Pars compacta of the sexually dimorphic area of the gerbil hypothalamus: postnatal ages at which development responds to testosterone.

The sexually dimorphic area (SDA) of the gerbil hypothalamus contains a dense cell group in males, the SDA pars compacta (SDApc), that is absent in females and that develops under the influence of testosterone. To determine how long SDApc development remains sensitive to testosterone and if it can be fully masculinized, female gerbils were injected with testosterone propionate on various days after birth. Control females and males received the vehicle. SDApc development was assessed when subjects were 2 weeks old. Exposing females to testosterone before they were 3 days old increased both the incidence and the size of the SDApc. Delaying exposure until 3 days after birth prevented these effects. In females that received testosterone daily for 5 days, starting on the day they were born, the incidence and size of the SDApc were the same as in males.