Intracellular diaphragmed fenestrae in cultured capillary endothelial cells

The endothelium of visceral capillaries is characterized by the occurrence of numerous fenestrae, which are usually bridged by a thin, single-layered diaphragm. Both in vivo and in vitro, diaphragmed fenestrae perforate the endothelial cell cytoplasm in the most attenuated regions of the cell. We report here that in capillary endothelial cells grown under experimental conditions promoting the development of intracellular lumina (for example, suspension within a three-dimensional collagen matrix), diaphragmed fenestrae can form in a unique, previously undescribed intracellular location - that is, within thin cytoplasmic septa separating contiguous luminal compartments.