Polyploidy induction by spherical size standard polystyrene particles in a Chinese hamster cell line CHL]

To investigate relationships between particle (as a model of aggregates) size in a nanomaterial test suspension and its cytotoxicity, a series of eleven sizes of polystyrene (PS) particles were tested in the cytotoxicity test and the chromosome aberration test by using a Chinese hamster cell line CHL. The PS particles were spheres with defined diameters ranging from 0.1 to 9.2 µm. A series of eight sizes of particles with diameters ranging from 0.92 to 4.45 µm showed stronger cytotoxicity than the others. There was a marked difference in cytotoxicity between the 4.45- and 5.26-µm particles. The 0.92- to 4.45-µm particles did not induce structural chromosome aberrations but induced a high frequency of polyploidy in the chromosome aberration test. The 5.26-µm particles showed very weak induction of polyploidy. The incorporation of the 4.45-µm particles into CHL cells was observed by scanning electron microscopy (SEM). Some cells incorporated more than 10 particles. The semi-quantitative measurement of incorporation of particles into cells was performed by flow cytometry with a parameter of side scattered light (SSC) intensity. It showed that CHL cells preferably incorporated the 4.45-µm particles to the 5.26-µm particles. These findings suggest that CHL cells may have a kind of size-recognition ability and incorporate a particular size of particles. The particles may prevent a normal cytokinesis resulting in polyploidy induction. Nanomaterials also may show size-dependent toxicity. Data on particle (or aggregate) size distribution in the test suspension should be provided to evaluate properly the results of toxicity tests of nanomaterials.