Urethral development in the fetal rabbit and induction of hypospadias: A model for human development

Purpose: To determine whether the development of the rabbit phallus would b e an appropriate model of human phallic development, we evaluated the forma tion of the fetal rabbit phallus and attempted to induce hypospadias pharma cologically. Materials and Methods: New Zealand rabbit fetuses were obtained on gestatio nal days 20 to 24, 26, 28 and 31. Sex was determined by gonadal morphology, and 6 fetuses were obtained at each age. The perineum was dissected, fixed , sectioned and stained with hematoxylin and eosin, and monoclonal antibodi es against neuronal specific enolase. Two pregnant rabbits were treated wit h 10 mg./kg. finasteride orally daily between gestational days 19 and 28. T he development of the external genitalia was compared in treated and untrea ted control rabbits. Results: The rabbit phallus contains 2 corpora cavernosa and dorsolateral n erves similar to the human. In male and female fetuses fusion of the urethr al folds progressed in a proximal to distal sequence forming a seam at the point of ventromedial fusion. In male fetuses urethral fold and ventral pre putial fusion continued more distally toward the glans compared to females. Thus, in mature males the urethral meatus and ventral prepuce extended to the tip of the phallus, whereas in females the urethral meatus opened on th e proximal phallus and the prepuce was deficient ventrally forming a dorsal hood. Male offspring had a significantly larger anogenital distance postna tally than female offspring. In male fetuses exposed to finasteride urethra l fusion did not extend distally and the prepuce was deficient ventrally. A lso, male offspring exposed to finasteride in utero had a significantly sho rter anogenital distance than females and untreated control males at all ag es (p <0.05). Conclusions: Fetal development of the rabbit phallus and urethra is homolog ous to the human. Although the gestational period is significantly shorter, the temporospatial pattern of external genitalia development is analogous in these species. Feminization of the rabbit urethra, hypospadias, can be i nduced by inhibiting 5 alpha-reductase. Use of this animal model will allow further study of molecular mechanisms involved in urethral fusion and the evaluation of the pathophysiological processes of hypospadias.