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When scanning electron micrographs of orthogonal transmural fracture surfaces of distended aortas were compared with appearances on corresponding semithin light microscopic and ultrathin transmission electron microscopic sections, medial lamellar units could be resolved into composites of overlapping, musculo-elastic fascicles lying parallel to tangential planes of section. Each fascicle consisted of a group of commonly oriented, elongated smooth muscle cells and an encompassing array of branching, similarly oriented elastic fibers. On longitudinal sections of straight segments of the aorta, fascicles appeared as closely packed, transversely transected smooth muscle cell groups within compartments formed by similarly transected elastic fibers. On transverse sections, fascicles appeared as groups of cells within cell strips or layers, each of which was bracketed on both its luminal and abluminal sides by straight elastic fibers. Fascicles were increasingly evident during growth as commonly oriented groups of cells within the already present concentric cell layers became demarcated by enlarging elastic fibers. Wavy collagen fiber bundles, distinct from the interlaced fibrils of the immediate pericellular matrix, were interposed mainly between the facing elastin systems within the fibrous regions between cell layers. Thus, the radial transmural disposition of cells and matrix fibers on transverse sections of the media in well developed aortas proved to be: elastin-cells-elastin--collagen bundles--elastin-cells-elastin--collagen bundles, etc. Medias of major branch arteries were also composed of musculo-elastic fascicles, but their encompassing elastic fiber systems were less prominent than in the aorta. In straight segments of aortas and arteries, fascicles were uniform in size at any given transmural level and oriented mainly circumferentially. At bends and branch points fascicles were smaller and less uniform in size and orientation. In relation to changes in vessel wall curvature, alignment of the fascicles was usually in the direction of presumed resultant tensile stress. The findings suggest that these subunits of medial organization correspond to the distribution and magnitude of tensile stresses.

Transmural organization of the arterial media. The lamellar unit revisited.