Colorado tick fever (CTF) virus was observed by ultrathin section and negative contrast electron microscopy. In sections of infected cultured cells and of mouse brain, virus particles (75 mμ in diameter) with electron dense cores were associated with intracytoplasmic granular matrices, arrays of intracytoplasmic filaments, and fine kinky threads. Occasionally, virus particles were enveloped by membranes of cytoplasmic organelles. Intranuclear filaments in dense arrays were frequently found in infected cells. In negative contrast preparations the virus was round, 80 mμ in diameter, with regularly spaced surface projections suggestive of cubic symmetry. Partial removal of surface components permitted the observation of an inner capsid 50 mμ in diameter. CTF virus was directly compared with reoviruses, which in several ultrastructural characteristics it resembles.
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