Fetal development of the urethral sphincter.

A morphological study was carried out on 25 human embryos (3-60 mm CR) and 20 human fetuses (15-40 weeks). The anatomical analysis was completed by immunocytochemical studies of different markers of muscle differentiation (vimentin, desmin, titin and isoforms of the myosin heavy chains). A mesenchymal condensation forms around the urethra after the division of the cloaca (E.H. 12-15 mm CR). The m. pubo-rectalis appears in 20-30 mm CR embryos, following the opening of the anal membrane. Striated muscle fibers can be clearly differentiated at 15 weeks. At this time, the smooth muscle layer also becomes thicker at the level of the bladder neck and forms the inner part of the urethral musculature. The urethral sphincter is a functional unit composed of central smooth muscle fibers and peripheral striated muscle fibers. In the human fetus, this musculature mainly develops in the anterior wall of the urethra. We analysed the expression of smooth and skeletal markers as an original approach to the muscular development in this region.