Arteriosclerosis in the African elephant: Part 2. Medial sclerosis

A type of spontaneous arteriosclerosis, described as medial sclerosis and quite distinct from atherosclerosis, was found in the aortas, coronary arteries and aortic branch arteries of free-living elephants (Loxodonta africana) in Uganda and Kenya. The lesions took the form of calcified fibrotic plaques in the inner tunica media. The calcification appeared to commence in the internal elastic lamina and was associated with atrophy of medial smooth muscle fibres and their replacement by fibrous tissue. In the aorta, medial sclerosis was found to be associated with aortic dilatation, decreased wall thickness and decreased extensibility. These changes were shown to result in substantial increases in the tangential stresses carried by the tissues of the aorta and coronary arteries. As with atherosclerosis, medial sclerosis increased progressively with age; and the approximate involvement of the aorta at different ages could be predicted from linear regression equations. There was no difference in the severity of lesions between male and female animals. Biochemically, the lesions of medial sclerosis were associated with decreased amounts of elastin and increased amounts of collagen in arterial walls. Arterial tissue showing medial calcification always contained less than 30% elastin by weight. In addition, the severity of medial sclerosis in individual elephants was found to be positively correlated with the concentration of calcium in their sera. The pathogenesis of these lesions is discussed and it is suggested that mechanical stress, medial anoxia and high serum calcium levels all contribute to the aetiology of medial sclerosis.