Developmental disorders of the mouse brain induced by murine cytomegalovirus: animal models for congenital cytomegalovirus infection.

Developmental disorders induced by congenital cytomegalovirus (CMV) infection mainly involve the central nervous system. The type and degree of the brain disorders seems to depend on infection time during gestation, virulence, route of infection and viral susceptible cells in each embryonal stage. Since transplacental transmission has been reported not to occur with murine CMV (MCMV), we developed mouse models for congenital CMV infection by surgical injection of MCMV into the mouse conceptus or embryo at different gestational stages. For the early stage, the mouse embryos were not infected with MCMV even after injecting the virus into the blastocysts, which were developed in the pseudo-pregnant mothers or cultured in vitro. Isolated whole mouse embryos of day 7.5 of gestation (E7.5), adsorbed with a high titer of MCMV and cultured for 3 days, were susceptible to MCMV infection. Therefore, the mouse embryo acquires the susceptibility around this period. Microphthalmia and cerebral atrophy were induced in mouse embryos after injection of MCMV into the conceptus on E8.5. Viral antigen-positive cells were widely distributed in the mesenchyme around the oral and nasal cavities and in the mesenchyme around the brain, especially the endothelial cells of vessels and the perivascular mesodermal cells, then infection extends to the eyes, brain or choroid plexus. This finding suggests that mesenchymal infection may be the critical step in disrupting organogenesis, resulting in brain disorders. For the late stage, mouse embryos were infected with MCMV by injecting the virus into the cerebral ventricles on E15.5. Brains of the offspring showed massive necrosis with gliomesodermal proliferation in the cerebral cortex. Viral antigen-positive cells were observed in laminar array in the lesion-free cortex and the hippocampus, suggesting that the infected cells migrate in association with the lamina formation. Immunohistochemical double-staining showed that brain cells susceptible to MCMV infection may be mainly neuronal and endothelial cells, resulting in cerebral atrophy with reduction of neuronal cells and cystic lesions, presumably due to ischemic vascular changes.