Qualitative and quantitative characterization of the Rhesus monkey (Macaca mulatta) penis.

Background: Knowledge of the anatomy of laboratory animals is important for experimental research. Erectile dysfunction has been studied using the penises of different laboratory animals such as rats, mice, rabbits, dogs, etc. However, these animals have penises with different characteristics to the human penis. If these differences are not taken into account, the conclusions may be questionable. The Rhesus monkey (Macaca mulatta), due to its similarities to humans, could be a good model.

Objective: To characterize and quantify the components of the penis of the Rhesus monkey (Macaca mulatta), qualifying it as a model for experimental studies.

Methods: Ten adult Rhesus monkey penises were fixed in 10% buffered formalin and processed for paraffin embedding. Histological sections 5-μm thick were made and stained using histochemical techniques. We assessed the thickness of the tunica albuginea, and in the erectile tissue, the following parameters were analyzed: in the corpus cavernosum (CC): total area, area densities of collagen fibers, muscle fibers and elastic system fibers; in the corpus spongiosum (CS): area densities of collagen fibers, muscle fibers and elastic system fibers. Histomorphometric analyses were carried out on photomicrographs by using ImageJ software.

Results: The penis of the Rhesus monkey (Macaca mulatta) has a single CC. The tunica albuginea was thicker in the dorsal region (1.11 ± 0.03 mm) than in the ventral region (0.87 ± 0.01 mm). The quantitative analysis of the CC showed the following values: total area (20.33 ± 5.67 mm²), collagen fibers (24.00 ± 4.00%), muscle fibers (31.52 ± 9.93%) and elastic system fibers (8.46 ± 3.20%). The quantitative analysis of the CS showed the following values: collagen fibers (52.50 ± 11.76%), muscle fibers (10.50 ± 6.36%) and elastic system fibers (15.07 ± 4.78%).

Conclusion: The predominance of muscle tissue over connective tissue in the corpus cavernosum, similar to what is observed in humans, qualifies the Rhesus monkey penis as a good experimental model for erectile dysfunction.