Electron microscopy reviews最新文献

Typically, models of bacterial structure combine biochemical data obtained from bulk analyses of cell populations with electron microscopic observation of individual cells. Recent development of a battery of cryotechniques specific for biological electron microscopy have begun to supercede routine procedures such as conventional thin sectioning. One of these cryotechniques, freeze-substitution, combines the advantages of ultrarapid freezing with standard microtomy methods. This technique is particularly well suited to the examination of bacterial structure and has yielded additional ultrastructural information consistent with biochemical data but often challenging models of cell structure obtained from conventional microscopical methods. In addition to retaining more accurately the spatial distribution of cell components, freeze-substitution has been successfully combined with immunochemical labelling techniques and has enabled identification and localization of specific molecules both within the cell and on the cell surface. In this review, I describe current ideas on bacterial ultrastructure, modified in accordance with new data obtained from recent freeze-substitution studies.

The biological importance of iron for most living cells has been under increasing attention during recent years. In addition to iron deficiency, iron overload has been recognized as a significant metabolic abnormality with potentially damaging consequences. The iron-storing compounds ferritin and hemosiderin have the unique quality of being ultrastructurally recognizable because of the electron-density of the iron concentrated within their particles. In this review, the electron microscopic features of iron overload are discussed, as found in various subcellular compartments and different types of cells and tissues. Defensive mechanisms against iron overload are exhibited by most cell lines and include: (1) the capacity of synthesis of the protein apoferritin by most cells whenever the concentration of ambient iron increases, (2) the capacity to bind toxic inorganic iron within the hollow shell of apoferritin; the transfer of the assembled iron-rich ferritin molecules into siderosomes and (3) the capability of further iron segregation within siderosomes by degradation of ferritin to hemosiderin. The study provides examples of cytosiderosis in different types of cells and tissues, as well as iron-related ultrastructural pathological changes.

Isolated gap junction plaques contain hexagonal crystalline arrays of membrane channels called connexons which are a suitable specimen for electron crystallography. Image analysis of gap junction lattices has shown that while there is sufficient lattice order for structural analysis to ∼25 Å, there is enough disorder in both the lattice and the connexon to create a family of related images. This review is focused on how these images can be interpreted in terms of what is known about both the connexon and its constituent protein, connexin.

Electron microscopical data is presented on the varying morphology of the complexes formed between DNA and a number of synthetic β-oligopeptides. In general, these peptides produce DNA compaction or condensation, resulting in two main types of complexes: toroids and rods. By controlling the ratio of peptide to DNA, interpretations of the possible packing of DNA within the various compact particles are advanced. Some understanding of the mechanism of peptide-induced DNA compaction in vitro may be of significance in relation to DNA condensation and the discrimination of gene domains in vivo.

Electron spectroscopic imaging (ESI) is a microanalytical technique which is being used to determine elemental distributions at the resolution limit of the electron microscope. Detection and mapping of phosphorus by energy filtered imaging makes it possible to determine the organization of the nucleic acid component in nucleoprotein complexes, because phosphorus is present at much higher levels in nucleic acids than in the associating proteins. ESI has provided a method for approaching numerous questions related to chromatic structure at the level of the specific protein-DNA interactions, at the nucleosome level at higher organizational levels of chromosome structure.

A great deal of information on the 3-dimensional structure of the protein assemblies involved in muscle contraction has been obtained using conventional transmission electron microscopy. In recent years, developments in cryo-electron microscopy have facilitated work with fully hydrated, non-chemically fixed specimens. It is shown how this technique can be used to visualize muscle sarcomere filaments in quasi-native conditions, to access hitherto inaccessible states of the crossbridge cycle, and to obtain new high resolution structural information on their 3-dimensional protein structure.

A short introduction to the crossbridge cycle and its biochemically accessible states illustrates the problems amenable to studies using the electron microscope, as well as the possibilities offered by cryo-microscopy on vitirifed samples. Work on vitrified cryo-sections and myosin filament suspensions demonstrates the acccessibility of crossbridge states and gives implications on the gross structural features of myosin filaments. Recent studies on actin filaments and myosin (Sl) decorated actin filaments provide the first high resolution data on vitrified samples. The use of photolabile nucleotide precursors allows the trapping of short lived states in the millisecond time range, thererby visualizing intermediate states of the crossbridge cycle.

Contact dependent cell mediated cytotoxicity has been found to be executed by lymphocytes, macrophages, and even granulocytes, Cytotoxic effector cells of the lymphatic lineage are divided into cytotoxic T lymphocytes (CTL), mediating MHC related cytotoxicity, and in effectors mediating non-MHC restricted cytotoxicity such as natural killer (NK) cells, T lymphocytes displaying NK-like activity and lymphokine activated killer (LAK) cells. In morphologic studies these cells are hardly to be distinguished: they all show features of large granular lymphocytes (LGLs), which are characterized by a low nuclear to cytoplasmic ratio and azurophilic granules. Ultrastructurally lysosomal granules, showing an electron dense core that is either surrounded by numerous small vesicles or by a small electron translucent halo, have been found. Pore-forming proteins such as perforin, as well as serine esterases and proteoglycans have been pointed out in these granules. Specialities are parallel tubular arrays (PTA) in NK cells and nuclear inclusion bodies in LAK cells.

Morphologically two types of killing event may be distinguished. In one way membrane lesions develop at the surface of target cells upon binding of effector cells and in advanced stages of cytolysis the target cells are sorrounded by a completely disintegrated membrane. The nuclei, however, show only minor changes. In the other way, called apoptosis, the cell membrane of the targets remains intact, but the nucleus and cell organelles very early disintegrate intracellularly. Whether these morphologically different types of cell killing correspond to the functionally different pathways of cell mediated cytotoxicity remains to be resolved.