Feminization of social play behavior depends on microglia.

The cellular mechanisms by which sex differences in the brain arise provide insight into the cellular basis of behavior. Most mechanistic studies have focused on the process whereby regions of the male brain are differentiated from the female in response to elevated gonadal steroid in development due to the tractability of inducing masculinization by blocking steroid action in males or providing exogenous steroids to newborn females. As such, feminization is usually defined as "not masculinized". Here, we demonstrate the active feminization of astrocyte density in a brain region modulating complex social behavior, rough-and-tumble play in juveniles. These findings indicate that lower levels of playfulness in females is an actively regulated process as opposed to simply being a lack of masculinization.