Morphological and quantitative studies were made on serotonin-containing paraneurons throughout the lower urinary tract in male and female dogs. Using an anti-serotonin antiserum, the cells were consistently demonstrated to be dispersed in the epithelium from the vesico-urethral junction to the external urethral ostium. They occurred most frequently in the urethra proximal to the urogenital diaphragm in both sexes. The total number of the serotonin-immunoreactive cells in the urethra was estimated to be 36.2 X 10(4) (SD 9.9 X 10(4] in the male (n = 3) and 15.6 X 10(4) (SD 2.1 X 10(4] in the female (n = 3). Besides the urethra, the prostate and vaginal vestibule contained several serotonin-immunoreactive cells. The urethral serotonin cells were basically bipolar basal-granulated cells that extended the basal cytoplasm to the basement membrane and reached the lumen with an apical process. Modified cell shapes were, however, also frequent, and included bifurcated apical and/or basal processes or a laterally directed basal process. Occasional serotonin cells possessed a threadlike basal process with varicosities and a terminal bouton, reminiscent of a neuronal process. Immunoreactivity for chromogranin A, a carrier protein common to endocrine paraneurons, was demonstrated in all of the urethral serotonin cells. The chromogranin A-immunoreactive granules accumulated more densely in the basal and perinuclear regions of the cell. It is hypothesized that the serotonin-immunopositive paraneurons may receive chemical and/or physical information from urine and, in response to it, secrete serotonin which presumably causes the contraction of the musculature of the urethra.

The vascular construction of the dog penis was investigated mainly by scanning electron microscopic observation of corrosion casts produced by injection of methylmethacrylate into the penile artery and its branches. The corpus cavernosum penis was supplied mainly by the penile deep artery; only its distal end received a few small branches from the dorsal artery. The helicine arteries were provided with polsters which protruded into the lumen so prominently that they might conspicuously reduce the lurninal space during the flaccid state of the penis. The

The present study deals with endocrine-like cells in the urethra of human penis. A large number of basal-granulated cells immunoreactive for serotonin were dispersed in the urethral epithelium. No cellular elements were stained positively with antisera against bioactive peptides. The serotonin-immunoreactive cells consisted of a small oval perikaryon and slender processes, and resembled neurons in shape. An apical process reached the urethral lumen. The basal processes frequently branched out in a dendritic fashion, some running laterally for a considerable distance. The number of cells immunoreactive for serotonin was remarkably reduced in subjects over 60 years of age.

A pressure-flow study of the corpus cavernosum penis in the dog was performed during the flaccid, tumescent, erect and rigid stages. Perfusion was selectively made into the penile deep artery. The erect stage was induced with the local administration of papaverine hydrochloride, and the rigid stage by electrostimulation of the ischiocavernous muscle during the erect stage. Concurrently, vascular casts of the penis at each stage of erection were observed under a scanning electron microscope for the anatomical analysis of the mechanisms controlling cavernous pressure. In the pressure-flow study, it was demonstrated that a combination of decreased arterial resistance and restricted venous outflow was essential to both cause and maintain the erect state. The rigid stage was generated by contraction of the ischiocavernous muscle tightly compressing the crus penis. In the rigid stage, the cavernous pressure was elevated as high as five times the perfusion pressure; nevertheless, no backflow from the cavernous sinus to the deep artery was recognized. By use of the casts, it was confirmed that venous outflow restriction in the erect and rigid stages was due to compression of the postcavernous venules and penetrating veins in the tunica albuginea. The deep artery was occluded in the tunica albuginea in the casts made during the rigid stage, suggesting that the backflow from the corpus cavernosum penis to the deep artery was prevented by this occlusion. Therefore, in the rigid stage, the arterial and venous occlusion isolated the cavernous sinus from the systemic circulation; this mechanism seems to account for the maintenance of the extremely high cavernous pressure.

The present study deals with morphological changes during erection both in the penile deep artery supplying the corpus cavernosum and in veins draining it. These vessels were shown to maintain wide lumina in the flaccid state. In the erect state induced by injection of papaverine, the postcavernous venules were compressed between the cavernous sinuses and the tunica albuginea. Simultaneously, the penile deep artery and the penetrating vein were strangulated by stretched collagen bundles in the inner layer of the tunica albuginea. Quite noteworthy was a wedge-shaped connective tissue mass between the penetrating vein and the postcavernous venules converging to it, and plugged, in the erect state, towards the penetrating vein to effectively stem the venous outflow. When the rigid state was induced by papaverine injection and following intracavernous injection of a fixative by high pressure, the arterial and venous obliteration mentioned above was so intense that the vessels almost lost their lumina, resulting in the cavernous sinuses becoming isolated from the systemic circulation.