tolerance varied with the age of culture.

The permeability barrier in mammalian epidermis

The structural basis of the permeability barrier in mammalian epidermis was examined by tracer and freeze-fracture techniques. Water-soluble tracers (horesradish peroxidase, lanthanum, ferritin) were injected into neonatal mice or into isolated upper epidermal sheets obtained with staphylococcal exfoliatin. Tracers percolated through the intercellular spaces to the upper stratum granulosum, where further egress was impeded by extruded contents of lamellar bodies. The lamellar contents initially remain segregated in pockets, then fuse to form broad sheets which fill intercellular regions of the stratum corneum, obscuring the outer leaflet of the plasma membrane. These striated intercellular regions are interrupted by periodic bulbous dilatations. When adequately preserved, the interstices of the stratum corneum are wider, by a factor of 5-10 times that previously appreciated. Freeze-fracture replicas of granular cell membranes revealed desmosomes, sparse plasma membrane particles, and accumulating intercellular lamellae, but no tight junctions. Fractured stratum corneum displayed large, smooth, multilaminated fracture faces. By freeze-substitution, proof was obtained that the fracture plane had diverted from the usual intramembranous route in the stratum granulosum to the intercellular space in the stratum corneum. We conclude that: (a) the primary barrier to water loss is formed in the stratum granulosum and is subserved by intercellular deposition of lamellar bodies, rather than occluding zonules; (b) a novel, intercellular freeze-fracture plane occurs within the stratum corneum; (c) intercellular regions of the stratum corneum comprise an expanded, structurally complex, presumably lipid-rich region which may play an important role in percutaneous transport.

Factors influencing the frequency of mesosomes observed in fixed and unfixed cells of Streptococcus faecalis

Mesosomes of Streptococcus faecalis (American Type Culture Collection 9790) were seen about 92% less frequently in freeze fractures of unfixed cells than in freeze fractures and sections of fixed cells. This difference in frequency was not related to any period of unbalanced macromolecular synthesis induced by chemical fixation. All measured synthetic processes (DNA, RNA, and protein synthesis, and glycerol incorporation) were halted with either osmium tetroxide (OS) or glutaraldehyde fixation. That fewer mesosomes were seen in freeze fractures of unfixed cells was probably due to the difficulty of observing cross-fractured mesosomes in this organism in the unfixed state. Unfortunately, mesosomes probably preferentially cross fracture in the unfixed state and therefore are usually only observed, infrequently, in those cases where the freeze fracture follows the surface layer of a mesosomal membrane. However, the addition of glycerol to unfixed cells, especially in the chilled state, greatly increased the frequency of observation of cytoplasmic mesosomes in freeze fractures. It is thought that glycerol, like chemical fixation, increases the number of surface-fractured mesosomes, which in turn increases the frequency of mesosome observation. It was also observed that cellular autolysis occurring during OS fixation seemingly reduced the number of mesosomes observed in thin sections and freeze fractures of OS-fixed cells.

Molecular alterations in the plasma membrane of sporangiospores of Phycomyces related to germination

Replicas of concave fractures of freeze-fractured plasma membrane of dormant sporangio­-spores ofPhycomycesshow particles of three types, namely, (i) small (5-8 nm†); (ii) large homogeneous (30 to 35 nm); (iii) compound (30 to 35 nm). The compound particles appear to be composed of four oblong subparticles (23 nm x 12 nm). The convex fracture shows only small particles (5 to 8 nm), and also depressions of two types that are complementary to the homogeneous and compound particles on the opposite (concave) face. Heat shocking initiates germination of spores. The compound particles are no longer seen by 6 h after heat-shocking. Aggregates of homogeneous particles appear upon heat-shocking, and these aggregates grow to large arrays by about 12 h. These aggregates and the homo­geneous particles disappear by the time the spore germinates, that is, about 20 h after heat-shocking. It therefore appears that both homogeneous and compound particles are related to controlled dormancy and germination of spores. The number of small particles increases on both the fractured faces as the spore germinates. Partial digestion of homogeneous particles by incubation with pronase suggests that these macromolecules may contain protein and are amphipathic in structure. The small particles are apparently unaffected by treatment with pronase. Evidence indicative of pinocytotic activity is seen in germinating spores.