The arteries of mammals contain large amounts of elastin arranged in concentric lamellae known as medial lamellar units (MLU). In adult mammals of a variety of species the number of lamellar units is roughly proportional to the radius of the artery and the tensioniMLU ratio is roughly constant in all species, but greater in the abdominal than in the thoracic aorta. Re-analysis of these data shows that the number of MLU of the abdominal aorta is linearly related to the pulse pressure, while the number of MLU in both the thoracic and abdominal aorta increases exponentially with stroke volume. Preliminary data are presented showing the decrease in number of MLU along the thoracic aorta of both fetal lambs and sheep, and evidence is provided that some of this elastin may be involved in the formation of the small arteries, such as the intercostals, which arise from the aorta.

Scanning electron microscopy showed that the elastin on the intimal side of the media was in the form of fenestrated sheets while that on the adventitial side was a fibrous network. The size and density of the fenestrations was greater in fetal lambs and may play a role in allowing growth of the artery.