Abstract
A line of cultured plant cells used to study cell differentiation and organ initiation over a period of several years has lost its ability to differentiate. Examination of these proliferating cells by electron microscopy reveals aggregates of 600-Å polyhedral particles in the nucleoli and heterochromatic regions of interphase nuclei. During mitosis and cell division the same particles are present on the chromosomes and are segregated into the daughter cells. Although the nucleoli and nuclear envelope are absent during mitosis, the particles remain associated with the chromosomes and do not appear in the cytoplasm. The morphological similarity of these particles to several known plant viruses suggests that the nuclear particles may represent a viral infection. The relationship between these particles and the loss of differentiation ability is discussed with reference to the production of carcinomas and the transformation of animal cells by viruses.
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