A to F, OPT of 6.5-week penis at indifferent stage of development stained with E-cadherin. G to K, histological sections correspond to horizontal lines in gross OPT specimen. G and H, note extensive proliferation (Ki67) along urethral plate (UP), urethral meatus (UM) and areas of urethral plate canalization (Canal), ie “opening zipper.” I and J, high power images of G and H. K, caspase 3 was not expressed, in contrast to Ki67 in I and J, supporting role of cellular proliferation over apoptosis as means of urethral plate/groove development. EB, erectile body.