[A scanning electron microscopic study on the arteriovenous anastomoses of rabbit ear using corrosive resin casts]

Arteriovenous anastomoses (AVAs) partake in the regulation of blood flow, and although it has been speculated that AVAs in the rabbit ear play a major role in the regulation of the body temperature, no documented work is available on the structural characteristics of AVAs in the rabbit ear. Accordingly, the purpose of this investigation was to determine the frequency of AVAs and their structural characteristics by using the vascular corrosive resin casts and further examination under a scanning electron microscope (SEM). This method permitted not only the visualization of a three-dimensional view of AVAs, but also depicted a clear differentiation between AVAs and arterioles or venules. Within the vascular architecture of rabbit ear, three distinctive vascular layers could be identified. The frontal and dorsal epithelial vascular layers are distributed underneath the epithelium and formed with nearly all capillaries, and the intermediate vascular layer which is established with main arteries, veins and their branches. The AVAs were observed in all three vascular layers, although various venous valves were concentrated primarily in the intermediate vascular layer. In tabulating the frequency of appearance of AVAs, the mean number was 81.5 per centimeter square. The mean number of AVAs in each of the three vascular layers was 31.1 in the frontal layer, 17.1 in the intermediate layer and 33.1 in dorsal layer, respectively. The vasculature of rabbit ear was further anatomically divided into tip, middle and root portions. The mean number of AVAs in each portion, from the tip to the root was 112.9, 49.4 and 84.4, respectively. Several essential types of AVAs were observed, such as the S-shaped and similar forms which were the predominate type. Others observed were U-shaped, straight, Y-shaped and tri-branching types. Most AVAs within the intermediate vascular layer, consisting of a few small vessels which were mainly capillaries and venules, connected to the anastomosing portion of AVA. The high occurrence and strategic location of AVAs in the rabbit ear strongly suggest that AVAs in the rabbit ear partake in thermal regulation.