NATURE OF SPECIES DIFFERENCES IN MEDIAL DISTRIBUTION OF AORTIC VASA VASORUM IN MAMMALS

Thoracic aortic segments of 12 mammalian species [man, mice, rat, guinea-pig, rabbit, cow, pig, cat, dog, horse, sheep, monkey] were fixed while distended at normal physiological pressures after the vasa vasorum were filled by a per-fusion mixture containing gelatin and carbon. Mammals whose aortas had 20 or fewer medial lamellar units had no demonstrable intra mural vascular channels; those whose aortas had more than 29 medial lamellar units, had medial vasa. Aortas with medial vasa vasorum always had a subintimal medial zone devoid of vasa vasorum. In growing animals the width of this avascular zone increased with age; in adults, the width increased slightly with increasing species body weight. However, the number of lamellar units in the avascular zone was independent of both age and species and equal to 29.0 [plus or minus] 2.5. All of the species with intramural aortic vasa vasorum as adults had 29 or more aortic medial lamellar units at birth; none of the species which had 29 or fewer lamellar units at birth had more than 29 lamellar units at maturity. In aortas with medial vasa vasorum, widening of the avascular zone during growth appeared to be due only to the uniform thickening of each of the approximately 29 lamellar units already present at birth; the vascularized outer zone widened both by enlargement of its lamellar units and the addition of new lamellar units. Species differences in medial distribution of vasa vasorum are due primarily to differences in thickness of the vascularized outer zone.