Novel insights on the progression of intermediate viral forms in the morphogenesis of vaccinia virus

•Description of a sequence of structural events for VACV assembly that provides key clues about VACV morphogenesis.•Electron microscopy (EM) studies with the WR mutants M65 and M101.•Intermediate stages in poxvirus morphogenesis.

Morphogenesis of vaccinia virus (VACV) is a complex structural process in which the capture of all cytoplasmic stages is difficult due to the rapid transition between the different viral forms. Taking advantage of two VACV mutants (M65 and M101) with defined genetic alterations, we described by transmission electron microscopy (TEM) of ultrathin sections novel potential transition viral forms (Ts) with reorganization of the immature virus (IV) membrane and construction of the internal core, and illustrated the envelopment steps from the mature virus (MV) to the wrapped virus (WV) stages. Our observations allowed us to propose a sequence of structural events for VACV assembly that provides key clues about VACV morphogenesis.