Envelopment and the envelopes of infectious bovine rhinotracheitis virus in ultrathin sections.

An electron microsopic study of cell cultures and bovine foetal tracheal organ cultures infected with infectious bovine rhinotracheitis (IBR) virus showed the following. a) The difference in the site at which the outer envelope is acquired in the infected cell is responsible for some morphological differences between the virions. Where envelopment of the capsids occurs by budding into cytoplasmic tubules, dense material adjacent to the tubules is often incorporated between the capsid and the outer envelope, giving a pleomorphic appearance to the latter. No dense material is seen in the same position if envelopment occurs in the nucleus. b) Dense material adjacent to the cytoplasmic tubules is not only incorporated in the virions, but may also bud into the cytoplasmic tubules without nucleocapsids, thus giving rise to dense bodies. c) Dense material in the virions is closely adjacent to the inner side of the outer envelope, but is separated from the capsid by a lucent zone. A similar zone can be seen around some nucleocapsids in the nucleus. d) The outer envelope of numerous extracellular virions and of those located in the cytoplasmic tubules shows the unit membrane structure with projections. The outer envelope of intranuclear particles and of those located in the perinuclear cisterna had the appearance of a dense membrane.