Spatial dispersion of foam cells in the human thoracic aorta.

The lateral walls of the thoracic aorta in the human have a valuable property that permits useful studies of atherogenesis. Each of the points in the sample is similar to all other points, and, therefore, a collection of such points can be treated as a random sample from a defined population. Lateral walls of 215 aortas from subjects of ages 15 to 69 years were sectioned in paraffin and stained with hematoxylin and eosin. These were evaluated at 40 anatomically defined sites, 20 on the left and 20 on the right. Presence or absence of atheronecrosis was noted. In the absence of necrosis, each 100-microns thick layer of intima was assessed for presence or absence of foam cell infiltration. In the presence of necrosis, the fibrous cap was assessed. The prevalence of foam cell infiltrates was strongly associated with intimal thickness, with depth within the intima, and with the presence of atheronecrosis. The association was alike in all 10-year age groups from 30 to 69 years, indicating the existence of a steady state in which intimal thickness determines the prevalence of foam cells without regard to age. Infiltrates were greatest at intimal thicknesses of 400 to 600 microns and were scarce at distances greater than 300 microns from the endothelium. In the absence of atheronecrosis, foam cells were found abundantly in the surface layers of the more thickened regions of intima. The deeper layers acquired that characteristic after the onset of atheronecrosis. An explanation for these findings is not immediately apparent. The focal character of atheronecrosis and of foam cell infiltrates demands that evaluation of a specimen for these features must use a large sample of tissue. Combining left and right thoracic aorta into strips of tissue of 22 to 30 cm total length yields estimates of foam cell involvement which have disappointingly large confidence intervals. Thus, the observed condition of a specimen evaluated at autopsy may be strongly influenced by random sampling variation.