Fig. 2. The morphology of the starting membranes dictates the biochemical requirements of cell-free nuclear assembly. Classically, nuclei were formed by mixing membrane fragments with sperm chromatin and cytosol (classical nuclear assembly). In this case, membrane fragments bind to the chromatin surface, fuse into a tubular network and expand to form the NE. Alternatively, when ER fragments are preformed into an ER network prior to chromatin addition, the network binds to chromatin and expands to enclose the chromatin. The addition of homotypic ER fusion inhibitors blocks nuclear assembly when fragmented membranes are mixed with chromatin (classical nuclear assembly). However, when these inhibitors are added after ER network assembly, there is no affect on NE formation.