The development of the male genitourinary system: III. The formation of the spongiose and glandar urethra.

It is generally agreed that the urethral plate disintegrates, resulting in the urethral groove. This is subsequently transformed into the urethra by fusion of the urethral folds, which flank its sides. Recently, the existence of such a groove and folds has been denied and this challenge to the long accepted existence of such folds is significant since hypospadias is considered to result from failure of their fusion. The present studies indicate that mesodermal fold formation and its subsequent subepithelial fusion across the midline plays an essential role in urethral tube formation. Disruption of this process readily explains common congenital abnormalities of the urethra.