Aggregation of melanosomes in melanophores is accompanied by the reorganization of microtubules and intermediate filaments

The structure of the cytoskeleton in cultured melanophores of the fish Gymnocorymbus ternetzi during aggregation of melanosomes was studied. It has been shown that the motion of pigment granules is accompanied by a reorganization of microtubules and intermediate filament systems. In melanophores with dispersed pigment granules, microtubules are wavy and form a loose network whilst intermediate filaments in such cells form a dense network around the dispersed melanosomes. During aggregation microtubules and intermediate filaments become radially oriented. It was also shown that the surface area of melanophores increased during aggregation.     

Sea urchin oocytes possess elaborate cortical arrays of microfilaments, microtubules, and intermediate filaments

Extensive arrays of microfilaments, microtubules and cytokeratin-type intermediate filaments were detected in the cortex of Strongylocentrotus droebachiensis oocytes using fluorescently labeled antibodies on both cortex and whole mount preparations. All three filament systems undergo dramatic structural reorganization during meiotic maturation of the egg. Microfilaments form a dense meshwork within the cortex of the oocyte. After meiosis, the filaments rearrange and shorten, resulting in a more loosely organized network. Both cortical microtubules and microtubules associated with a microtubule-organizing center are observed within the oocyte. After meiosis, the number and length of the cortical microtubules gradually diminish. A microtubule organizing center is found situated between the germinal vesicle and the plasma membrane in many oocytes. A network of filaments extends from the microtubule organizing center and radiates peripherally toward the germinal vesicle, presumably marking the animal pole. Cytokeratin-like intermediate filaments form a reticular network within the oocyte cortex, then solubilize during meiosis. In whole mounts of oocytes there is a single focal center of cytokeratin staining from which filaments radiate. Indirect immunofluorescence experiments, using anti-tubulin and anti-cytokeratin antibodies simultaneously, reveal the intermediate filament focal center to be localized within the microtubule organizing center. These results demonstrate the presence of a complex cortical cytoskeleton in premeiotic eggs of the sea urchin, Strongylocentrotus droebachiensis.     

Heat shock gene expression and cytoskeletal alterations in mouse neuroblastoma cells

The cytoskeleton of neuroblastoma cells, clone Neuro 2A, is altered by two stress conditions: heat shock and arsenite treatment. Microtubules are reorganized, intermediate filaments are aggregated around the nucleus, and the number of stress fibers is reduced. Since both stress modalities induce similar cytoskeletal alterations, no thermic denaturation of one or more cytoskeletal components can be involved in this process. Heat shock proteins are induced both by heat and by arsenite. However, cells treated with arsenite synthesize hsp28 which is not detected in heat-treated cells. Synthesis of all hsps is prevented by addition of actinomycin D or cycloheximide. Under these conditions no alterations are observed in the organization of microtubules and intermediate filaments during heat or arsenite treatment. However, these drugs are not able to prevent the rapid loss of stress fibers. A re-formation of the cytoskeleton during the recovery period proceeds within 3 h and is also found to occur in the presence of a protein synthesis inhibitor. These data suggest that reorganization of microtubules and intermediate filaments during a stress treatment requires the synthesis of a new protein(s), probably hsp(s).     

Cytoplasmic fibrils in living cultured cells

Summary By a combined light and electron microscopic study, the structure and behavior of the stress fibers of cultured rat embryo cells are described. From an analysis of movie records of living cells it is seen that the stress fibers are in a state of flux, continually altering their dimensions and dispositions within the cell. However, compared to most other cellular movements, these rates of change are slow. By electron microscopy it is shown that the stress fibers consist of bundles of close packed elongate 75 Å filaments, arranged just beneath the plasma membrane adjacent to the cell's plane of attachment and that similar filaments, forming a loose framework, permeate the cytoplasmic matrix. On the basis of careful light and electron microscopic comparisons, it is concluded that the filamentous structure shown within the cytoplasm of glutaraldehyde/osmium-fixed cells is a generally accurate representation of the structure of the living cell cytoplasm. It seems likely that the stress fibers are concerned in stabilizing areas of cellular attachment as well as with resisting forces that stretch the cell. The suggestion is made that, by controlling cytoplasmic viscosity and responding to cytoplasmic microtubules, the diffuse framework of filaments helps to determine the form of the cell and that, by a coordinate dynamic activity of its filaments, it provides the motive power for the various forms of cellular and intracellular movements.     

