Electron microscopic comparison of the tunica media of the thoracic aorta between species.

Previous observations indicate that the tunica media of the thoracic aorta of chickens contains two populations of cells (smooth muscle cells and connective tissue cells), whereas that of the mammalian species only contains one cell type (smooth muscle cells). It was considered that this difference in chickens: a) might occur in other avian species; b) might be a common feature in other classes from phylum chordata which are inferior to mammalia; and c) might be a special adaptation for flying animals. In order to test these hypotheses, the cytology of the tunica media of thoracic aorta was studied in five mammals (human, pig, bat, rabbit, and rat), three avian species (chicken, turkey, and pigeon), one reptile (fresh water turtle), and one amphibian (frog). All animals exhibited smooth muscle cells in the tunica media of the aorta, but only the avian species demonstrated the second major cell type (interlamellar connective tissue cells). In the avian aorta the tunica media was composed of alternating layers of smooth muscle cells and fibroblast-like connective tissue cells separated by thick elastic lamellae. It is concluded that the presence of connective tissue cells in the tunica media of the aorta may be a distinctive characteristic of birds.