Morphogenesis of bovine parvoviruses and associated cellular changes

Morphogenic features of bovine parvovirus replication and associated cellular alterations in bovine fetal lung and spleen cells were examined by electron microscopy. Morphological evidence of parvoviral replication was detected first at 24 hr after inoculation when empty viral capsids were found in association with the smooth endoplasmic reticulum, in the lumina of nuclear pores, and in the nucleus. At 36 hr empty capsids were found in the parafibrosa of the nucleolus. Few complete virions were seen in the nucleus at 24 and 36 hr postinfection. The ratio of complete to empty virions increased and complete virions predominated 48, 60, and 72 hr after infection. Empty capsids and complete virions measured 20–22 nm in diameter. At 48 and 60 hr complete virions accumulated within the nucleus near nuclear pores, and similar 20–22 nm particles were seen on the cytoplasmic side of the nuclear membrane. The cytoplasm was severely degenerated in cells 72 hr after inoculation, the nuclear membranes were fragmented, and complete virions replaced the heterochromatin. Crystals composed either of empty capsids or complete virions were occasionally seen in the nuclei of different infected cells.     

Replication characteristics and core size of intranuclear herpes simplex virus (HSV-1) in genital skin lesions: electronmicroscopy studies of a biopsy from a female patient

Herpes simplex virus (HSV) type 1 genital infection, leading to ulcerating lesions in a female patient, was studied by electronmicroscopy. Infection had probably been recent, through oro-genital contact with a cold sore on the husband's lip. Cell-culture typing and serological tests indicated that the patient currently had an HSV-1 secondary infection. Aspects studied in a skin biopsy from an ulcerating labium majus were epidermal cell types infected, stages in virus genesis, virus core diameter in intranuclear capsids and extracellular appearance of virus. Different stages in virus genesis, in virus envelope formation and in nuclear and cytoplasmic degeneration were observed in the few remaining, rounded and swollen, epidermal (?) spinosum cells. Their nuclei, some with marginated chromatin, harboured besides dense-cored or empty capsids, electron-dense blobs possibly representing clones of immature virus and falling apart into aggregates of small granules. In other nuclei, large clusters of dense-cored capsids, some distinctly hexagonal in shape, had accumulated in wide gaps in the nuclear membrane whereas remaining nuclear membrane portions were quadruple and often engaged in viral envelope formation. Partially enveloped capsids and naked dense-cored capsids were seen extracellularly indicating their survival outside cells. An occasional virion was present in dermal blood vessel lumina. Measurements of the electron-dense core (nucleoid) of intranuclear capsids in electronmicrographs showed that the HSV-1 core diameter differs very significantly from the core of intranuclear HSV-2 capsids, thus allowing a clear distinction by electronmicroscopy between the two HSV subtypes in plastic-embedded biopsies.     

Electron microscopic study of the development of herpesvirus saimiri.

Electron microscopic study of herpesvirus saimiri in thin sections of infected OMK and Vero cells showed the apparent intranuclear envelopment of capsids by membranes of vesicles. Clusters of filaments were also encountered. Numerous unenveloped intracytoplasmic capsids were observed in cells devoid of nuclear changes, raising the possibility that differentiation can occur within the cytoplasm.     

Demonstration of stepwise coiling of nucleoprotein in adenovirus core by application of critical point drying

Mild destruction of a virus particle to observe the organized structure of the nucleoprotein complex in a virion was achieved by application of the critical point drying method. Adenovirus type 12 (ad12) virions have been treated by this method after the particles had been fixed with glutaraldehyde on an electron microscope grid. With 15 min prefixation, the capsids (shells) and the cores were in various stages of unfolding. The core was unfolded in the filamentous structure. The thickness of these filaments was 6.7, 13.3, +23 nm, or more. Some pictures showed that the thicker filaments consisted of super-coiling of two thinner filaments, for example two 6.7-nm filaments coiled up to give the 13.3-nm filaments. This suggests that the nucleoprotein complex of a circular double-stranded DNA and inner proteins of ad12 virus was folded in a stepwise fashion to produce the compacted form of the core.     

