The abdominal aorta-inferior mesenteric artery branchings of 65 human subjects younger than 39 years old were examined at autopsy to study the mechanical effect of the blood flow on the localization of early atherosclerotic lesions and their precursors.
(1) At the apex of the flow divider, the apical pad, a thin but extremely fibrous fibrocellular intimal thickening, was almost always observed, even from subjects younger than 1 year old. However, no atherosclerotic lesions were found here even in the subjects older than 30 years.
(2) At the proximal wall of the bifurcation, the proximal pad was observed after around 10 years old. This was a sparse fibrocellular intimal thickening in abundance of smooth muscle cells and oedematous extracellular ground substance accompanied by various degree of abnormalities of elastic tissues, such as fragmentation, thinning, and duplication. In the subjects older than 10 years, an extensive lipid deposition was almost always observed in the intima at the bottom layer of the intimal thickening. The lipid deposition was found extracellularly at the early stage, and intracellularly at the late stage. The foamy cells were frequently found in this portion. This type of fibrocellular intimal thickenings were, therefore, thought to be potential to develop into the atherosclerotic plaque.
(3) From the fluid mechanical view points, the apical portion of a branching is considered to receive a high wall shear, whereas the proximal portion of the branching to have a low wall shear, or sometimes even a flow separation. These mechanical factors are concluded to play a major role on the differentiation of the fibrocellular intimal thickenings through a modification of the elastic tissues in the intima. Of them., the proximal-padtype of fibrocellular intimal thickening is thought of as the early atherosclerotic lesion at this branching, so that its specific localized patterns in the human abdominal aorta result.