Strains of Newcastle disease virus (NDV) induce cell fusion by two different mechanisms. The first, called ‘fusion from without’ (Bratt & Gallaher, 1969), is independent of virus multiplication, can be induced equally well by infective and non-infective virus and does not require host-specific or virus-specific macromolecular synthesis. This type of cell fusion is completed within 1 to 3 hr of infection at high multiplicity and is caused by the direct interaction of the virus envelope with the plasma membrane of the cell. Fusion from without is a laboratory phenomenon and is similar to the rapid cell fusion induced by high multiplicities of Sendai virus, SV5, and measles virus (Poste, 1970). The second type of NDV-induced poly-karyocytosis, called ‘fusion from within’, is characterized by cell fusion beginning several hours after infection at moderate or low multiplicity. This type of cell fusion is related to the intracellular growth of virus and requires virus-specific macromolecular synthesis (Reeve & Poste, 1971a; Reeve et al. 1971).
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