Mullerian duct regression and its hormonal control in the American alligator, Alligator mississippiensi

The purpose of the present research was three-fold: (1) to document the timing and morphological pattern of mullerian-duct regression in the American alligator, (2) to determine the role of the testis in reptilian regression and (3) to learn how testosterone and estradiol affect regression in a reptile as compared with their effects on regression in mammals and birds.;In the first experiment, male alligator embryos and hatchlings were sampled over a three-week period. The mullerian ducts were examined histologically and the following ductal parameters were quantified: luminal area, epithelial area, stromal area, length of the mesosalpinx and length of the ridge. Histologically, regression in the alligator follows the same pattern seen in mammals and birds. Regression begins approximately two weeks before hatching and occurs in a caudocranial direction. In the second experiment, female hatchlings were castrated and implanted with either a testis or muscle graft, and the ducts were examined histologically and quantified. The testis grafts caused a significant reduction in all the ductal parameters, and only testis-grafted females exhibited regression which was morphologically identical to that seen in males. Therefore, a testicular substance is responsible for mullerian-duct regression.;Finally, in the third experiment castrated female hatchlings were either implanted with a testis graft or were sham-operated. They then received one of three treatments via subcutaneous, sustained-release pellets: testosterone, estradiol, or a placebo. Eight weeks after surgery, the females were sacrificed and the urogenital system removed for histology. The testis-grafted females demonstrated a significant reduction in all parameters of the mullerian ducts indicative of regression. Estradiol caused hypertrophy of the mullerian ducts in sham-operated females and prevented regression in testis-grafted females. Testosterone had no effect on the mullerian ducts; it neither enhanced graft-induced regression nor caused regression itself. In conclusion, the hormonal control of mullerian-duct regression in the alligator is similar to that in mammals and birds, except that testosterone did not enhance graft-induced regression at the dosage used.