Penile bulb and its relationship with the pelvic urethra and the penile urethra in the rat: Light and scanning electron microscopical observations

Abstract

In male rats the urethral tract consists of the following parts: the pelvic urethra, the transitional urethra, the urethral diverticle, and the penile urethra. Perusal of the literature results in only some more general descriptions and a very few which go more into detail. None of the latter deals with all the compartments nor with the relationships between each other. Aim of this study is to give a detailed morphological study of all the urethral compartments and their relationships to provide a platform for further experimental investigations. From 40 adult male Wistar rats the urethral tract has been investigated by histology and scanning electron microscopy using intact tissue and casting techniques. Most striking is the existence of the sinusoids in the spongious tissues in the pelvic urethra into a system of two perpendicular layers of parallel sinusoids. In the spongious tissues of the penile corpus spongiosum the same organization can be found although less prominent. Smooth muscle cells are very unequally present along the urethral tract. In its pelvic part they do not form a continuous layer around the spongious tissue and compared to the distal part they appear proximally more abundant. In the bulbal part of the corpus spongiosum the smooth muscles form a massive sac around the spongious tissue. In the corpus spongiosum in the penile shaft they are absent. The smooth muscle cells are separated from the striated urethral muscle by a very thin layer of connective tissue, in the penile bulb a prominent tunica albuginea separates them from the striated penile muscles. In the spongious tissues of the pelvic urethra and the penile bulb large amounts of compound urethral glands are present. Their ducts open into the respective lumen. Helicine arteries are common in the pelvic urethra, especially in the glandular tissue. They are also normally present in the wing tips of the penile bulb. The transitional urethra, the urethral diverticle and surrounding spongious tissue are structurally closely related to the other parts, however their microscopical organization is intermediate between the pelvic part of the urethra and the corpus spongiosum. The highly ordered sinusoids in the spongious layers especially in the pelvic part together with a similar organization of the urethral muscle provide a possible basis for a peristaltic driving mechanism, suggesting these structures play a role in the high speed transport of sperms during ejaculation.