Development of the human male urethra: a histochemical study on human embryos.

PURPOSE Controversy persists regarding the formation of human penile urethra. The classic fusion theory for the development of the spongy urethra and ectodermal ingrowth or endodermal transformation theories for the development of the glanular urethra do not explain the wide spectrum of anomalies seen in patients with hypospadias. This histological study was made to clarify the mechanism of urethral development. MATERIALS & METHODS 15 human male embryos ranging from 6 to 14 weeks were studied. The phalluses were examined microscopically and photographed. Tissues were prepared as serial histological sections and stained with haematoxylin and eosin and with special immuno-histochemical stains. RESULTS 1) The penile urethra: At 6 weeks of gestation, the urethral plate which is solid distally and partially grooved proximally becomes grooved distally and has fused proximally by 8 weeks. At 14 weeks of gestation; the urethral opening migrates only to the middle of the shaft. 2) The glanular urethra: At the 6th week of gestation, a solid epithelial plate reached the tip of the genital tubercle, and a glans cannot be identified. At the 7th week, a central vacuolation appears and the penile urethral groove does not reach the tip of the phallus. At the 8th week; coronal sulcus starts to appear, and a well defined blind central canal was evident in the 13th week. During the 14th week, the floor of the glanular canal degenerated to form a glanular groove. CONCLUSIONS Our observations suggest that the spongy urethra passes through 3 stages of development: a solid epithelial plate, deep urethral groove, and fused urethra. The glanular urethra passes through 4 developmental stages: a solid epithelial plate, a blind central canal, a deep glanular groove, and the floor from the preputial lamella. There was no evidence of ectodermal ingrowth. These observations raise serious questions to the current theories for human urethra development. Further studies on fresh human embryos are needed.