Development of the human anterior urethra.

To further our understanding of the development of the anterior urethra a series of 38 normal human fetuses ranging from the end of the embryonic period proper to the third trimester of gestation were studied. Tissues prepared as serial histological sections were examined and appropriate specimens were reconstructed. The formation of the bulbar and spongy urethra occurred by proliferation of mesenchyme underlying the epithelium of the urethral folds, causing the creation of a tubular urethra by epithelial fusion in the ventral midline. The tubular urethra becomes invested by mesenchyme, the future corpus spongiosum. This process of fusion of the urethral folds extends to the glans penis. At this period of development the glans contains the urethral plate, a lamina of epithelium lacking a lumen. The mechanism of the subsequent connection of the spongy urethra to the canalized urethral plate has been the point of controversy. Our observations support the idea that 3 processes must act in synchrony to produce successful completion of the anterior urethra: 1) the prepuce forms by overgrowing the glans due to proliferation of the penile skin and subcutaneous tissues, and in so doing it continues the progression of closure of the urethral folds into the glans and forms the frenulum by its fusion on the ventral aspect, 2) the mesenchyme surrounding the prolongation of the urethra fuses with the mesenchyme of the glans, which before this time had existed as a mesenchymal structure distinct from the corpus spongiosum or corpus cavernosum and 3) the epithelium of the urethral plate within the glans, which underlies the epithelial tag, becomes canalized and develops continuity with the lumen of the spongy urethra.