Morphological and morphometric characterization of the aorta of Brazilian horses (Equus ferus caballus, Linnaeus-1758): Comparative analysis of the aortic bulb and ascending aorta.

We examined 52 horse aortas to characterize the morphological aspects of the aortic bulb wall and the ascending aorta. The morphometric data were analyzed using ImageJ®-Fiji 1.5 software. The Tunica intima was composed of endotheliocytes with scarce cytoplasm and ellipsoid nuclei, supported by a collagenous subendothelial stratum. The Tunica media was composed entirely of fascicles of vascular smooth muscle cells interspersed with bundles of collagen fibers and elastic lamellae, characterizing the aortic medial lamellar unit. The Tunica adventitia consisted of two distinct sub-layers: the external lamina, which contained the specific vascular-nervous plexus, and the internal lamina, which was composed of dense, unformed connective tissue. In absolute values, the total thickness of the aortic wall, as well as the thickness of the Tunica media, was significantly less in the aortic bulb than in the ascending aorta. The fibro molecular content of both segments was also analyzed. Thus, 19.1% of the wall in the aortic bulb was composed of vascular smooth muscle cells, 13.9% of elastin, 25.5% of collagen I, and the content of collagen III was 15.6%. The wall of the ascending aorta contained 17.5% vascular smooth muscle cells, 13.8% elastin, 23.5% collagen I, and 15.1% collagen III. The increase in the thickness of the Tunica media determined the increase in the total wall thickness of the aortic bulb compared to the ascending aorta. Furthermore, the increase in wall thickness observed between the segments analyzed did not affect the relative amount of fibro-cellular elements in each segment.