Organization and relative content of smooth muscle cells, collagen and elastic fibers in the corpus cavernosum of rat penis

Purpose: The corpus cavernosum smooth muscle and extracellular matrix are essential for normal penile erection and are implicated in erectile dysfunction. Although investigations of these issues have used the rat corpus cavernosum, organization of its components is to date not well known. We characterized and quantified the smooth muscle cells and the main extracellular matrix components of the rat corpus cavernosum. Materials and Methods: Collagen, elastic fibers and smooth muscle cells were stained on paraffin sections of rat penises using sirius red and Gomori's reticulin, Weigert's resorcin-fuchsin and an anti-smooth muscle cells alpha-actin antibody, respectively. Stained components were then quantified by computer aided morphometry. Results: Smooth muscle cells were restricted to the subendothelial space of corpus cavernosum and had a volumetric density of 9.1%. Collagen was thick, usually in transversely oriented bundles and was the most abundant component of the trabeculae with a volumetric density of 62.7%. Gomori's reticulin disclosed a meshwork of fibrils also in the subendothelial space but did not stain the thicker bundles. Volumetric density of elastic fibers was 4.9%, and at the periphery of the corpus cavernosum the fibers were parallel to the long axis of the penis, while in deeper regions most of them were transversely oriented and at different directions from those of collagen. Conclusions: Rat corpus cavernosum differs from that of humans by lesser amounts of smooth muscle cells, greater amounts of collagen and the presence of fibrillar collagen and smooth muscle cell subendothelial layers. Therefore, these differences should be considered when using the rat penis for studies on erection.