Comparison of aortic intima and inner media in young adult versus aging rats. Stereology in a polarized system.

Age-related ultrastructural changes in the intima and inner media of rat thoracic aorta were examined by new morphometric techniques. Young adult male rats, 10 weeks old, were compared with 1-year-old male rats. The most marked changes were found in the sub-endothelium, which increased in thickness more than five-fold. Basement-membrane-like and granular material accounted for the bulk of this thickening. Certain other structures were increased sevenfold or more in subendothelium. These structures and the volume fractions they occupied in 1-year-old rats were as follows: banded collagen, 4.3%; mononuclear leukocytes, 4.5%; cystic structures, 3.3%; and fibrillar elastin, 1.0%. Changes were also demonstrated in the fenestrae of and at selected depth levels below the innermost, or alternatively the internal, elastic lamina. Collagen increased strikingly within fenestrae and just below the elastic lamina. This was associated with a 28% increase in the thickness of the elastic lamina and a recession of smooth muscle cytoplasm to a deeper position within the first musculoelastic medial layer. The alterations in subendothelial tissues imply an altered basis for mechanical support for aortic endothelium in aging rats. These results mark the successful application of micro-computer-based stereology to a situation of polarized geometry.