Morphogenesis of the human external male genitalia.

The morphogenesis of the external genitalia of human fetuses (16-250 mm crown-rump [CR] length, 6-26 weeks of gestation) obtained after medical termination of pregnancy were studied. Differential development (male/female) started after 50 mm CR length (9 weeks). At that time the external genitalia consisted of a cylindrical genital tubercle 2 mm in length with a visible coronary sulcus and glans and genital swellings on either side. A groove on the ventral aspect of the genital tubercle extended to the coronary sulcus; the lateral boundaries of this groove separated to form the urethral folds. In male fetuses the free edges of the urethral folds fused, starting from the proximal end, to form a tunnel over the ventral aspect of the phallus. The pelvic urethra opened into this tunnel, slightly distal to its origin. The mesodermal tissue forming the genital swellings migrated ventrally and then medially. As medial migration started, the skin in the midline between the genital swellings was raised up as a skin fold, which subsequently, as the genital swellings migrated further, became elevated. The proximal part of the tunnel formed by fusion of the urethral folds (proximal to the point of entry of the pelvic urethra) also was compressed and pushed out as the genital swellings fused in the midline over the root of the phallus. These changes took place at between 80 and 110 mm CR length (12-13 weeks' gestation); at this stage the phallus appeared short and was bent ventrally. With further growth and caudal migration of the scrotum, the phallus lost its ventral curvature. The appearance of the external genitalia at different gestational ages bore a close resemblance to that in children with hypospadias. We therefore conclude that hypospadias can be explained on the basis of an embryological arrest due to the absence of the required stimulus for male phenotypic development at the appropriate time.