MORPHOGENESIS OF THE HUMAN EXTERNAL MALE GENITALIA

The morphogenesis of the external genitalia of human fetuses (16-250 m m crown-rump [CR] length, 6-26 weeks of gestation) obtained after medi cal termination of pregnancy were studied. Differential development (m ale/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 tubercl e extended to the coronary sulcus; the lateral boundaries of this groo ve separated to form the urethral folds. In male fetuses the free edge s of the urethral folds fused, starting from the proximal end, to form a tunnel over the ventral aspect of the phallus. The pelvic urethra o pened into this tunnel, slightly distal to its origin. The mesodermal tissue forming the genital swellings migrated ventrally and then media lly. 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 t o the point of entry of the pelvic urethra) also was compressed and pu shed out as the genital swellings fused in the midline over the root o f the phallus. These changes took place at between 80 and 110 mm CR le ngth (12-13 weeks' gestation); at this stage the phallus appeared shor t 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 res emblance to that in children with hypospadias. We therefore conclude t hat hypospadias can be explained on the basis of an embryological arre st due to the absence of the required stimulus for male phenotypic dev elopment at the appropriate time.