The present studies were undertaken to investigate the ultrastructure and development of virus particles detected in a spontaneous mammary tumor of a rhesus monkey. Thin-section electron microscopy of the tumor tissue has revealed 2 types of particles, viz., an intracytoplasmic, electron-dense, ring-shaped particle measuring about 70 mµ in diameter, and an extracellular particle with an outer unit membrane and a central dense nucleoid measuring about 110 mµ in diameter. From the electron micrographs, the intracytoplasmic development and virus maturation by a process of budding at the level of the cell membrane are reconstructed. The observations are discussed in view of the known oncogenic RNA-type virus particles.


Supported by Breast Cancer Task Force Contract PH 43-67-1176, NIH.

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