Normal development of the male anterior urethra

A histological study was performed on serially sectioned human and mouse em bryos to study the influences of programmed cell death (PCD) during morphog enesis for clarifying the existing controversies on the morphology and basi c processes involved in the embryonic development of the male anterior uret hra. The following new insights into the development of the anterior urethr a could be established. The formation of the urethra starts with the early adhesion of the arms of the genital tubercle. In this way an epithelial pla te is formed, located in the ventral midline, that is in continuity with th e cloacal membrane. Male sex differentiation takes place following rupture of this cloacal membrane through programmed cell death. Fusion of the uroge nital swellings with primary luminization gives rise to the penile urethra, whereas the glandular part of the urethra is formed through secondary lumi nization of the epithelial cord that is formed during fusion of the arms of the genital tubercle, i.e., the glans. In both fusion processes, apoptosis plays a key role. The consequence of fusion of the urogenital swellings is that their mesodermal cores unite on the ventral aspect of the penile uret hra, where they differentiate into the integumental structures. The prepuce starts to develop as a fold of ectoderm with a mesodermal core after compl ete fusion of the entire urethra. Finally, the scrotum was found to develop through merging of the labioscrotal swellings and not by fusion. (C) 2000 Wiley-Liss, Inc.