The urethral plate--does it grow into the genital tubercle or within it?

OBJECTIVE: To determine whether the urethral plate grows into the genital tubercle or develops within it.
MATERIALS AND METHODS: Sprague-Dawley rat embryos were examined at 11-21 days of gestation and compared with available human embryo sections. Rat embryos were examined by a combination of light microscopy, dissection, serial histological section and immunohistochemistry.
RESULTS: In rat embryos the urethral plate extended to the tip of the genital tubercle from the earliest stages of genital tubercle development. The plate formed from adherence in the sagittal plane of the inner walls of the primitive cloacal cavity. The urethral plate continued to extend to the tip of the genital tubercle throughout penile development in the male and developed a lumen ahead of the advancing perineal mesenchyme. In the female the perineal side of the urethral plate disintegrated by massive apoptosis. This loss of perineal substance allowed the genital tubercle to collapse on itself and left the urogenital opening at the base of the genital tubercle. Human embryos also showed the presence of the urethral plate at the tip of the genital tubercle throughout genital tubercle development.
CONCLUSION: The urethral plate forms ab initio within the genital tubercle and extends to the tip; it does not grow into the genital tubercle as is often described. The contribution of ectodermal ingrowth to urethral development at the tip of the penis is minimal.