الفهرس 
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Abstract
Aorta is the largest artery in the body. It carries blood from the heart to different body organs (Waugh and Grant, 2006). Aorta emerges from the left ventricle of the heart as the ascending aorta, arches over the heart (the aortic arch), and passes downward as the descending aorta (Heylings et al., 2007).
Histologically, the wall of the aorta consists of three layers, tunica intima, tunica media and tunica adventitia. The tunica intima is formed of endothelial layer and subendothelial connective tissue layer. The tunica media is formed of concentrically arranged perforated elastic laminae, smooth muscle fibers, and collagen fibers. The tunica adventitia is formed of few elastic and collagen fibers, nutrient blood vessels, and lymphatics (Mescher, 2010).
Within the tunica adventitia of the aortic arch, the aortic bodies’ chemoreceptors are especially present. They are formed of Glomus and sheath cells embedded in highly vascular connective tissue stroma. They respond to low oxygen tension, high carbon dioxide concentration and low arterial blood pH. Signals transmitted from these bodies via afferent nerves to the respiratory center in the brain to control respiration (Junqueira and Carneiro, 2005).
Aging is the accumulation of changes over time resulting in progressive loss of physiological capacity of the organisms (Bass, 2006).
Thirty male Albino rats were used in this study divided into three groups, ten animals each as follows: group I; the control group (3 months), Group II (1.5 years), and group III (2 years or more). Specimens were obtained form the aortic arch. They were stained by Hematoxylin and Eosin, Orcein, and Masson trichrome stains, then they were examined microscopically.
With the advancement of age, many histological changes were detected in the wall of the aorta including thickening of its three layers, breakdown and fragmentation of the medial elastic laminae and elastic fibers, significant increase in the collagen content that replaced the significant decrease in the volume percent of elastin. By aging the smooth muscle cells acquired irregular morphology and orientation that was associated with elastic fragmentation. There were areas of apparent loss of smooth muscle nuclei defined as medionecrosis (Ailawadi et al., 2003, Challah et al., 2003, Schlatmann and Becker, 2009, and Gupta et al., 2011).
The effects of these changes might be the development of many pathophysiological changes such as aortic stiffness, decrease in the aortic compliance, and aortic aneurysm formation. These changes may attribute to many clinical manifestations such as progressive increase in systolic blood pressure, left ventricular hypertrophy, left ventricular failure and myocardial ischemia (Longo et al., 2002 and Zieman et al., 2005).
The aortic bodies showed many changes by aging, some of the glomus cells became larger and darker in staining. Other glomus cells appeared degenerating or dying off. The sheath cells increased in number. The connective tissues around chemoreceptor cells became thick fibrosed with congested sinusoidal vessels. These changes in chemoreceptors may attribute to the hypofunction of the aortic bodies associated with aging hence; the decreased peripheral drive of ventilation is commonly observed in aged human (Pokorski et al., 2004 and Valladolid and Spain, 2006).