Confocal analysis of cytoskeletal organisation within isolated chondrocyte sub-populations cultured in agarose

This study reports the cytoskeletal organisation within chondrocytes, isolated from the superficial and deep zones of articular cartilage and seeded into agarose constructs. At day 0, marked organisation of actin microfilaments was not observed in cells from both zones. Partial or clearly organised microtubules and vimentin intermediate filaments cytoskeletal components were present, however, in a proportion of cells. Staining for microtubules and vimentin intermediate filaments was less marked after 1 day in culture however than on initial seeding. For all three cytoskeletal components there was a dramatic increase in organisation between days 3 and 14 and, in general, organisation was greater within deep zone cells. Clear organisation for actin microfilaments was characterised by a cortical network and punctate staining around the periphery of the cell, while microtubules and vimentin intermediate filaments formed an extensive fibrous network. Cytoskeletal organisation within chondrocytes in agarose appears, therefore, to be broadly similar to that described in situ. Variations in the organisation of actin microfilaments between chondrocytes cultured in agarose and in monolayer are consistent with a role in phenotypic modulation. Vimentin intermediate filaments and microtubules form a link between the plasma membrane and the nucleus and may play a role in the mechanotransduction process.     

Association of acetylated microtubules, vimentin intermediate filaments, and MAP 2 during early neural differentiation in EC cell culture

Pluripotent P19 embryonal carcinoma cell cultures can be induced to differentiate into neurons and glial cells by the addition of 10(-6) M retinoic acid. During early neural differentiation, a bundle of colchicine-stable, acetylated microtubules is formed. This acetylated microtubule array apparently extends to form neurites during neurogenesis. In this paper, we analyze changes in vimentin and MAP 2 distributions during neural differentiation with respect to the changes in the acetylated microtubule array. During a brief period early in differentiation, indirect immunofluorescence staining shows the colocalization of colchicine-stable acetylated microtubules, vimentin, and MAP 2. Using acrylamide to disrupt the organization of vimentin intermediate filaments and estramustine to disrupt the binding of MAP 2 to microtubules, we show that acetylated microtubules, MAP 2, and vimentin intermediate filaments are arranged in an interdependent cytoskeletal array. We suggest this array may serve to stabilize processes in neural stem cells, before the final decision to differentiate into neurons or glia is made.     

The Effect of Colchicine on the Induction of the Stellate Configuration in Dorsal Iris Epithelial Cells of the Newt Notophthalmus viridescens, in vitro

Neuroblastoma cells, grown in monolayer, transform, emit cytoplasmic processes, and acquire morphological and functional properties resembling those of mature neurons, whereas in suspension culture they remain in the undifferentiated anaplastic form. The appearance of intermediate (10 nm) filamentous structures in neuroblastoma cells is generally considered to indicate a state of cellular differentiation, one of a progressive sequence of maturing phases which lead the cell to the final differentiated state.
We have examined by electron microscope murine C 1300 neuroblastoma cloned cells, grown in suspension or in monolayer cultures in the presence or absence of BrdU as an inducing agent and have compared the expression of intermediate filaments. These filaments were present in five clones of cells grown in suspension still in undifferentiated anaplastic form. One clone in particular showed a massive expression of filaments, particularly visible in the perinuclear region. One hundred per cent of the cells observed presented filaments whose number apparently increased when cells were grown in the presence of BrdU in suspension or in monolayer. One clone never showed intermediate filaments under any circumstances. The original line from which clones were derived showed poor expression of filaments which were visible only in cells grown in monolayer. These results suggest that the expression of intermediate filaments in neuroblastoma cells should be viewed as the result of a positive genetic control of phenotype expression rather than the result of a progressive sequence of differentiating events.     

