Three-dimensional distribution of substance P-like immunoreactivity in the urinary bladder of rat

The overall distribution of substance P-like immunoreactive (SPI) fibers in the rat urinary bladder was examined by means of frozen sections and whole mount preparations. Two types of SPI fibers entered the urinary bladder from the neck; one forming thick fiber bundles and the other around the blood vessels. These SPI fiber bundles branched into several thinner segments which often project many collaterals to the smooth muscle and submucosal layers. In the smooth muscle layer, single SPI fibers were seen running parallel to both longitudinal and circular muscle bundles. They appeared to be evenly distributed in the smooth muscle layer. SPI fiber bundles located in the circular muscle layer dissociated SPI fibers which entered the submucosal layer, where they directed to the epithelium and formed a meshwork just beneath it. SPI fibers often left the meshwork to enter the epithelium where abundant arborization of fine SPI fibers was seen. The density of SPI fibers in the submucosal layer and epithelium was not even. The highest density was identified in the neck and the trigonum area.