An electron microscope study of the aorta in young and in aging mice

Abstract

The thoracic aortas of mice ranging in age from 3 days to 17 months were studied in thin sections by means of the electron microscope. A few small arterial blood vessels from mouse lung and the aorta of a young rabbit were also examined. All tissues were fixed in osmium tetroxide. The length of fixation and the embedding procedures were varied in different preparations. The mouse aorta consists of an endothelium (tunica intima), a tunica media composed of about five elastic laminae and alternating layers of smooth muscle cells, and a tunica adventitia containing fibroblasts and fibers. Notable aging changes in the tunica media are: a gradually increasing amount of collagen, rarifications and interruptions of elastic laminae, disconnections between smooth muscle cells and elastic laminae, and a peculiar fraying or fragmentation of the innermost elastica. In recessive-obese mice, subendothelial deposition of collagen was also found. A subendothelial proliferative “lesion” consisting of elongated cells and newly formed collagen and elastin is interpreted as a possible parallel to the initial lesions seen in atherosclerosis. Additional evidence demonstrates that there is probably a filamentous component present within elastic fibers and elastic laminae. These filaments can be seen at the surface of small elastic fibers and, less frequently, within the elastic laminae. In addition, it appears that collagen fibrils and possibly cell fragments are incorporated into the outermost elastic laminae in the young aortas. The cells within the media are recognized as a variety of smooth muscle cells which probably arise from fibroblasts present during embryonic development. It is assumed that these smooth muscle cells are responsible for the gradual production of collagen, although the fine structure of their cytoplasm is not typical of cells engaging in fibrogenesis.