The influence of testosterone on the sex-dependent structural asymmetry of the medial habenular nucleus in the chicken.

The volume of the left and right medial habenular nuclei of male and female chicks was analysed for evidence of structural asymmetry. The influence of exogenous testosterone on habenular asymmetry was also examined. Male and female chickens were injected intramuscularly with either testosterone enanthate or an oil vehicle on day 2 (15 animals per group) and were perfused at days 5, 12, and 19 respectively. Paraffin sections (8 microns) containing the medial habenular nucleus were stained with cresyl violet and both left and right medial habenular nuclei were measured by planimetry. In control chicks, males had structural asymmetry at day 12, whereas females did not show any structural asymmetry. Testosterone did not appear to influence asymmetry in the male chick but induced structural asymmetry at all three ages in the female to favour the right hemisphere. This study demonstrates a role for testosterone in influencing structural asymmetry of a nucleus in the vertebrate brain.