Cytoskeletal Architecture of Dermal Chromatophores of the Freshwater Teleost Oryzias latipes

Cytoskeletal construction of dermal chromatophores of Orgzias latipes was studied by immunofluorescence microscopy. A microtubule system was most prominent in melanophores where a large number of microtubules emanated from the center of the cell. Xanthophores had an arrangement basically similar to that of melanophores, though the radial pattern became more irregular in the peripheral region where intersecting wavy microtubules were quite frequent. Oval-shaped leucophores exhibited the least-developed microtubule system, where the limited number of microtubules formed a loose basket-like architecture. Intermediate filaments were ubiquitously present in all types of chromatophores and were found to be vimentin-immunoreactive. Examination of doubly-labeled cells indicated that vimentin filaments had similar distribution patterns with microtubules. Orderly arranged bundles of actin filaments were found only in xanthophores, while in melanophores and xanthophores, actin expression was diffuse without displaying a conspicuous filamentous organization. Colchicine treatment induced depolymerization of microtubules and retraction of dendrites in varying degrees in cells in culture and in situ. Melanophores in culture are very sensitive to the treatment while xanthophores appeared to be more resistant in respect to the maintenance of cell morphology.     

An alternatively spliced exon links intermediate filaments to adhesions

Anchorage of the contractile actomyosin apparatus to the plasma membrane at discrete sites in muscle and non-muscle cells enables the transmission and conversion of force into work, such as muscle contraction and membrane deformation to regulate cell and tissue shape. Assembly, stabilization and turnover of adhesion sites are complex processes that involve structural components, a variety of signalling and adapter molecules, diverse kinases and phosphatases, and phospholipids. The dynamic turnover of adhesions also requires the frequent interaction with other filament systems of the cytoskeleton, in particular with microtubules. How the delivery and activation of all the required components is co-ordinated, however, remains to be fully understood. In the current issue of Biochemical Journal, Sun et al. provide evidence that a specific exon that is exclusively present in the alpha variant of the type IV intermediate filament protein synemin interacts directly with the focal adhesion protein vinculin in its active state. Interaction of adhesion components with intermediate filaments could serve as a general mechanism to regulate cell- and tissue-specific cytoskeleton-membrane attachment.     

Structural interaction of cytoskeletal components

Three-dimensional cytoskeletal organization of detergent-treated epithelial African green monkey kidney cells (BSC-1) and chick embryo fibroblasts was studied in whole-mount preparations visualized in a high voltage electron microscope. Stereo images are generated at both low and high magnification to reveal both overall cytoskeletal morphology and details of the structural continuity of different filament types. By the use of an improved extraction procedure in combination with heavy meromyosin subfragment 1 decoration of actin filaments, several new features of filament organization are revealed that suggest that the cytoskeleton is a highly interconnected structural unit. In addition to actin filaments, intermediate filaments, and microtubules, a new class of filaments of 2- to 3-nm diameter and 30- to 300-nm length that do not bind heavy merymyosin is demonstrated. They form end-to-side contacts with other cytoskeletal filaments, thereby acting as linkers between various fibers, both like (e.g., actin- actin) and unlike (e.g., actin-intermediate filament, intermediate filament-microtubule). Their nature is unknown. In addition to 2- to 3-nm filaments, actin filaments are demonstrated to form end-to-side contacts with other filaments. Y-shaped actin filament “branches” are observed both in the cell periphery close to ruffles and in more central cell areas also populated by abundant intermediate filaments and microtubules. Arrowhead complexes formed by subfragment 1 decoration of actin filaments point towards the contact site. Actin filaments also form end-to-side contacts with microtubules and intermediate filaments. Careful inspection of numerous actin-microtubule contacts shows that microtubules frequently change their course at sites of contact. A variety of experimentally induced modifications of the frequency of actin-microtubule contacts can be shown to influence the course of microtubules. We conclude that bends in microtubules are imposed by structural interactions with other cytoskeletal elements. A structural and biochemical comparison of whole cells and cytoskeletons demonstrates that the former show a more inticate three-dimensional network and a more complex biochemical composition than the latter. An analysis of the time course of detergent extraction strongly suggests that the cytoskeleton forms a structural backbone with which a large number of proteins of the cytoplasmic ground substance associate in an ordered fashion to form the characteristic image of the “microtrabecular network” (J.J. Wolosewick and K.R. Porter. 1979. J. Cell Biol. 82: 114-139).     

