Transmedial zonation and regional variations in the tunica media of the sheep (Ovis aries)

Abstract

The sheep is a valuable model for cardiovascular studies and its aorta is frequently afflicted by atherosclerosis and aneurysms. The structure of sheep aorta is, however, only seldom reported. This study, therefore, aimed at describing the microscopic organization of the tunica media of the aorta in sheep. Specimens obtained from the aortae of six healthy young adult sheep were fixed in 10% formaldehyde solution and routinely processed for paraffin embedding and sectioning. Five micron sections were stained with Weigert’s Resorcin Fuchsin/Van Gieson and examined with a light microscope. Micrographs were taken with a high resolution digital camera. The tunica media in the ascending, arch and thoracic regions comprised two zones namely a luminal elastic zone with continuous uniform elastic lamellae and adventitial musculo-elastic one where there were muscle islands frequently interrupting the elastic lamellae. The proportion of musculo-elastic zone with the muscle islands declined caudally. In the abdominal region, however, the tunica media comprised regular continuous concentric elastic lamellae. These results show that the aorta in sheep displays regional variations characterized by transmedial zonation of the thoracic segments into an elastic luminal and musculo-elastic adventitial zones. These differences may underpin the regional differences in physicomechanical properties and vulnerability of the aorta to diseases such as atherosclerosis and aneurysm.

Keywords: Tunica media, aorta, muscle islands, sheep