Three-dimensional ultrastructure of the brush border glycocalyx in the mouse small intestine: a high resolution scanning electron microscopic study

The three-dimensional ultrastructure of the filamentous glycocalyx of the brush border in the mouse small intestine was successfully demonstrated by high resolution scanning electron microscopy (SEM). The specimens were fixed with 2% glutaraldehyde in a 0.1M phosphate buffer (pH 7.4), and rinsed with buffered solutions with differently adjusted pH values (pH 3.0, 7.0 or 11.0). They were then osmicated, dried, spatter-coated with gold (1.0-1.5 nm), and observed under a high resolution SEM. The glycocalyx on the luminal surface of the intestinal villi covered the top of the microvilli of the epithelial cells and were well preserved in the specimens treated with an alkaline buffer (pH 11.0). The glycocalyx was observed as filamentous structures, 7 to 15 nm thick in diameter. These filaments repeatedly branched and anastomosed with neighboring ones to form an actual network or plexus as a whole, in contrast with superimposed images in transmission electron microscopy (TEM) which suggested that such anastomoses were pseudo-networks. The filaments thickened globularly at the sites of the filament bifurcation or branching. On the other hand, specimens rinsed with an acid or neutral buffer showed no glycocalyx on their microvilli, whose naked top had knob-like structures. Thus, the pH values of the washing buffer solutions were considered to affect the preservation of the surface coat due to molecular characteristics.     

Histopathologic and ultrastructural studies of disseminated cytomegalovirus infection in strain 2 guinea pigs

Inbred strain 2 guinea pigs developed severe disseminated disease during acute experimental guinea pig cytomegalovirus (GPCMV) infection. A high mortality rate (100%) resulted, with most animals dying between 10 and 14 days after high dose (7.5 X 10(5) TCID50) virus inoculation. Infectious virus was recovered from many tissues, including spleen, lungs, liver, pancreas, heart, adrenals, kidneys, and salivary glands. The rate of GPCMV isolation from these tissues ranged from 50 to 100%. Gross lesions were observed in the spleen, liver, and lungs. On histologic examination, lesions were also seen in many other organs, including heart, pancreas, kidneys, adrenals, brain, intestines, and salivary glands. Intranuclear viral inclusions were present in many cell types of various organs. Under electron microscopic examination, cells with viral inclusions were easily found in the spleen, and liver, but less readily in the lungs, kidneys, salivary glands, and other organs. Most of the intranuclear inclusions consisted of electron-dense fibrils (10 nm diameter), viral nucleocapsids (100 nm), and tubular structures (60 nm diameter). Dense bodies and enveloped dense virions containing single or multiple capsids were present in the cytoplasm of many infected cells. The morphologic developments of GPCMV in these visceral tissues of strain 2 guinea pigs resembled those seen in GPCMV-infected cultured guinea pig cells but differed from those observed in the infected salivary gland duct cells. Strain 2 guinea pigs are a useful animal model for studying disseminated infection in CMV-associated human diseases.     

Structure of cytomatrix and nuclear matrix revealed by embedment-free electron microscopy