Endothelial adherence under shear stress is dependent upon microfilament reorganization

In response to externally applied shear stress, cultured endothelial monolayers develop prominent, axially-aligned, microfilamentous bundles, termed "stress fibers" (Dewey: Journal of Biomechanical Engineering 106:31-35, 1984; Franke et al.: Nature 81:570-580, 1984; Franke et al.: Klin. Wochenschr 64:989-992, 1986; Wechezak et al.: Laboratory Investigation 53:639-647, 1985). It is unclear, however, whether similar stress fibers develop in noncontiguous endothelial cells and whether these structures are necessary for adherence of individual cells under shear stress. It also is unknown what alterations occur in microtubules, intermediate filaments, and focal contacts as a consequence of shear stress. In this study, endothelial cells, free of intercellular contact, were exposed to 93 dynes/cm2 for 2 hr. With the aid of specific labeling probes and interference reflection microscopy, the distributional patterns of microfilaments, microtubules, intermediate filaments, and focal contacts were examined. Following shear stress, microfilament bundles and their associated focal contacts were concentrated in the proximal (relative to flow direction) cell regions. In contrast, microtubules were distributed uniformly within cell contours. Intermediate filaments displayed only an occasional tendency for accumulation at proximal edges. When cells were shear-tested in the presence of cytochalasin B to inhibit microfilament assembly, considerable cell loss occurred. Following inhibition of tubulin polymerization, no increase was observed in the percentage of cells lost due to shear over nontreated controls. Nocodazole-treated cells, however, were characterized by prominent stress fibers throughout the cell. These results indicate that stress fiber and focal contact reorganization represent major responses in isolated endothelial cells exposed to shear stress and that these cytoskeletal structures are necessary for adherence.     

Structural and functional properties of the non-muscle tropomyosins

Summary The non-muscle tropomyosins (TMs), isolated from such tissues as platelets, brain and thyroid, are structurally very similar to the muscle TMs, being composed of two highly -helical subunits wound around each other to form a rod-like molecule. The non-muscle TMs are shorter than the muscle TMs; sequence analysis demonstrates that each subunit of equine platelet TM consists of 247 amino acids, 37 fewer than for skeletal muscle TM. The major differences in sequence between platelet and skeletal muscle TM are found near the amino and carboxyl terminal ends of the proteins. Probably as the result of such alterations, the non-muscle TMs aggregate in a linear end-to-end manner much more weakly than do the muscle TMs. Since end-to-end interactions are responsible for the highly cooperative manner in which TM binds to actin, the non-muscle TMs have a lower affinity for actin filaments than do the muscle TMs. However, the attachment of other proteins to actin (e.g. the Tn-I subunit of skeletal muscle troponin or the S-1 subfragment of skeletal muscle myosin) can increase the affinity of actin filaments for non-muscle TM. The non-muscle TMs interact functionally with the Tn-I component of skeletal muscle troponin to inhibit the ATPase activity of muscle actomyosin and with whole troponin to regulate the muscle actomyosin ATPase in a Ca⁺⁺-dependent manner, even though one of the binding sites for troponin on skeletal TM is missing in non-muscle TM. A novel actomyosin regulatory system can be produced using Tn-I, calmodulin and non-muscle TM; in this case inhibition is released when the non-muscle TM detaches from the actin filament in the presence of Ca⁺⁺. Although it has not yet been demonstrated that the non-muscle TMs participate in a Ca⁺⁺-dependent contractile regulatory system in vivo it does appear that they are associated with actin filaments in vivo.     

Interrelationships of endoplasmic reticulum, mitochondria, intermediate filaments, and microtubules--a quadruple fluorescence labeling study.

To study the interrelationships of endoplasmic reticulum, mitochondria, intermediate filaments, and microtubules, we have developed a quadruple fluorescence labeling procedure to visualize all four structures in the same cell. We applied this approach to study cellular organization in control cells and in cells treated with the microtubule drugs vinblastine or taxol. Endoplasmic reticulum was visualized by staining glutaraldehyde-fixed cells with the dye 3,3'-dihexyloxacarbocyanine iodide. After detergent permeabilization, triple immunofluorescence was carried out to specifically visualize mitochondria, vimentin intermediate filaments, and microtubules. Mitochondria in human fibroblasts were found to be highly elongated tubular structures (lengths up to greater than 50 microns), which in many cases were apparently fused to each other. Mitochondria were always observed to be associated with endoplasmic reticulum, although endoplasmic reticulum also existed independently. Intermediate filament distribution could not completely account for endoplasmic reticulum or mitochondrial distributions. Microtubules, however, always codistributed with these organelles. Microtubule depolymerization in vinblastine treated cells resulted in coaggregation of endoplasmic reticulum and mitochondria, and in the collapse of intermediate filaments. The spatial distributions of organelles compared with intermediate filaments were not identical, indicating that attachment of organelles to intermediate filaments was not responsible for organelle aggregation. Mitochondrial associations with endoplasmic reticulum, on the other hand, were retained, indicating this association was stable regardless of endoplasmic reticulum form or microtubules. In taxol-treated cells, endoplasmic reticulum, mitochondria, and intermediate filaments were all associated with taxol-stabilized microtubule bundles.     

