INVESTIGATIVE UROLOGY

Concentration of Elastic System Fibers in the Corpus Cavernosum, Corpus Spongiosum, and Tunica Albuginea in the Rabbit Penis
Maia RS, Babinski MA, Figueiredo MA, Chagas MA, Costa WS, Sampaio FJ
Urogenital Research Unit, State University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil
Int J Impot Res. 2006; 18: 121-5

• The corpus cavernosum (CC) extracellular matrix is essential for normal penile erection and is implicated in erectile dysfunction. Although investigations of these issues have used the rabbit CC, organization of its components is not well known to date. We characterized and quantified the volumetric density (Vv) of the elastic system fibers in the corpus spongiosum (CS), CC and tunica albuginea (TA) of the rabbit penis. Adult New Zealand rabbits (n = 10) were used. The penile mid-shaft fragments were fixed with 4% phosphate-buffered formalin solution and/or Bouin’s liquid for 24-48 h, and processed using standard histological techniques. The sections were stained with Weigert’s Fucsin-Resorcin with previous oxidation. The elastic system fibers Vv (%) was determined in 25 random fields of each fragment, using the M-42 test grid. The histochemical methods detected elastic system fibers in CS, CC and TA of all animals. The Vv of elastic fibers average was 25.03+/-2.0% for CC, 32.23+/-1.41% for CS and 22.38+/-3.61% for TA. Results for CC and CS were not significantly different. The great amount of elastic fibers distribution beneath the endothelium suggests that these fibers may have an important role in the erection process in rabbits. The present data should therefore provide important information for devising experiments and interpreting results when using the rabbit penis as a model for penile dysfunctions, especially when making comparisons with humans.
  
  
• Editorial Comment
  The general understanding of the morphological changes and physiology of penile erection has been obtained considering different animal models such as rats, domestic animals, primates and rabbits. Therefore, normative data on the erectile tissue of these animals are important when studying diverse physiological situations and experimental pathological conditions, and comparing the findings obtained with findings in humans.
  The purpose of this study was to better understanding the rabbit penis using morphometrical analysis of the elastic fibers in the corpus spongiosum (CS), corpus cavernosum (CC) and tunica albuginea (TA).
  A previous study demonstrated that the volumetric density (Vv) of elastic system fibers in the rat CC was 9%, and therefore, it was concluded that the cellular and matricial components of the rat CC differ markedly from those of humans in content and organization (1). Consequently, inferences and correlations based on physiological and pathological findings derived from experiments that use the rat as an erection model may be misleading if these differences are not considered.
  In mammals, the classification of different penis types is based on erectile or connective tissue. In animals with vascular penis (rabbit or man), erection is a consequence of increase in size and hardening of the organ. In animals with a fibroelastic penis, the erection is essentially a result of length increasing, with the penis emerging from the prepuce due to sigmoid flexure straightening (2).
  Interesting, the present study showed that the elastic system fibers were abundant in the CS of the rabbit, demonstrating a greater Vv in contrast to the CC and TA. It was demonstrated that the New Zealand rabbit penis is a vascular organ with prominent elastic fibers in the CS (Vv = 32.3%) and CC (Vv = 25.1%), as well as in the TA (Vv = 22.4%). It was observed a larger amount of elastic fibers in the rabbit penis than in human penis components. As the rabbit has been used as the better animal model for studying erectile function, this information is of utmost importance and should be taken into account when comparing the experimental findings with those of humans.

References
1. Pinheiro AC, Costa WS, Cardoso LE, Sampaio FJ: Organization and relative content of smooth muscle cells, collagen and elastic fibers in the corpus cavernosum of rat penis. J Urol. 2000; 164: 1802-6
2. Babinski MA, deBrito-Gitirana L, Chagas MA, Abidu-Figueiredo M, Costa WS, Sampaio FJ: Immunohistochemical analysis of smooth muscle cells and volumetric density of the elastic system fibers of wild boar (Sus scrofa) penis. Anim Reprod Sci. 2005; 86: 317-28.

Dr. Francisco Sampaio
Full-Professor and Chair, Urogenital Research Unit
State University of Rio de Janeiro
Rio de Janeiro, Brazil