• song;
  • sexual differentiation;
  • in situ hybridization;
  • steroid


Male zebra finches sing and females normally do not. This sexually dimorphic behavior is mediated by a sexually dimorphic series of interconnected nuclei that are larger and more developed in males. Estradiol administered to females as early as the day of hatching (P1) causes profound masculinization of this song system. The exact timing of estrogen action is unknown, and there is little information concerning the times and sites of expression of estrogen receptors and aromatase before P5. We measured the expression of mRNAs encoding these proteins in brain during late embryogenesis and on P1 to determine if estrogen synthesis or receptor-mediated actions on the song system, as part of the program of sexual differentiation, might be possible during this period. Using highly sensitive and specific in situ hybridization procedures for mRNAs encoding ERα, ERβ, and aromatase, we detected mRNA for ERs in archistriatal regions as early as embryonic stage 34, and in diencephalic regions as early as embryonic stage 30. ERα mRNA was also detected in the dorsal mesencephalon at P1. Aromatase mRNA expression was present as early as embryonic stage 30 in diencephalic and mesencephalic regions. No obvious sex differences in the spatio-temporal pattern of mRNA expression were detected. Our results suggest that estrogen can influence cell growth and differentiation in zebra finch brain well before hatching and into posthatching life. The results fail to provide support for the hypothesis that sexual differentiation of the song system is mediated by sex differences in the expression of these mRNAs at these ages. © 2003 Wiley Periodicals, Inc. J Neurobiol 55: 204–219, 2003