Unique microtubules in luteal cells from superovulated rats

Luteal cells of immature female rats treated with gonadotropins contain microtubules with a number of interesting features. Many of the microtubules of these cells are arranged in bundles in which they are separated one from another by strands of material (i-MT bands) of unknown composition. The microtubules within the bundles assume a hexagonal packing pattern with i-MT bands between any two microtubules. The bundle microtubules and their i-MT bands are further connected via crosslinking filaments: pattern obtained from densitometer scans (measuring the arrangement of the crosslinking filaments) suggest that the filaments may represent microtubule-associated proteins. The complex arrangement of the microtubules within a bundle does not appear to extend for the entire length of the individual microtubules, and occasionally one sees profiles of single microtubules fanning out from the ends of the bundle: whether the same microtubules are regrouped at some other point in the cell is not known. Structures similar to the i-MT band and the crosslinking filaments have also been observed connecting microtubules to segments of the luteal cell plasma membrane: in these instances the i-MT-like band is found between the longitudinally sectioned microtubule and the membrane, with filaments connecting the two structures via the intermediate band. It is of interest that the microtubules of these luteal cells are not sensitive to treatment with antimicrotubule drugs and we suggest that the complex bundling arrangement provides their unusual stability.     

Filament arrangements in negatively stained cultured cells: the organization of actin

Treatment of spread, cultured cells with Triton X-100 followed by negative staining reveals the organization of the unextracted intracellular filamentous elements: actin, microtubules and the 100 angstrom filaments. The present report describes the organization of the actin-like filaments in human skin fibroblasts and mouse 3 T 3 cells. As shown in earlier studies, the cytoplasmic stress fibres were seen to be composed of bundles of colinear actin-like filaments. In addition to these large stress fibres much smaller bundles of thin filaments as well as randomly oriented thin filaments were also observed. A thick bundle of thin filaments, 0.2 microm to 0.5 microm in diameter, was found to delimit the concave cell edges most prominent in well-spread stationary cells. The leading edge and ruffled border of human skin fibroblasts appeared as a broad web, of meshwork of diagonally oriented thin filaments interconnecting radiating, linear bundles of thin filaments about 0.1 microm in diameter. These bundles corresponding to the microspikes described earlier ranged from about 1.5 microm in length and were separated by 1 microm to 3 microm laterally. The leading edge of 3 T 3 cells showed a similar organization but with fewer radiating thin filament bundles. Both the filaments in the bundles and in the meshwork formed arrowhead complexes with smooth muscle myosin subfragment - 1 which were unipolar and directed towards the main body of the cell. The findings are discussed in relation to the mechanisms of non-muscle cell motility.     

Phenotype modulation in primary cultures of arterial smooth-muscle cells: reorganization of the cytoskeleton and activation of synthetic activities

During primary culture, arterial smooth-muscle cells (SMCs) undergo transition from a contractile to a synthetic phenotype. As a consequence, they lose the ability to contract and, instead, acquire the ability to synthesize DNA, divide and produce extracellular-matrix components. In the present study, we used cytochemical and electron-microscopic methods to study the organization of the cytoskeleton in primary cultures of adult rat and human arterial SMCs. Freshly isolated cells were all in contractile phenotype and stained intensely with NBD-phallacidin, a fluorescent marker for F-actin. Diffuse, positive staining was also obtained using indirect-immunofluorescence microscopy with antibodies against tubulin and vimentin, which are subunit proteins of microtubules and intermediate filaments, respectively. Fine structurally, the cytoplasm of these cells was mainly filled with microfilament bundles coalescing in dense bodies. After a few hours in culture, the SMCs attached to the substrate and started to extend processes in various directions. These stained with antibodies to tubulin and vimentin, but not with NBD-phallacidin. Within 1-3 days of culture, the cells spread out on the substrate and developed a system of actin-containing stress fibre bundles spanning their entire length, as well as a radiating system of microtubules and vimentin filaments, originating in the juxtanuclear region. Fine structurally, these changes corresponded to a marked decrease in the number of microfilaments, an increase in the number of microtubules and intermediate filaments, and the formation of an extensive rough endoplasmic reticulum and a large Golgi complex. The morphological transformation of the cells was accompanied by the coordinated activation of DNA, RNA and protein synthesis.     

