Aortic rupture is a well recognized cause of
sudden death in thoroughbred horses. Some microscopic lesions, such as those caused by cystic medial
necrosis and medionecrosis, can lead to
aortic rupture. However, these microscopic lesions are also observed in normal horses. On the other hand, a previous study of
aortic rupture suggested that underlying
elastin and
collagen deposition disorders might be associated with
aortic rupture. Therefore, the purpose of this study was to compare the structural components of the tunica media of the aortic arch, which is composed of
elastin,
collagen, smooth muscle cells and
mucopolysaccharides (MPS), in fetal to mature thoroughbred horses. The percentage area of
elastin was greatest in the young horses and subsequently decreased with aging. The percentage area of
collagen increased with aging, and the elderly horses (aged ≥20) exhibited significantly higher percentage areas of
collagen than the young horses. The percentage area of smooth muscle cells did not change with age. The percentage area of MPS was inversely proportional to the percentage area of
elastin. The fetuses exhibited a markedly larger percentage area of MPS than the mature horses. We concluded that the medial changes seen in the aortic arch, which included a reduction in the amount of
elastin and increases in the amounts of
collagen and MPS, were age-related variations.