PLATE 33 • AORTA The aorta, the main systemic artery of the body, is an elastic artery. The presence of numerous fenestrated elastic lamellae allows it to resist the pressure variations caused by rhythmic contraction of the left ventricle. The intima is comparatively much thicker than that seen in muscular arteries. The subendothelial layer of the in-tima consists of connective tissue with both collagen and elastic fibers. The cellular component consists of smooth muscle cells and fibroblasts. The external border of the intima is bounded by an internal elastic membrane that represents the first layer of the many concentric fenestrated laminae in the media of the vessel. The media constitutes the bulk of the wall. Between the elastic laminae are collagen fibers and smooth muscle cells. The latter are responsible for the synthesis of collagen and elastic fibers. With age, the number and thickness of elastic laminae in the wall in-creases. By 35 years as many as 60 laminae are found in the thoracic aorta. At ap-proximately 50 years, individual laminae begin to show signs of degeneration and gradually become replaced by collagen leading to a gradual loss of elasticity of the aortic wall. The adventitia consists of irregular dense connective tissue with intermixed elas-tic fibers that tend to be organized in a circumferential pattern. It also contains small blood vessels that supply the outer portion of the media. They are the vasa vasorum of the aorta. Also present in the adventitia are lymphatic capillaries. ORIENTATION MICROGRAPHS: The upper micrograph shows a cross section of an H&E stained human aorta from a child. The intima (I) stains considerably lighter than the adjacent media (M) The adventitia (A) contains an abundance of collagenous fibers and stains more densely than that of either the media or intima. The lower micro-graph is from an adult and has been stained to reveal the elastic component of the vessel wall. The intima (I) is very lightly stained, in this case, due to the paucity of elastic material. The media (M) is heavily stained due to the presence of large amounts of elastic laminae. The adventitia (A) contains in addition to the dense connective tissue a moderate amount of elastic fibers. MMMMAAAAIIIIMMAAII
Aorta, human, H&E, ×365; inset ×700.
This micrograph shows the layers of the aortic wall. The intima consists of an endothelium (En) overlying loose connective tissue (LCT). The thickest portion of the vessel wall is the media (M). The wavy eosinophilic material is the collagenous fibers. The eosin stain does not reveal the elastic laminae. The nuclei are those of smooth muscle cells. Fibroblasts are absent. The outer
Aorta, human, iron hematoxylin and aniline blue, ×255; inset ×350.
The specimen shown here has been stained to distinguish collagen from elastic material. The intima (I) consists mostly of collagenous fibers. The endothelium (En) represented by several nuclei is just barely evident. The media (M) contains nu merous elastic lamellae that appear as the black wavy lines. The intervening blue stained material consists of collagen fibers. Careful examination of the layer of the vessel wall is the adventitia (A). The eosinophilic material here consists of dense connective tissue. The nuclei that are evident belong to fibroblasts. Also note the small blood vessel (BV) in the adventitia. The inset shows the intima at higher magnification and includes part of the media. Note the endothelium (En). The eosinophilic material in the intima consists of collagenous fibers (CF). The main cell type here is the smooth muscle cell (SMC).