A paracrystalline inclusion body in normal human vascular endothelial cells

Peculiar crystalloid containing cytoplasmic inclusions are described for human umbilical vein endothelial cells (EC) which have never been reported for normal human blood vascular EC before. The inclusions are distinctly membrane-bound, their diameters range from 0.4 to 0.8 micron, and they tend to form clusters. In the latter case the corpuscles are pressed together very tightly and their membranes are often cross-connected by a thin electron dense thread traversing the intermembrane space in a very regular zigzag fashion. The contents of these inclusions vary from finely granular and homogeneous to more or less coarsely vesicular and inhomogeneous, finally culminating in the appearance of stacks of regularly arrayed electron-dense bands. These crystalloids vary in size and either one or more of them occur within the same corpuscle, thereby occupying different amounts of its volume. The various contents of these inclusions together with their close proximity and their actual fusion with defective mitochondria already point to a lysosomal and thus an organelloid nature of these structures. This assumption is strongly supported by the demonstration of acid phosphatases in the supernatant of cultures of human umbilical vein EC as well as by the ultrastructural localization of this marker enzyme within these corpuscles. These findings induced us to hypothesize that this crystalloid containing endothelial component appears to be species specific (human) with a distinctive preference for endothelial cells of various origins.