Vascular Histology (II): The Great Vessels.

Aorta and pulmonary artery. A. Low magnification micrograph of the aorta of a 1 year old individual. The intima is a single layer of endothelial cells. The media is formed by alternating layers of smooth muscle cells and elastic lamellae. The adventitia is thin and composed of loose connective tissue (Movat pentachrome, 25X). B. The pulmonary artery of the same individual as in A. The major difference in the microscopic appearance of the two vessels is in the structure of the elastic lamellae. These lamellae are interrupted and form gaps, as opposed to the uniformly parallel array shown in A (Movat pentachrome 25 X). C. At higher magnification, the parallel arrangement of the elastic lamellae of the aorta is evident. Note the abundance of collagen and proteoglycan material. This well organized structure is the basis of the great strength of the aorta and its major elastic branches. Molecular defects of individual components (namely, fibrilin and collagen) of this network are known to occur in pathologic conditions (Movat pentachrome 300X). D. The pulmonary artery at the same magnification as in C, shows that the elastic lamellae are thinner and arranged in a haphazard fashion. A small artery (vasa vasorum) is seen in the lower portion of the field (Movat pentachrome 300 X). E. Aorta of a newborn showing a well defined single layer of endothelial cells in the intima. The media shows the alternating layers of smooth muscle cells. Also note the vasa vasorum in the adventitia as they begin to penetrate into the media. At the bottom of the field there is a single layer of mesothelial cells lining the adventitia of this intrapericardial segment of aorta (H&E 50X).

In addition to the structural components to the great vessels there are specialized tissues around the aorta which are pressure sensors