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In density-inhibited 3T3 fibroblast cultures F-actin is localized almost exclusively at the cell-to-cell borders. Depolymerization of microtubules results in a rearrangement of the F-actin pattern. Numerous actin bundles are formed, most of them are anchored to the cell borders. This rearrangement is preceded by a transient rise in F-actin content. Copyright 1993, 1999 Academic Press

Mammalian cells in culture (BHK-21, PtK2, Friend, human flia, and glioma cells) have been observed by reflection contrast microscopy. Images of cells photographed at two different wavelengths (546 and 436 nm) or at two different angles of incidence allowed discrimination between reflected light and light that was both reflected and modulated by interference. Interference is involved when a change in reflected intensity (relative to glass/medium background reflected intensity) occurs on changing either the illumination wavelength or the reflection incidence angle. In cases where interference occurs, refractive indices can be determined at points where the optical path difference is known, by solving the given interference equation. Where cells are at least 50 nm distant from the glass substrate, intensities are also influenced by that distance as well as by the light's angle of incidence and wavelength. The reflected intensity at the glass/medium interface is used as a standard in calculating the refractive index of the cortical cytoplasm. Refractive indices were found to be higher (1.38--1.40) at points of focal contact, where stress fibers terminate, than in areas of close contact (1.354--1.368). In areas of the cortical cytoplasm, between focal contacts, not adherent to the glass substrate, refractive indices between 1.353 and 1.368 were found. This was thought to result from a microfilamentous network within the cortical cytoplasm. Intimate attachment of cells to their substrate is assumed to be characterized by a lack of an intermediate layer of culture medium.

The characterization of the fibrillar collagens and the cellular control of their spatial deposition were studied in fish scales using immunofluorescence, electron microscopy, electrophoretic and HPLC analyses, immunoprecipitation and hybridization with cDNA probes. This study was carried out on undisturbed and regenerating scales in situ and in organ and cell cultures from regenerating scales. The hyposquamal scleroblasts forming a pseudoepithelium show an apico-basal polarization and synthesize thick collagen fibrils (100 nm) organized in a plywood pattern as long as the integrity of the cell-cell and cell-collagenous matrix contacts are preserved. In culture, scleroblasts become fibroblast-like and produce an unordered meshwork of thin collagen fibrils (30 nm). Comparison of the synthesized collagens in culture with those extracted from the scales indicates that culture conditions modify fibrillogenesis but do not change the expression of fibrillar collagen genes. Type I collagen, the prédominent component, is associated with the minor type V collagen. Type HI collagen was not present. In type I collagen, a third chain, α3 chain, was identified. The ratio between the 3 chains suggests the coexistence of two heterotrimers (αl(I))2 α2(1) and αl(I) α2(1) α3(I). Analysis by HPLC and electrophoresis of the cyanogen bromide-derived peptides obtained from the purified α3 chain support the hypothesis that αl(I) and α3(I) chains are encoded by two different genes. The presence of the two types of heterotrimers in vivo as well as in vitro could correspond to an innate property of the goldfish scleroblasts. Despite the fact that teleost cyanogen bromide-derived peptides differ from those of higher vertebrates, homologies with the mammalian collagen genes (human, for example) are sufficient to allow the detection of mRNA transcripts for al(I), α2(I) and α2(V) from confluent scleroblast cultures with human probes.

Mitochondrial movements have been followed by phase-contrast microscopy in living XTH-cells (Xenopus laevis tadpole-heart cells) in tissue culture. The same organelles have been viewed subsequently in electron micrographs. Locomotion of mitochondria proceeds at velocities up to 100 μm/min. Formation of branches of mitochondria and other shape changes may occur with the same speed. Mitochondrial motility can be classified into 4 types: (1) Alternating extension and contraction at the two ends of rod-shaped mitochondria. (2) Lateral branching. (3) Alternate stretching and contraction of arbitrary parts of a mitochondrion amounting to a kind of peristaltic action. (4) Transverse wave propagation along the organelle. Types 1 to 3 can be reduced to the same underlying principle; they cause locomotion. Formation of mitochondrial extensions is due to elongation of cristae. The observations are discussed in terms of 4 specific proposals. (1) Intracellular locomotion of mitochondria is caused by local enlargements and contractions of the organelles. (2) The shape changes are correlated with alterations in the arrangement of the cristae. (3) Such arrangements are not associated with overall swelling or shrinkage of the mitochondrion; they are local features. (4) Estimates of the time required for rearrangement of the inner compartment amount to less than 0·3 s for single crista arrangements during the fastest shape changes, and less than 1–3 s during slower alterations. This high velocity is in good accord with the hypothesis of energy conservation by conformational events during oxidative phosphorylation.