Morphological analysis for two types of viral particles in vacuoles of SARS-CoV-2-infected cells

In this study, we analyzed the morphological structure of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in human cells. We identified the two types of viral particles present within the vacuoles of infected cells. Using transmission electron microscopy, we observed that SARS-CoV-2 particles exhibited both low- and high-electron-density structures, which was further confirmed through three-dimensional reconstruction using electron tomography. The budding of these particles was exclusively observed within these vacuoles. Intriguingly, viral particles with low-electron-density structures were confined to vacuoles, whereas those with high-electron-density structures were found in vacuoles and on the cell membrane surface of infected cells. Notably, high-electron-density particles found within vacuoles exhibited the same morphology as those outside the infected cells. This observation suggests that the two types of viral particles identified in this study had different maturation status. Our findings provide valuable insights into the molecular details of SARS-CoV-2 particles, contributing to our understanding of the virus.