The structure and innervation of the male urethra: histological and immunohistochemical studies with three-dimensional reconstruction.

The structure of the striated urethral sphincter, the so-called rhabdosphincter, remains the subject of controversy. There are two main concepts regarding its structure: either it is a part of the urogenital diaphragm, or it extends from the base of the bladder up to the urogenital diaphragm and is an integral part of the urethra. It is also uncertain whether it possesses a somatic innervation or a mixed innervation (i.e. autonomic and somatic). The purpose of this study was to show the precise location of the nerves running to the urethra, and to try to determine their exact nature. Histology and immunohistochemistry were performed in the external urethral sphincter of ten male fetuses (114-342 mm crown-rump length, or between 14 and 40 weeks of gestation). A three-dimensional (3D) reconstruction of the urethral structure and its innervation was made from serial sections. The 3D reconstruction of the same section levels with different strains allowed us to identify the precise structure of the muscle layers (smooth and striated muscle fibres) and the nature of the nerve elements (myelinated and unmyelinated), their distributions and their relationship to the urethral wall, the prostate and the seminal vesicles. Histological and immunohistochemical 3D reconstruction of the anatomical elements of the urethral sphincter helps us to understand the 3D arrangement of the sphincter muscle layers. It also provides a better understanding of the origin and nature of the nerve elements that play a role in urinary continence.