Human Cytomegalovirus: Development and Progression of Nuclear Inclusions by Primary Clinical Isolates and Laboratory-Adapted Strains

The morphogenesis of cytomegalovirus (CMV) nuclear inclusions (NIs) was investigated using unadapted clinical isolates and adapted laboratory strains. Both adapted and unadapted strains of CMVs induced NIs whose morphologic appearance was similar in human fibroblastic cells. Early NIs appeared as ring-like structures composed of dense granular and fibrillar material, while late NIs appeared to consist of multiple electron-lucent areas containing coarse granules and bounded by electron-dense fibrillar material (cellulae). Capsids and nucleocapsids were associated primarily with the electron-dense fibrillar material; however, developing nucleocapsids were most often observed at the interface of the electron-dense and -lucent areas. Although there was some variation in the rate of development and maturation of the NIs with the intensity of infection, all CMVs examined produced late NIs with similar organizational patterns consisting of cellulae. Substitution of human fibroblastic cells derived from various tissues as cellular substrate did not appreciably affect the results. Thus, the unique organization of the CMV late NI, consisting of multiple cellulae, appears to be an intrinsic feature of CMV replication since it seems to be independent of the extent of laboratory adaptation, the virus strain, the intensity of infection, or the cell type.