Embedment-free electron microscopy (EFEM) is a new method which allows the visualisation of cytoskeleton in whole-mounted cells. In this study we employed EFEM to investigate the structure of cellular scaffolds in glioma C6 cell line. The cells were extracted with Triton X-100 that dissolves phospholipids in the membranes and removes most of cytoplasmic soluble proteins. The DNA and nuclear histones were removed with DNase I and high-salt buffer, respectively. The remaining cellular frameworks were temporary embedded in diethylene glycol distearate (DGD), sectioned and observed in transmission and scanning electron microscope after the removal of DGD. The predominant structure was the extensive meshwork of 10-20 nm filaments in the cytoplasm (cytomatrix) and 15-30 nm filaments in the nucleus (nuclear matrix). The 5 nm filaments, presumably corresponding to the actin filaments, were present in the cytomatrix, but not in the nuclear matrix. Moreover, the ultrathin (3 nm) filaments, connecting other cytoskeletal components were detected. Those are possibly identical with the previously described plectin filaments. For the first time we report the occurrence of ultrathin filaments in the nuclear matrix. Thus, in a addition to the well known cytoskeletal components (microtubules, intermediate filaments, actin microfilaments) EFEM showed a new type of filaments (the ultrathin filaments) in the cytomatrix and nuclear matrix. Further immunocytochemical studies are needed to determine the biochemical identity of the filaments observed in EFEM.     

Localization of immunoreactive keratins in cyst epithelium of chick ultimobranchial glands

The cyst structures of chick ultimobranchial glands were studied by electron microscopy and immunocytochemistry to characterize the type of intermediate-sized filaments present in the cells lining cyst lumina. Electron microscopy showed that the majority of the lumen-bordering cells contained extensive meshworks of intermediate-sized (7-11 nm) filaments, many of which were arranged in bundles. Apical regions of C cells directly bordering on cyst lumina were also filled with thinner (5-6 nm) filaments. Immunoperoxidase staining showed that the majority of cyst epithelial cells were stained intensely with anti-keratin antiserum, but not with anti-neurofilament antiserum, which is a specific marker for neuronal differentiation. The cyst epithelium also showed moderate-to-weak immunoreactivity for actin. Subsequently, the differentiation and maturation of cyst structures related to intermediate filament expression were studied. In 18-day-old chick embryos, keratin immunoreactivity began to appear in the cell clusters destined to form cysts and in the primordial cysts with small cavities. At this time, fine networks of intermediate filaments were already detected in the cells lining the cystic cavities. At 1 day after hatching, the cysts became a consistent feature of ultimobranchial glands. Intermediate filaments associated in bundles were observed, and the intensity of immunostaining for keratins increased. Thereafter, with progressive enlargement of cysts, numbers of intermediate filaments and intensity of keratin immunoreactivity gradually increased with age. Thus, the data indicate that in cyst epithelium keratin filaments are highly organized and may confer the structural strength necessary for cells lining cyst lumina.     

Ultrastructural organization of cell characteristics relevant to diagnosis in rhabdomyosarcomas

Rhabdomyosarcomas may be easily diagnosed by light microscopy when tumor cells exhibit cross striations typical of skeletal muscle cells. But in many cases the histological diagnosis is difficult because the tumors are predominantly composed of undifferentiated elements and only single cells can be seen which show an eosinophilic cytoplasm suggesting different steps of a possible rhabdomyoblastic differentiation. Using the electron microscope seven embryonal rhabdomyosarcomas, one alveolar rhabdomyosarcoma including one lymph node metastasis and one pleomorphic rhabdomyosarcoma were analysed in order to study the submicroscopical organization of diagnostic cellular features, resp. the distribution and arrangement of cytoplasmic filaments. The following lines of cellular differentiation could be distinguished: 1. Development of differentiated myoblasts and satellite cells: -- primitive, undifferentiated tumor cells: small round cells with scanty cytoplasm containing few thin filaments (4 to 6 nm) and intermediate-type filaments (10 nm), -- round or slightly spindle-shaped myoblast-like tumor cells: cells with moderate cytoplasm exhibiting irregularly arranged thin and intermediate-type filaments and only some thick filaments (15 nm), -- myotube-like cells: long extended cells (strap shaped cells) revealing thin and thick filaments, Z-line material and different stages of myofibrillar organization, -- well-differentiated myoblasts: long extended cells showing typical cross-striations which correspond to well-formed sarcomeres with I-bands (Z-lines with extending thin filaments) and A-bands, which are subdivided into H- and M-bands, -- satellite cells of typical ultrastructure associated with differentiated myoblasts by a common basement membrane present in one case. 2. Development of aberrant myoblasts and giant cells: -- round myoblasts: cells with increased cytoplasm containing thin and thick filaments, primitive Z-lines, which were not organized into sarcomeres as well as unaligned sarcomeres, -- large round myoblasts and giant cells: cells with abundant cytoplasm containing irregularly distributed thin and thick filaments, primitive Z-lines and haphazardly arranged sarcomeres, which did not appear as cross-striations by light microscopy. The ultrastructure of tumor cells is discussed with regard to the degree of differentiation and their light microscopic appearance. The scale of cellular features in rhabdomyosarcomas could be also correlated with normal fetal myogenesis. Furthermore this confirms the important role of electron microscopy in the differential diagnosis of other small, primitive and dark-cell tumors.     

