The longitudinal orientation of structures in the subendothelial space of rat aorta

The aortas of rats were sectioned parallel to the endothelial surface for the purpose of studying the orientation of structures in the endothelium and subendothelial space relative to the long axis of the aorta. Microfilament bundles within endothelial cells, and the associated anchoring filaments of the subendothelial space, lay parallel to the long axis. Other filamentous structures of the subendothelium, namely, the bundles of very fine microfibrils and the somewhat larger elastic fibers, also showed a generally longitudinal orientation, although they frequently deviated from this arrangement, and were not perfectly straight. Another group of filaments in the deepest part of the subendothelial space lay in the transverse axis of the aorta. These filaments appeared to represent the most superficial component of the internal elastic lamina. In addition to the longitudinally orientated fibrillar structures, the subendothelium also contained longitudinal cytoplasmic folds, or rugae, projecting from the deep surface of the endothelial cells. It is concluded that contact‐guidance by structures having a pronounced longitudinal orientation within the subendothelial space may be responsible for creating the longitudinal arrangement of smooth muscle cells seen in this layer of the arterial wall with aging, and with certain types of pathological change.