Ultrastructural analysis of 3Cpro-induced vacuoles. Cytoplasmic vacuoles in A549/3Cpro and Calu-1/3Cpro cells were analyzed by electron microscopy. A. Vacuoles bounded by a single membrane layer. B. Vacuoles with blurred boundaries. C. Vesicles neighboring vacuoles. Vesicles inside vacuoles are indicated by triangles; multimembrane formations, m; small vesicles neighboring vacuoles, *. Scale, 0.5 μm.