Scanning electron microscopy of cytoplasmic filaments in rat anterior pituitary cells

In order to demonstrate the occurrence of cytoplasmic filaments and their relationship to secretory granules, rat anterior pituitaries perfused with a detergent solution containing 0.5% Triton X-100 were observed with the scanning electron microscope. Thin section images and quick-freeze, deep-etching replica images were also studied. Scanning electron microscopy clearly demonstrated complicated networks of numerous cytoskeletal filaments in the cytoplasm of all the secretory cells of the anterior pituitary. These filaments can be classified into two groups on the basis of their diameter: thick filaments measuring 30-40 nm in diameter, and thin filaments 15-25 nm in diameter, each including the thickness of the platinum coat. The thick filaments, which run rather straight, are considered to be microtubules. The thin filaments attached to the surface of a secretory granule may connect it with an adjacent secretory granule, with cyto-organelles, with the nucleus, or with the plasma membrane. Transmission electron microscopic images of thin sections and quick-freeze, deep-etching replicas confirm the occurrence of filamentous structures associated with the limiting membrane of the secretory granule. The fine filaments associated with the limiting membrane of the secretory granule might participate in the support and transport of the granule.     

Mouse thymic virus-mediated immunosuppression: association with decreased helper T cells and increased suppressor T cells

Mouse thymic virus (MTV) is a herpesvirus which, when administered to newborn mice, induces an extensive but temporary thymic necrosis associated with immunosuppression. In the present study, the T cell subsets in the thymus of MTV infected newborn C57Bl/6 mice were evaluated at 4, 7, 14, 28, 56, and 84 days after infection, using labeled monoclonal anti-CD4 and anti-CD8 antibodies with two-color flow cytometry. At 7 and 14 days, the percentages of CD4+8- and CD4+8+ cells were significantly decreased whereas the percentage of CD4-8+ cell was increased. At days 28 and 56 percentages had returned to normal. These results indicate that the virus has an affinity for CD4+ T cells (helper cells and their precursors). Increased percentage of CD4-8+ T cells (suppressor cells) is also associated with depressed immune functions in MTV infected newborn mice.     

Ultrastructural instability of paired helical filaments from corticobasal degeneration as examined by scanning transmission electron microscopy.

