Visualisation of microalgal lipid bodies through electron microscopy

In this study, transmission electron microscopy (TEM) and cryo-scanning electron microscopy (cryo-SEM) were evaluated for their ability to detect lipid bodies in microalgae. To do so, Phaeodactylum tricornutum and Nannochloropsis oculata cells were harvested in both the mid-exponential and early stationary growth phase. Two different cryo-SEM cutting methods were compared: cryo-planing and freeze-fracturing. The results showed that, despite the longer preparation time, TEM visualisation preceded by cryo-immobilisation allows a clear detection of lipid bodies and is preferable to cryo-SEM. Using freeze-fracturing, lipid bodies were rarely detected. This was only feasible if crystalline layers in the internal structure, most likely related to sterol esters or di-saturated triacylglycerols, were revealed. Furthermore, lipid bodies could not be detected using cryo-planing. Cryo-SEM is also not the preferred technique to recognise other organelles besides lipid bodies, yet it did reveal chloroplasts in both species and filament-containing organelles in cryo-planed Nannochloropsis oculata samples.

Gap junction formation between reaggregated Novikoff hepatoma cells

We have combined freeze-fracture and electrophysiological methods in a study of gap junction formation between reaggregated Novikoff hepatoma cells. Cell clumps are dissociated with EDTA, and the resulting single cells are allowed to reaggregate (5-180 min) in loose pellets in the presence of calcium at 37 degrees . The earliest electron microscopic evidence for the genesis of new junctions is the appearance of flattened regions of the plasma membrane with a relative paucity of small intramembranous particles. These regions contain instead loosely organized groupings of 9- to 11-nm intramembranous particles, which are seen on the A face of the fractured plasma membrane, while corresponding pits occur on the membrane B face. We have termed the specialized membrane regions "formation plaques." They are seen as early as 5 min after reaggregation and are quite numerous by 30 min. Larger plaques are observed at later times. Plaques seen at 30 min are consistently matched with other plaques on apposed cells, although the extracellular space separating these structures still exceeds 10 nm. By 60 min, some matched plaques display a reduced extracellular space, resembling that of normal gap junctions. Between 30 and 60 min, aggregates of closely packed particles on A faces and hexagonally arranged pits on B faces frequently appear in the formation plaques. The aggregates, which are indistinguishable from small gap junctions, appear to enlarge over the subsequent 2-hr period as the number of unaggregated 9- to 11-nm particles declines. Microelectrode studies demonstrate progressive increases in the percent of interfaces containing lowresistance junctions and in the degree of elctrical coupling in preparations incubated up to 2 hr. Coupling is first detected at about the same time as particle aggregates (or formation plaques with reduced extracellular spaces), and increases as aggregate sizes increase.

Gap junctions between photoreceptor cells in the vertebrate retina

In the outer plexiform layer of the retina the synaptic endings of cone cells make specialized junctions with each other and with the endings of rod cells. The ultrastructure of these interreceptor junctions is described in retinas of monkeys, rabbits, and turtles, in thin sections of embedded specimens and by the freeze-fracturing technique. Cone-to-rod junctions are ribbon-like areas of close membrane approximation. On either side of the narrowing of the intercellular space, the junctional membranes contain a row of particles located on the fracture face A (cytoplasmic leaflet), while the complementary element, a row of single depressions, is located on fracture face B. The particle rows are surrounded by a membrane region that is devoid of particulate inclusions and bears an adherent layer of dense cytoplasmic material. Cone-to-cone junctions in some places are identical to cone-to-rod junctions, while in other places they closely resemble typical gap junctions (nexus). Interreceptor junctions, therefore, represent a morphological variant of the gap junction, and probably mediate electrotonic coupling between neighboring photoreceptor cells.

Response of Dictyostelium plasma membranes to adenosine 3':5'-cyclic monophosphate

Myxamoebae of the cellular slime mold Dicytostelium discoideum aggregate in response to a chemotactic substance identified as adenosine 3':5'-cyclic monophosphate. Upon aggregation cell division is suppressed, the cells become adhesive, and differentiation is initiated. Freeze-fracture studies of myxamoebae were conducted to determine the effect of cyclic AMP and calcium on plasma membrane ultrastructure. The inner surfaces of the plasma membranes exhibited particulate structures whose sizes (43-187 A) and frequency distribution were determined. Cyclic AMP and calcium induced within 2 hr the formation of particles having average diameters 1.7-times and 1.5-times greater, respectively, than those of the vegetative myxamoebae controls. These data suggest that cyclic AMP mobilizes the intracellular calcium which may be effective in changing plasma membrane structure.