Histology of normal aortas in birds with emphasis on intimal thickening

Published studies of normal aortic structure have been infrequent in birds despite a high prevalence of spontaneous atherosclerosis relative to mammals, and a feeling that this prevalence of atherosclerosis might be related to the pecularities of aortic structure in birds. We describe aortic structure in 26 birds, including 22 species and 12 families. All aortas had a complex structure similar to hat of the White Carneau pigeon which has been carefully studied. Diffuse intimal thickening (DIT) was found in the elastic zones in 22 of the 26 aortas. This lesion has not been previously described as such, and consisted of multiple longitudinally oriented elastic laminae apparently synthesized by interlamellar connective tissue cells, the latter being peculiar to the avian aorta. Focal intimal thickenings were very common in the non-elastic zones of the aortas, and were composed of smooth muscle cells and elastic laminae similar to the DIT of mammals. Their location suggested that blood flow patterns might be an important influence in their development. The fact that atherosclerotic lesions usually occurred in the non-elastic zones of the avian aorta suggested that some property of the elastic zone or the interlamellar connective tissue cells might inhibit the development of atherosclerosis.