OBJECTIVES: Minor variants of a spectrum of anomalies are usually more common than the major variants. Penile epispadias is rare compared with the relatively more common exstrophy-epispadias complex, but it has been assumed that the former is simply a lesser form of the latter. This assumption is challenged and an alternative explanation is proposed for the embryogenesis of isolated epispadias. MATERIALS AND METHODS: Descriptions of the normal and abnormal development of the cloaca, the cloacal membrane, the external genitalia, the pubis, and in particular the tail were studied. RESULTS: The tail was found to begin its elongation at about 21 days of development, when it overlies the cloacal membrane; it then impacts upon the body stalk and turns aside to lie upon the ventral abdominal wall. By 8 weeks all 10 tail segments have undergone apoptosis. CONCLUSION: The tail may briefly compress and separate the merging second sacral pair of mesodermal streams that sever the cloacal-allantoic connection and form part of the phallus. These streams then come together to form the phallus behind the allantoic connection which then lies on the dorsal slope of the epispadiac penis. If the compression by the tail occurs after the first paired streams have met and formed the penis, the next pair that form the inner genital folds and penile urethra may be held apart by compression of the tail creating extreme forms of hypospadias on the ventral slope of the penis. This tail compression theory may explain the rarity of epispadias compared to the relatively more common exstrophy-epispadias complex.
OBJECTIVES: Minor variants of a spectrum of anomalies are usually more common than the major variants. Penile epispadias is rare compared with the relatively more common exstrophy-epispadias complex, but it has been assumed that the former is simply a lesser form of the latter. This assumption is challenged and an alternative explanation is proposed for the embryogenesis of isolated epispadias. MATERIALS AND METHODS: Descriptions of the normal and abnormal development of the cloaca, the cloacal membrane, the external genitalia, the pubis, and in particular the tail were studied. RESULTS: The tail was found to begin its elongation at about 21 days of development, when it overlies the cloacal membrane; it then impacts upon the body stalk and turns aside to lie upon the ventral abdominal wall. By 8 weeks all 10 tail segments have undergone apoptosis. CONCLUSION: The tail may briefly compress and separate the merging second sacral pair of mesodermal streams that sever the cloacal-allantoic connection and form part of the phallus. These streams then come together to form the phallus behind the allantoic connection which then lies on the dorsal slope of the epispadiac penis. If the compression by the tail occurs after the first paired streams have met and formed the penis, the next pair that form the inner genital folds and penile urethra may be held apart by compression of the tail creating extreme forms of hypospadias on the ventral slope of the penis. This tail compression theory may explain the rarity of epispadias compared to the relatively more common exstrophy-epispadias complex.