Whole-mount transmission electron micrograph of an LSEC treated with 100 nM dihydrohalichondramide for 1 hour, showing the dark nuclear area and surrounding extracted cytoplasm. Note the presence of small cytoplasmatic areas of intermediated density within the fenestrated cytoplasm. In several of these areas a very peculiar structure could be observed, consisting of rows of fenestrae with increasing diameter, fanning out into the surrounding cytoplasm, connected to the small cytoplasmatic areas with their smallest fenestrae. These structures are suggestive of de novo fenestrae formation and we therefore named them "fenestrae-forming center" (FFC) . Scale bar, 5 μm.