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A hitherto unknown rod-shaped cytoplasmic component which consists of a bundle of fine tubules, enveloped by a tightly fitted membrane, was regularly found in endothelial cells of small arteries in various organs in rat and man. It is about 0.1 micro thick, measures up to 3 micro in length, and contains several small tubules, approximately 150 A thick, embedded in a dense matrix, and disposed parallel to the long axis of the rod. In some of these cells, the cisternae of the endoplasmic reticulum are greatly distended by the accumulation of a dense, finely granular material. The nature and significance of these cytoplasmic components are yet unknown.
Weibel et al. (Thu,) studied this question.