Comparison of human astrocytes (1181N1 line) and sheep choroid plexus cells (SCP) infected with visna virus (V-1181N1 and V-SCP) shows that the mode of virus assembly and the morphology and size of the free virions are identical in the 2 systems. V-SCP cells usually cytolyse soon after infection; V-1181N1 cells do not. This is considered to be due to interference with membrane assembly in the former. Syncytial giant cells from both V-1181N1 and V-SCP cultures have a much looser attachment to the growth surface than their uninfected parents. This is shown to be associated with a loss of certain microfibrillar arrays close to the plasmalemma. Uninfected SCP cultures have cilia and desmosomes and retain these after infection till cytolysis; 1181N1 cells do not have these structures, but have a well developed microtubular system, which the SCP cells lack.

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