Paired helical filaments (PHFs) accumulate in the brains of subjects affected with Alzheimer's disease (AD) and certain other neurodegenerative disorders, including corticobasal degeneration (CBD). Electron microscope studies have shown that PHFs from CBD differ from those of AD by being wider and having a longer periodicity of the helical twist. Moreover, PHFs from CBD have been shown to be primarily composed of two rather than three highly phosphorylated polypeptides of tau (PHF-tau), with these polypeptides expressing no exons 3 and 10. To further explore the relationship between the heterogeneity of PHF-tau and the appearance of abnormal filaments, the ultrastructure and physical parameters such as mass per unit length and dimensions were compared in filaments from CBD and AD using high resolution scanning transmission electron microscopy (STEM). Filament-enriched fractions were isolated as Sarcosyl-insoluble pellets and for STEM studies, samples were freeze-dried without prior fixation or staining. Ultrastructurally, PHFs from CBD were shown to be a heterogeneous population as double- and single-stranded filaments could be identified based on their width and physical mass per unit length expressed in kilodaltons (kd) per nanometer (nm). Less abundant, double-stranded filaments had a maximal width of 29 nm and a mass per unit length of 133 kd/nm, whereas three times more abundant single-stranded filaments were 15 nm wide and bad a mass per unit length of 62 kd/nm. Double-stranded filaments also displayed a distinct axial region of less dense mass, which appeared to divide the PHFs into two protofilament-like strands. Furthermore, these filaments were frequently observed to physically separate along the long axis into two single strands or to break longitudinally. In contrast, PHFs from AD were ultrastructurally stable and uniform both in their width (22 nm) and physical mass per unit length (104 kd/nm). The ultrastructural features indicate that filaments of CBD and AD differ both in stability and packing of tau and that CBD filaments, composed of two distinct protofilaments, are more labile under STEM conditions. As fixed and stained filaments from CBD have been shown to be stable and uniform in size by conventional transmission electron microscopy, STEM studies may be particularly suitable for detecting instability of unstained and unfixed filaments. The results also suggest that molecular heterogeneity and/or post-translational modifications of tau may strongly influence the morphology and stability of abnormal filaments.     

Squamous differentiation in human breast epithelial cells in culture, with comparative observations on intermediate filaments from intact epithelium

Intact, grossly normal human mammary tissue and cultured breast epithelial cells were examined by transmission electron microscopy for indicators of squamous differentiation. Intact epithelium contained 7-10 nm filaments which we interpreted as cytokeratin intermediate type filaments; many formed bundles which lacked conspicuous electron density. Two types of cell process were present: one was luminal and covered in a glycocalyx, whereas the other lacked a coat and was confined to lateral membranes. In culture, mammary epithelial cells contained intermediate filament bundles which were conspicuous by their enhanced electron density compared with those of intact epithelium. These were acceptable as dense tonofibrils i.e. indicators of squamous differentiation. Cultured cells also possessed cell processes broadly comparable to those of intact epithelium. This study confirms the phenomenon of squamous differentiation consequent upon culture, in a system which differed methodologically from previous studies, and makes hitherto neglected comparisons between intact and cultured breast epithelium.     

Assembly of viral particles in the cells infected with endotheliotropic or fibroblastotropic strains of human cytomegalovirus

The ultrastructure of endothelial cells and fibroblasts infected with VHL/E or AD169 strains was examined. The cells producing the virus 25 days after infection with VHL/E strains (0.01 PFU/cell) contain numerous cytoplasmic-located filament-associated capsids in addition to standard infectious and noninfectious viral particles. Only single filament-contacting capsids were found in the cytoplasm of the cells producing the virus 20 days after infection with AD169 strain (0.001 PFU/cell). In situ DNA hydridization on the VHL/E-infected cells has indicated that the capsid-contacting filament is human cytomegalovirus DNA. The findings suggest that premature release of viral DNA from the capsids occurs during their maturation in the cytoplasm.     

Morphology of adenovirus type-3 infection of human respiratory epithelial cells in vitro

The adenovirus is a non-enveloped DNA virus which may lead to severe diseases of the respiratory tract. In order to study the influence of virus infection on primary cultured peribronchial submucosal gland cells, we performed in vitro infection with human adenovirus type 3. Peribronchial submucosal glands are the main source of tracheobronchial mucus and, therefore, play a major pathophysiological role in common pulmonary diseases such as bronchial asthma, chronic obstructive pulmonary disease and cystic fibrosis. The success of infection was verified by means of immunofluorescence and transmission electron microscopy. Infection follows a certain timetable with a climax of paracristalline intranuclear virus inclusions after 48 h of infection. Virus particles could be detected in the nucleus as well as in peripheral and perinuclear cytoplasmatic vacuoles. The release of virus capsids from the nucleus could be visualized using transmission electron microscopy and immunofluorescence with antibodies against hexon proteins. Two different kinds of mechanisms of transition of newly synthesized virus capsids from the nucleus into the cytoplasm could be identified. Due to an increasing cytopathic effect, viruses spread from cytoplasm after longer terms of infection. Cytopathic effects and cytoskeleton aspects under this virus infection could be characterized using immunofluorescence with several monoclonal antibodies against different cytokeratins.     