The apoptotic process of human bladder carcinoma T24 cells induced by retinoid

Breakdown of the cytoskeletal network and redistribution of cytoplasmic organelles are early events of programmed cell death. Previous studies showed that retinoic acid induces programmed cell death in many tumor cell lines and that cytokeratins, particularly cytokeratin 18, are affected in the early events of apoptosis. In this study, patterns of cytoplasmic intermediate filaments (cytokeratin 18), actin filaments, and microtubules, as well as Bax and Bcl-2 proteins in human bladder carcinoma T24 cells were examined before and after retinoic acid treatment by immunocytochemistry and conventional electron microscopy. Our results demonstrate that the redistribution of Bax and Bcl-2 proteins in the subcellular compartment of T24 cells is correlated with reorganization of the cytoplasmic intermediate filament network and that cytokeratins are cleaved by caspases, as revealed by the M30 antibody which recognizes a specific caspase cleavage site within cytokeratin 18. The cytoskeletal architectures of microtubules are not significantly affected in the early apoptotic process, from our observations. We suggest that the breakdown in the intermediate filament network associated with the aggregation of mitochondria and lysosome may be a crucial event in the apoptotic process and that aggregation of cytoplasmic Bax may accelerate apoptotic death.     

Distribution of microtubules and other cytoskeletal filaments during myotube elongation as revealed by fluorescence microscopy

Summary Distribution of microtubules and other cytoskeletal filaments in growing skeletal muscle cells (myotubes) was studied in vitro by fluorescence microscopy using fluorescin-labeled antibodies and phalloidin, a specific antiactin drug. In the distal elongating tips of myotubes, microtubules were the major cytoskeletal elements; actin and intermediate filaments were much less abundant. On the other hand, colcemidand nocodozole-treatments caused disruption of microtubules and also prompt retraction of growth tips to form myosacs, a type of deformed myotube. Actin filaments remained unaffected during the retraction. The difference in the distribution of the 3 cytoskeletal filaments in the region of growth tips was most remarkable in the case of those myotubes in the process of recovery from myosacs. In an early phase of recovery, the cellular processes extending from myosacs were enriched with both microtubules and intermediate filaments, but not with actin filaments. Later, when the processes became further developed, intermediate filaments were scarce at the extreme ends. Fluorescein-labeled actin introduced by a micro-injection method was minimally incorporated into filaments in the cellular processes. We conclude that microtubules make up the cytoskeletal element which is most responsible for elongation or spreading of growth tips of myotubes in vitro.     

An immunofluorescence study of the effects of ultraviolet radiation on the organization of microfilaments, keratin intermediate filaments, and microtubules in human keratinocytes.

Indirect immunofluorescence microscopy has been used to investigate the ultraviolet (UV) radiation induced disruption of the organization of microfilaments, keratin intermediate filaments, and microtubules in cultured human epidermal keratinocytes. Following irradiation, concurrent changes in the organization of the three major cytoskeletal components were observed in cells incubated under low Ca2+ (0.15 mM) conditions. UV irradiation induced a dose-dependent condensation of keratin filaments into the perinuclear region. This collapse of the keratin network was accompanied by the reorganization of microfilaments into rings and a restricted distribution of microtubules, responses normally elicited by exposure to high Ca2+ (1.05 mM) medium. The UV induced alteration of the keratin network appears to disrupt the interactions between keratin and actin, permitting the reorganization of actin filaments in the absence of Ca2+ stimulation. In addition to the perinuclear condensation of keratin filaments, UV irradiation inhibits the Ca2+ induced formation of keratin alignments at the membrane of apposed cells if UV treatment precedes exposure to high Ca2+ medium. Incubation of keratinocytes in high Ca2+ medium for 24 hours prior to irradiation results in the stabilization of membrane associated keratin alignments and a reduced susceptibility of cytoplasmic keratin filaments to UV induced disruption. Unlike results from investigations with isogenic skin fibroblasts, no UV induced disassembly of microtubules was discernible in irradiated human keratinocytes.