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Asbestos and other mineral fibers have long been known to induce lung cancer and mesothelioma. However, the primary mechanisms of fiber-induced carcinogenesis still remain unclear. We investigated the occurrence of mitotic disturbances induced by asbestos (amosite, crocidolite, chrysotile) in an in vitro approach using Syrian hamster embryo (SHE) fibroblast cells. The following endpoints were investigated: micronucleus formation as a result of mitotic disturbances and characterization of the induced micronucleus population by kinetochore staining and visualization of the spindle apparatus. Supravital UV-microscopy was used to analyze changes in interphase chromatin structure, impaired chromatid separation, and blocked cytokinesis. All three asbestos fiber types induced a high frequency of micronucleus formation in SHE cells (> 200/2000 cells) in a dose-dependent manner (0.1-5.0 micrograms/cm2), with a maximum between 48 hr and 66 hr exposure time. At higher concentrations (more than 5.0 micrograms/cm2) the micronucleus formation decreased again as a result of increased toxicity. Kinetochore staining of micronuclei revealed that 48 +/- 2% of asbestos-induced micronuclei reacted positively with CREST (antikinetochore) serum. Furthermore, spindle apparatus deformations occurred in cells with disturbed metaphases and anaphases, while the spindle fiber morphology appeared unchanged. Our results show that asbestos fibers may cause both loss and breakage of chromosomes in the absence of direct interaction with spindle fibers.