BACKGROUND INFORMATIONVACV (vaccinia virus) is one of the most complex viruses, with a size exceeding 300 nm and more than 100 structural proteins. Its assembly involves sequential interactions and important rearrangements of its structural components.RESULTSWe have used electron tomography of sections of VACV-infected cells to follow, in three dimensions, the remodelling of the membrane components of the virus during envelope maturation. The tomograms obtained suggest that a number of independent 'crescents' interact with each other to enclose the volume of an incomplete ellipsoid in the viral factory area, attaining the overall shape and size characteristic of the first immature form of the virus [IV (immature virus)]. The incorporation of the DNA into these forms leads to particles with a nucleoid [IVN (IV with nucleoid)] that results in local disorganization of the envelope in regions near the condensed DNA. These particles suffer the progressive disappearance of the membrane outer spikes with a change in the shape of the membrane, becoming locally curled. The transformation of the IVN into the mature virus involves an extreme rearrangement of the particle envelope, which becomes fragmented and undulated. During this process, we also observed connections between the outer membranes with internal ones, suggesting that the latter originate from internalization of the IV envelope.CONCLUSIONSThe main features observed for VACV membrane maturation during morphogenesis resemble the breakdown and reassembly of cellular endomembranes.