Kidney cells, predominantly from Cercopithecus monkeys but also from baboons, were infected in vitro with the SV40 virus. The infectious cycle was studied with the electron microscope by means of thin sections of cells fixed from 3 hours up to 11 days after infection. The frequency of virus formation and various nuclear and cytoplasmic lesions in relation to the infection are described. The virus particles appear in the nucleus in close contact with the chromatin. In a small number of cells they have been observed as early as l0 to 12 hours after infection, but most often they appear 24 to 48 hours afterward. Their mean diameter is 33 m#. They have no membrane and are frequently arranged as crystaMike structures. In addition to the appearance of virus, one observes various lesions in the nucleoplasm and particularly in the nucleolus, which shows an early hypertrophy and produces unusual, dense condensations in contact with the nucleolonema. The importance of these nucleolar lesions and the relationship between the SV40 virus and the polyoma, common wart, and Shope papilloma viruses are discussed.The SV40 virus was discovered by Sweet and Hilleman (38) in Macacus rhesus kidney cells, being identified by the cytopathic effect it produces on the kidney cells of Gercopithecus aethiops sabaeus, a monkey which is not naturally a carrier of this viral agent. The cytopathic effect is characterized by an intense cytoplasmic vacuollzation after several days of infection ("vacuolating agent").Our preliminary electron microscope study on Cercopithecus kidney cells in vitro infected by SV40 virus revealed that the site of development of the virus is in fact entirely nuclear, that the cytoplasmic vacuolization is actually the consequence of its multiplication in the nucleus, and, finally, that its morphological structure is similar to that of the polyoma virus (40). We also observed the importance of nuclear and nucleolar lesions (alterations produced by virus infection) which appear in the course of the infection. These first results are in agreement with the observations made by Gaylord and Hsiung (t5). When it was discovered that the inoculation of this agent into newborn hamsters produced tumors of the sarcoma type (13,17), this virus acquired importance in tumor virus research. A more detailed morphological study of the multiplication of this virus and the cellular lesions it induces appeared necessary to provide a better understanding of the cell-virus interaction following infection with an oncogenic agent.In the course of our study we have paid particular attention to nucleolar alterations during the first stages of infection. In addition, we have been much interested in the comparison of the evolution of this virus with that of the polyoma agent.
MATERIAL AND METHODS
Cell CulturesPrimary cultures were prepared from kidneys of the African green monkey (Cercopithecus aethiops sa-
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