Estrogen- and temperature-induced medullary cord regression during gonadal differentiation in a turtle

Gonadal differentiation associated with estrogen-induced female sex determination was examined in a turtle with temperature-dependent sex determination, and was compared to ovarian differentiation at a female-producing temperature. Freshly ladi eggs of the red-eared slider, Trachemys scripta, were incubated at a male-producing temperature (26 degrees C) and were experimentally manipulated at one of three embryonic stages: stage 15, 17, or 20 (i.e. early, midway, or late in the temperature-sensitive and estrogen-sensitive periods). At those developmental stages, groups of eggs were either: (1) treated with a control solution (95% ethanol) and placed back at the male-producing temperature, (2) treated with 10 micrograms of estradiol-17 beta and placed back at the male-producing temperature, or (3) shifted to a female-producing temperature (31 degrees C). Additionally, a control group of freshly laid eggs was continually incubated at 31 degrees C throughout embryonic development. To examine morphological events occurring after the treatments, a subset of embryos from each group was examined at the time of the treatment and at 1-2 stage intervals following the treatments. The results indicate that estradiol-17 beta as well as female-producing temperature may ensure female sex determination by facilitating medullary cord regression. Further, the results reveal a chronology of differentiation in which medullary cord regression temporally precedes cortical proliferation.