One of the locations of renal artery atherosclerosis is at the orifice of the renal artery, therefore the
structure of this orifice was assessed in 6 normal, adult male dogs by light microscopy (LM) and
transmission electron microscopy (TEM). For the LM study, processed tissues were embedded in paraffin
and sectioned serially into 6-μm thickness. Sections were stained with orcein. For the TEM study, after
fixation and dehydration, samples were embedded in resin, and ultrathin sections stained with uranil acetate. In the LM study, two edges (proximal and distal) at the orifice of the renal artery were observed. Extra formation of elastin was clearly seen at the tip of the distal edge compared with the proximal edge. The curve from the aorta wall into the renal artery at the distal edge of the orifice was sharper than that of the proximal edge. In the TEM study, the tunica intima in the distal and lateral edges was thicker than that in the proximal edge. At the proximal edge of the orifice, there was only one layer of endothelial cells, and the internal elastic membrane was situated directly beneath it. In the distal edge, besides the endothelial cells, smooth muscle cells (SMCs) were found in the tunica intima that were separated from each other with elastic and collagen fibers. In the tunica intima of the lateral edge, two layers of SMCs were observed, between which were collagen fibers. This distance between the internal elastic membrane and endothelial cells at the distal and lateral edges can be important in diseases such as atherosclerosis. These data indicate that there are differences between the proximal edge and the distal and lateral edges of the orifice, and the involvement of special structures in atherosclerosis is suggested.