Studies on freeze-etching of cell membranes

The fluid-mosaic membrane theory in the context of photosynthetic membranes: Is the thylakoid membrane more like a mixed crystal or like a fluid?

Since the publication of the fluid-mosaic membrane theory by Singer and Nicolson in 1972 generations of scientists have adopted this fascinating concept for all biological membranes. Assuming the membrane as a fluid implies that the components embedded in the lipid bilayer can freely diffuse like swimmers in a water body. During the detailed biochemical analysis of the thylakoid protein components of chloroplasts from higher plants and algae, in the '80 s and '90 s it became clear that photosynthetic membranes are not homogeneous either in the vertical or the lateral directions. The lateral heterogeneity became obvious by the differentiation of grana and stroma thylakoids, but also the margins have been identified with a highly specific protein pattern. Further refinement of the fluid mosaic model was needed to take into account the presence of non-bilayer lipids, which are the most abundant lipids in all energy-converting membranes, and the polymorphism of lipid phases, which has also been documented in thylakoid membranes. These observations lead to the question, how mobile the components are in the lipid phase and how this ordering is made and maintained and how these features might be correlated with the non-bilayer propensity of the membrane lipids. Assuming instead of free diffusion, a "controlled neighborhood" replaced the model of fluidity by the model of a "mixed crystal structure". In this review we describe why basic photosynthetic regulation mechanisms depend on arrays of crystal-like lipid-protein macro-assemblies. The mechanisms which define the ordering in macrodomains are still not completely clear, but some recent experiments give an idea how this fascinating order is produced, adopted and maintained. We use the operation of the xanthophyll cycle as a rather well understood model challenging and complementing the standard Singer-Nicolson model via assigning special roles to non-bilayer lipids and non-lamellar lipid phases in the structure and function of thylakoid membranes.

The Freeze-Fracture Technique: Application to the Study of Animal Plasma Membranes

During recent years, the freeze-fracture (FF) technique has become one of the most useful procedures available for the ultrastructural analysis of cell components, particularly for the study of biological membranes. The method has gradually evolved from a highly specialized and technically complex procedure to a reasonably accesible one, mainly as a consequence of improvements in commercial FF equipment, the understanding of the fracturing process and the artifacts induced, and the development of ancillary techniques for the study of cell membrane organization. Due to these advances, the FF method can be considered at present as an almost standard procedure for biological electron microscopical laboratories.

Isolation and electron microscopic observations of intracytoplasmic inclusions containing Chlamydia psittaci

Intracytoplasmic inclusions containing Chlamydia psittaci were isolated by a newly established method. Infected L-cells at 20 h after infection were suspended in 0.25 M sucrose-tris(hydroxymethyl)aminomethane buffer containing ethylene-diaminetetraacetic acid, homogenized in a Dounce tissue grinder, and filtered through a 2,000-mesh screen. Isolated inclusions were stabilized in 5% bovine serum albumin in 10 mM tris(hydroxymethyl)aminomethane buffer. Electron microscopic observations revealed the presence of surface projections on the vegetative, reticulate bodies and a direct connection between the reticulate bodies and the inclusion membrane by means of projections.

Demonstration of the outer surface of freeze-etched normal human platelets: correlation with buoyant density

The external surface of the normal human platelet shows clusters of spherical particles in a reticular pattern with smooth and regular surface membrane in adjacent areas. Labelling with cationic ferritin demonstrates a diffuse distribution of ferritin particles which stop at the fracture face of the intramembranous surface. The intramembranous surface is not labelled. Previously discrepant observations on the plasma membrane surface are probably due to different techniques of platelet preparation for freeze-etch electron microscopy. Density-dependent platelet sub-populations do not differ with respect to the ultrastructural features of their external surface and intramembranous portions of the plasma membrane. Based upon the relationship between platelet density and age, it appears that the ultrastructure of the normal platelet plasma membrane does not change with in vivo ageing.

