A sequential study is reported of the morphological changes occurring after herpes B virus infection of cells as revealed in ultrathin sections under the electron microscope. Monolayer cultures of renal epithelial cells prepared from the natural host of the virus, the monkey, were infected, and the cellular alterations were correlated with the appearance of infective virus in the culture fluids.

The morphological changes consisted in swelling of the cells and disappearance of the nucleolus, followed by margination and gradual decrease of the nuclear chromatin. The inclusion material corresponded to the clear central areas of the nucleus, where the chromatin had disappeared. In the late stages of infection this inclusion material filled the nucleus and formed a classical type A inclusion body.

Characteristic particles appeared in the nucleus and cytoplasm of the infected cells a few hours after inoculation. They had a dense center surrounded by one or two membranes. Those with one membrane ranged in size from 60 to 100 mµ and those with two from 120 to 180 mµ. Particles showing the same wide variation in size and structure were seen both in the nucleus and in the cytoplasm. They were first visible on the external surface of the swollen but intact cells at about the same time new infective virus became detectable in the culture fluid.

A small number of the extracellular, and cytoplasmic, virus particles appeared "binucleated," containing two central bodies, each having its own membrane, both being surrounded by a single external coat. About 180 mµ in diameter, they were randomly distributed among the "mononucleated" particles.

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