Human diploid BAMB cells with epitheloid morphology, which had been derived from amniotic fluid cells, were capable of supporting the replication of human cytomegalovirus (CMV), without prior treatment of the cells with halogenated pyrimidines. The growth of this virus in BAMB cells and in human diploid fibroblastoid (LEP) cells was compared in parallel tests. Virus replication was slower and less efficient in the former than in the latter system. The most characteristic morphological feature of the CMV-infected BAMB cells was the formation of multinucleated giant cells which frequently contained more than a hundred nuclei; such cells were not seen in LEP cultures. The development of ultrastructural changes was slower in BAMB cells than in LEP cells. The additional most marked differences concerned the place of viral envelopment and the production of cytoplasmic dense bodies. While in LEP cells most nucleocapsids were enveloped from the inner leaflet of the nuclear membrane, in the other system a great majority of the particles acquired their envelopes by budding into vacuoles. Cytoplasmic dense bodies were rare in infected LEP cells but very frequent in BAMB cells. Budding of these structures into vacuoles was also observed.

Ultrastructural changes developing in chick embryo fibroblast cultures infected with a wild-type strain of fowl plague virus (FPV) or one of six FPV temperature-sensitive (ts) mutants belonging to different complementation groups were studied. Cells infected with wild-type FPV and incubated at optimal (36 degrees C) or nonpermissive temperature (42 degrees C) displayed changes similar to those described for orthomyxoviruses. The same patterns of changes were observed at 36 degrees C in cells infected with ts mutants belonging to five of the complementation groups. Mutant ts 303, possessing mutation-altered haemagglutinin, induced at 36 degrees C the formation of virions carrying a considerably reduced number of spikes on their surfaces. At 42 degrees C, cells infected with ts mutant 131, with a defective primary transcription stage, showed no morphological changes and no formation of electron-dense inclusions. Cells infected with ts mutants with defective secondary transcription or replication displayed nuclear inclusions but no formation of filamentous cytoplasmic structures or virions. Mutant ts 5 with defective late morphogenesis induced formation of considerably enhanced numbers of nuclear inclusions.

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SummaryMorphological changes developing in BI{K-21 cells infected with the S/N virus (a recombinant of Ap/Singapore and A/NWS viruses), at the approximate m. o. i. of 1 TCDs0 per cell, were studied by electron microscopy. Substantial alterations were observed in both the nucleus and cytoplasm. Changes in the nuclei were first detected eight hours after infection. The nucleoli substance was reduced and dense inclusions consisting of fibrillar structures 40--60 • in diameter were formed. The greatest number of inclusions and their distribution over the whole nuclear area were found 24 hours after infection. Electron-dense inclusions were also detected in the cytoplasm. They were identical in shape and size with those observed in the nuclei but they consisted of fibrils 35--40 A in diameter. These inclusions were rare eight hours after infection. Their number increased with the incubation period, reaching a maximum 24 hours after infection. Six hours after infection filamentous structures 80--90 A in diameter were observed in the cytoplasm. It is assumed that they represent viral ribonueleoprotein. They were at first (hour 6) distributed diffusely through the whole cytoplasm and were later (hour 8--10) found mainly near the cytoplasmic membrane. Twenty four hours after the infection the filamentous structures were abundant both in the central part of the cytoplasm and in the proximity of the cellular membrane. Sometimes they formed dense agglomerations 2000--4200 A in width. Naturation of the virion took place at the plasma membrane and in cytoplasmic vacuoles in whose proximity the filamentous structures were found. The particles either budded directly from the smooth cell membranes or from processes formed on the cell surface. Numerous rod-like particles were observed.

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