Integrated stereological and biochemical studies on hepatocytic membranes. III. Relative surface of endoplasmic reticulum membranes in microsomal fractions estimated on freeze-fracture preparations

New methods are required for identifying membranes in subcellular fractions with respect to their origin, if such preparations are to be evaluated morphometrically. One method is freeze-fracturing which reveals intramembrane particles whose size, pattern, and numerical density differ for various membrane types. The question is examined whether the differences in numerical particle density per square micrometer of membrane (alpha) can be used to differentiate membrane vesicles found in microsomal fractions from liver cells with respect to their origin in the hepatocytes. It is found that the range of alpha for the protoplasmic face (PF) of endoplasmic reticulum (ER) membrane (1,900 less than alpha less than 3,250) is intermediate between those for plasma and mitochondrial membranes. Since PF(ER) should appear in the outer leaflet of microsomal vesicles, alpha was estimated on concave profiles of freeze-fracture preparations; the numerical frequency distribution of vesicles with respect to alpha was trimodal, with a major peak around 2,900/micrometer2 and 66% of the vesicles in the range determined for PF(ER). Using a new stereological method, it was calculated that 63% of the membrane surface in these microsomal fractions was of ER origin by this criterion. On the same preparations, an attempt was made to label the ER-derived membranes cytochemically for glucose-6-phosphatase. A line intersection count revealed 62% of the membrane surface to be of ER origin on the basis of marker enzyme labeling. These findings indicate a smaller part of ER membranes in microsomal fractions than would be predicted from biochemical data (77%). The possible reasons for such discrepancies are discussed; shifts in particle densities due to the preparation procedure could lead to an underestimate by freeze-fracturing, whereas the prediction from biochemical data could be overestimates if marker enzymes were not homogeneously distributed.

Cell wall enzymatic lysis of the yeast form of Pullularia pullulans and wall regeneration by protoplasts

During the process of degradation of the cell wall of the yeast form of Pullularia pullulans by the lytic system of micromonospora chalcea samples were withdrawn at different times and observed under phase contrast and electron microscope. The progressive lysis of the walls reveals a fibrillar component inside the apparently amorphous wall. Freeze etched preparations of cells during the formation and regeneration of protoplasts show that the cellular membrane is split and this method allows the smooth external face of the membrane and other internal face covered by particles to be seen. The fact that the smooth face of the membrane is only visible during the preparation or the regeneration of protoplasts and very rarely when intact cells are fractured, suggests a strong adherence between cell wall and this external layer of the membrane. During the regeneration which takes place as in most of the yeasts and moulds, a special study of the extension of the cell wall is made and a possible mechanism for this extension of the regenerated cell wall is proposed.

A peculiar substructure in the postsynaptic membrane of Torpedo electroplax

Freeze-etching of Torpedo marmorata electroplax reveals an unusual substructure in one of the fracture faces of the postsynaptic membrane. This substructure consists of a lattice of closely spaced particles that are smaller than the usual 80- to 90-A particles present in other membrane types. The lattice occurs exclusively on the B-face of the postsynaptic membrane.

Ultrastructural organization of chloroplast thylakoids of the green alga Oocystis marssonii

Intact cells of "Oocystis marssonii" were thin sectioned and freeze-etched, using conventional and double-recovery techniques. Thylakoids extend the length of the single chloroplast and occur in stacks of three to five. The peripheral thylakoids in a stack often alternate between adjacent stacks. Interpretation of double-recovery results suggests that membranes in unstacked regions are asymmetrical, with one face smooth and the matching face covered with closely packed 85-90 A diameter particles. Adjacent membranes in stacked regions evidently share 170 A diameter particles, and either membrane in a stacked region may fracture. The two fracture planes thus made possible may expose nearly entire 170 A particles or only the upper portion of such particles, creating in the latter case images of 125-135 A diameter particles. Fracture planes in all cases appear to occur through the interior of the membrane, in the plane between the hydrophobic ends of the lipid bilayer proposed in numerous membrane models.