Serological identification of viral and virus-related antigens on DBA/2 mouse leukemia lymphocytes.

Allogenic, semisyngeneic and syngeneic sera of animals immunized with ML-positive leukemia L1210 cells, besides anti-ML antibodies, contain antibodies which react with Gross cellular surface antigen. ML antigen and Gross cellular surface antigen were shown by the immunoferritin test in electron microscopy, and by the blocking test, to be situated on different parts of the cell surface. No budding viral particles were found on the areas occupied by these antigens. By distinguishing the ML antigen identified on the surface of leukemia L1210 cells from the Gross cellular antigen, it was shown that the MTV present in leukemias of DBA/2 mice has no leukemogenic properties. Demonstration of core and envelope antigens of the MTV and Gross MuLV, besides C particles and intracytoplasmatic A particles, which are precursors of B particles, is proof of existence of genomes of both viruses in leukemia L1210 cells. The ability of leukemia L1210 cells to absorb activity from the anti-ML sera and reaction between anti-ML sera and isolated B particles of the MTV in immunoprecipitation, indicate probable existence of an antigenic component of the MTV within the ML antigen.     

Basal cell adenoma with parallel tubules in stromal cells of the parotid gland

A case of basal cell adenoma in the right parotid region of a 51 years old male was reported. The tumor measured 2.5 cm x 3 cm, was spherical and covered with a fibrous capsule. Histologically, it was a tubular monomorphic adenoma with scant edematous interstitial tissue. The stromal cells stained positively by the PAP method using anti-S-100 protein serum. Electron microscopically, the tumor cells forming tubular had many microvilli at the luminal surface, many filaments in the cytoplasm and well developed desmosomes in the intercellular junctions. Ordinary intracellular organelles of the tumor cells were small in number, and their nuclei were oval with shallow indentation. In the dilated rough endoplasmic reticulum of the stromal cells, many straight parallel tubules were found. The tubules measured from 15 nm to 25 nm thick and 3.5 micrometers long in the longitudinal sections and from 25 nm to 30 nm in diameter with electron lucent core and poor coat in the cross sections. Other cell organelles of the stromal cells were small in number, and filaments and dense attachments were found in the ectoplasm. Around the stromal cells there was a discontinuous basement membrane.     

Alterations in virus protein synthesis and capsid production in infection with DI particles of herpesvirus

High multiplicity, undiluted passage of equine herpesvirus type 1 (EHV-1) in L-M cells resulted in the rapid production of virus particles whose genome was genetically less complex, contained more reiterated DNA sequences and exhibited a greater buoyant density (rho = 1.724 g/ml) than the DNA (rho = 1.716 g/ml) of standard virus. These data and the finding that these particles inhibited the replication of standard virus in interference assays confirmed that these were defective interfering (DI) particles (Henry et al. 1979). Additional evidence for this has been obtained from the pattern of cyclic fluctuation in infectious virus titre through 17 serial passages as well as from the pronounced variation in the particle to plaque ratio for each passage. Total particle production was markedly reduced in cells infected with virus preparations containing DI particles and quantification of major cell-associated EHV-1 capsid species by electron microscopy and analysis in Renografin density gradients indicated that this reduction occurred at the level of capsid assembly. Although total capsid production was reduced in cells infected with DI particle preparations, the synthesis of I (immature) capsids increased relative to that of L (empty) capsids and these alterations in the assembly of capsid species could be related to changes in the synthesis of capsid proteins. In cells infected with EHV-1 preparations rich in DI particles, the synthesis of major capsid protein 150000 was greatly reduced, whereas core protein 46000, a major component of I capsids, was overproduced as compared to standard virus infection. Capsids produced in cells infected with virus preparations rich in DI particles were identical in polypeptide composition to those made in standard virus infection.     

