Journal of ultrastructure research最新文献

We have determined the mass-per-length (MPL) and the width of unstained freeze-dried reconstituted human epidermal keratin filaments by scanning transmission electron microscopy (STEM). Filaments were reassembled from keratins extracted from four different sources: cultured human epidermal cells (CHEC), human callus (CAL), and the living layers (LL) and stratum corneum (SC) of normal human epidermis. MPL histograms of all four keratin filament types could be fitted by a superposition of two or three Gaussians, with their respective major peaks located between 17 and 20 kDa/nm. We interpreted the multiple MPL peaks to represent different polymorphic forms of the reconstituted filaments. The number of subunits per filament cross section calculated from MPL peak positions, average subunit molecular weight, and an axial repeat of the subunits within the filament of 46.5 nm revealed an average difference between polymorphic variants of 7.5 ± 0.9 subunits. These data suggest that reconstituted human epidermal keratin filaments are made of two to four 8-stranded “protofibrils” (i.e., made of two laterally aggregated 4-stranded protofilaments), in agreement with earlier observations. The average widths of unstained freeze-dried keratin filaments were larger than those of negatively stained filaments: 12.6 nm (9.6 nm) for CHEC, 12.3 nm (9.7 nm) for CAL, 11.6 nm (8.3 nm) for LL, and 11.3 nm (7.9 nm) for SC keratin filaments, with the values in brackets corresponding to negatively stained samples. Assuming the MPL to be proportional to the square of the filament width, there is a good correlation between the MPL and width measurements both for filaments within a given type as well as among those reconstituted from different types of keratin extracts.

The cell wall and plasma membrane of four strains of Butyrivibrio (anaerobic rumen bacteria) have been studied by freeze-etching and thin sectioning. Some strains have a wall of gram-positive ultrastructure, which is vulnerable to lysozyme, while others have a wall which resembles the outer envelope of a gram-negative bacterium and is less susceptible to lysozyme. The plasma membrane of all strains is rather rigid, and its lipids have a very high phase-transition temperature.

The structure of human enamel crystallites has been studied at a near atomic level by high-resolution electron microscopy. Electron micrographs have been obtained from crystallites present in human enamel with a structure resolution of 0.2 nm in the [0001], [1̄21̄0], [12̄13], [1̄100] and [45̄10] zone axes directions. In most cases it was possible to match the experimental images with images calculated using the atomic positions of mineral hydroxyapatite. However, in some cases a discrepancy between calculated and experimental image detail was observed in the c direction of the [1̄21̄0] and the [1̄100] images. This shows: (i) a structural heterogeneity of the crystals, and (ii) a loss of hexagonal symmetry of the structure. The resolution required to distinguish individual atomic sites in the different zones has been determined, and this will provide a useful basis for future work. As the determination of the “real structure” of biological crystals is of prime importance for the study of calcification mechanisms (crystal growth), biological properties and destructive phenomena of calcified tissues (i.e., dental caries and bone resorption).

The development of Sonchus yellow net virus was studied in ultrathin sections of protoplasts sampled at various intervals after inoculation. The sequence of major ultrastructural changes consisted of appearance in the nucleus, 10 hr after inoculation, of a granular matrix with coiled nucleocapsid strands at its edge, budding of virus particles at the inner nuclear membrane followed by the release of virus particles into the perinuclear space and their spread in the endoplasmic reticulum. Fusion of viral envelopes with endoplasmic reticulum membranes occurred from 24 hr after inoculation and was followed by release of coiled nucleocapsid strands in the cytoplasm. Incubation of infected protoplasts in the presence of tunicamycin prevented budding of virus particles and cores accumulated in the nucleus. Neither virus particles nor cores were detected in the cytoplasm.

To the common aspects of spermiogenesis in Reptilia, several new organelles are described in the lizard. One or two nuclear pouches, formed by the invagination of the nuclear envelope, contain a substance similar to that of the perinuclear substance. They are formed near the acrosome and follow an helical pathway at the surface of the nucleus. The pouches could be driven by a nuclear ribbon made of six to seven microtubules engaged in the mouth of each pouch. The perinuclear substance, transported by the pouches, is deposited near the posterior part of the nucleus. The evolution of the nuclear pores, and the attachment of the chromatin to the nuclear envelope suggest that the chromatin is twisted inside the nucleus, following the displacement of the nuclear pouches. Adjacent pouches are separated by a lamellar plate, possibly originating from a modified mitochondrion. Untwisting of the nucleus and chromatin is evident at the time the manchette is present. The chromatin fibers become coarser and are progressively aligned parallel to the nuclear axis. The midpiece of the flagellum is very short; it wears 15 mitochondria disposed into three crowns. The ribs of the fibrous sheet are synthesized from the posterior part of the flagellum, anteriorly. A tegosomial sheet is described. It is composed of lipidic droplets that are injected between the two nuclear membranes, prior to the spermiation. A classification of the steps of spermiogenesis is proposed.

In this study, two processes of ciliary resorption in the sessile heterotrich ciliateEufolliculina uhligi are presented. (1) Prior to division,E. uhligi resorbs its oral apparatus. This occurs by the incorporation of axonemes of single cilia into the cytoplasm, and by the formation of a large vacuole, in which the membranellar cilia of the buccal cavity are enclosed. The cilia inside the vacuole are continuously resorbed. After some minutes, this vacuole divides into several smaller ones which accumulate in the opisthe. They are characterized by a progressive lamination and infolding of their membrane; the cilia inside the vacuole degenerate. Morphological similarities and dissimilarities to digestive vacuoles are discussed. (2) Transformation of the motile swarmer into the sessile cell includes lorica secretion as well as resorption of the anterior membraneller cilia. The lorica material is secreted by exocytosis. The vesicular membrane is incorporated into the plasma membrane. The cilia are resorbed by retraction of the axonemes or by lateral fusion with the plasmalemma. The fate of the additional plasma membrane has been studied using cationized ferritin as an exogeneous marker. It could be demonstrated, that at the sites of ciliary resorption and exocytosis plasma membrane is retrieved endocytotically via coated pits. Endocytosis is not restricted to the parasomal sacs, but also occurs at the septae between adjacent alveoli. The internalized membrane moves as small vesicles and/or flattened cisternae from the periphery toward the center of the cell. No fusion of these membraneous elements with secondary lysosomes has been observed 1 h after application of ferritin. The further fate of the internalized membrane is discussed.

The parameters which might play a role in the compression of plastic-embedded objects are studied. The compression is measured on spherical polystyrene latex particles, used as markers in the grid sectioning technique. By changing independently the hardness of the latex particles through a controlled electron irradiation and the hardness of the embedding medium, it is shown that compression is a local event depending only on the mechanical properties of the sectioned object and not on the properties of its surrounding materials. It is demonstrated on one hand that the intrinsic resin compression diminishes as resin hardness increases, and on the other hand that the latex compression can be completely eliminated after a preliminary irradiation by electrons (the electron-induced vulcanization is equivalent to a hardening). It is thus concluded that compression could be greatly reduced or eliminated if objects were sufficiently hardened during their preparation. Several preparation procedures for biological specimens are suggested. For comparison latex has also been irradiated 3½ days near a 25 000-Ci⁶⁰Co gamma source and near the core of an 8-MW nuclear reactor: Neither of these irradiations was sufficient to produce a hardening equivalent to that of the electron irradiation.