Normal Development of the Penis and Urethra
The embryology of the penis and urogenital region has been controversial and confusing. This is related to the fact that in literature, observations have been extrapolated to the human from many different species including mice, rats, red squirrels, pigs and dogs. Classic theories suggest that the penile urethra (columnar epithelium) forms as a result of fusion of the medial edges of the endodermal urethral folds like closing a zip in the midline. The ectodermal edges of the urethral groove fuse to form the median raphe. The glanular urethra (stratified squamous epithelium) is supposed to develop due to an ingrowth of ectoderm from the tip of the glans or due to endodermal cellular differentiation. Recent publications suggest that the human penile urethra develops due to distal migration of the ventral mesenchyme, including the urethral orifice (The Migration Hyposthesis). Hypospadias is probably due to imperfect deficient distal migration (The Disorganisation Hyposthesis).