Morphological Study on the Development of Medial Lamellar Unit in the Wall of Aorta and its Major Branches in Human Fetuses

The structure of the arterial wall is formed and controlled by various hemodynamic factors in the adults and fetuses. Furthermore, recent studies of human fetuses at varying stages confirmed that the number of elastic medial lamellar unit(MLU) increased with age and decreased along it length from arch to lower abdominal aorta. Using human fetuses between 9 to 28 week of development, we traced the change in the number of MLU along the arch of aorta from which 3 major branches, the brachiocephalic trunk, left common carotid artery, left subclavian artery, arose. The number of MLU along the arch of aorta increased with age, but there were regional differences in the increase. Increase in the number of MLU was prominent in the proximal part of the arch before 15 week of development, but in the distal part after 15 week of development. Branch with greater diameter had more MLU. So the brachiocephalic trunk had the most MLU among 3 branches. When compare the sum of number of MLU in one branch (A) and in the arch after origin of branch (B) to the number of MLU in the arch before origin of brahch (C), A＋B was always greater than C. This means that the MLU does not form a complete sheet along the aorta and new MLU is added to the wall of branch probably due to shear stress.restrictio