Ultrastructural, biochemical, and immunologic characterization of Mallory bodies in livers of griseofulvin-treated mice. Fimbriated rods of filaments containing prekeratin-like polypeptides.

Mallory bodies (MBs) induced in hepatocytes by long term feeding of mice with griseofulvin were isolated, purified, and examined by electron microscopy in ultrathin sections and negatively stained preparations. The major structural component of the MBs was randomly oriented, unbranched rods of filaments; these were usually 175 to 250 nm. long and 14 to 20 nm. thick and covered by a dense fimbriate coat of laterally projecting 1.5- to 3-nm. thick threads. Such lateral threads could extend for more than 20 nm. and seemed to be involved in the interconnection of adjacent filaments and their association and aggregation into MBs. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified filament material showed six major polypeptide bands with apparent molecular weights ranging from 48,000 to 66,000. When the portion of the MB filament material that was soluble in solutions containing 8 M urea was allowed to reaggregate upon removal of the urea, an enrichment of one of the polypeptide components (approximate molecular weight, 64,000) was observed. When frozen sections of liver tissue MBs were subjected to indirect immunofluorescence microscopy, they were specifically revealed by guinea pig antibodies directed against purified bovine prekeratin. No significant accumulation of MBs was observed with a series of other antisera, including those containing antibodies against tubulin, actin, and vimentin, the major protein of the intermediate sized filaments predominant in mesenchymal cells. The observations suggest that MBs in livers of griseofulvin-treated mice, and probably also of human alcoholic hepatitis, contain large amounts of prekeratin-like polypeptides which are assembled into a special form of fimbriated rods of 14- to 20-nm. filaments. These filaments are morphologically different from other forms of intermediate sized and thick filaments, including the prekeratin-containing 6- to 11-nm. tonofilament-like filaments present in various epithelial cells.     

Taxol-induced neuropathy: further ultrastructural studies of nerve fibre changes in situ

Summary Nerve fibre changes have been further monitored morphologically in rat sciatic nerves which had been locally injected with taxol, an antimitotic drug known to promote microtubule assembly. Taxol caused a slowly progressive accumulation of microtubules over a three week period of experimentation, the effect being more pronounced in Schwann cells. Schwann cells of myelinated fibres became detached from nodes of Ranvier and were applied to naked internodes as bulbous cells replete with microtubules and smooth endoplasmic reticulum (ER). In some cases, the smooth ER was believed to arise from rough ER after the displacement of ribosomes. These cells also contained myelin fragments from the original internodes. Microtubules displayed unique relationships with cytoplasmic membranes and were seen to exist as regular 22 nm microtubules, as obliquely sectioned profiles or as incomplete, trough-shaped structures. Internodes devoid of myelin were common, and naked axons covered only by basal lamina appeared swollen and invariably contained an abundance of microtubules. Schwann cells lacking axons had multilobate nuclei and possessed complex arrays comprising smooth ER membranes and microtubules. In areas where microtubules were absent, these membranes compacted to form intracellular myelin. Intermediate filaments existed in lower numbers than normal within both axons and Schwann cells. Arrested mitoses were occasionally seen and it is speculated that together with the immobilization of the cell by the over-abundant polymerization of tubulin, incomplete mitosis was an underlying cause for the observed lack of remyelination over the 21-day period of study. These results suggest that perturbations in microtubule synthesis might dramatically affect Schwann cell behaviour and myelin proliferation.