Scientists have known for a while that male and female brains in many mammals differ in both form and function, but how they become that way is still a mystery. However, a new study published in Nature Neuroscience sheds some light on how gender may be determined in the brain.
Previous studies have shown that the presence or absence of androgen hormones, such as testosterone, during the first 12 weeks of pregnancy determine many of the physical characteristics of gender. Studies that examined human fetuses in the first 26 weeks of pregnancy also found differences in the brain: By using an ultrasound scanner, researchers from Israel discovered that female fetuses generally had thicker corpora callosa when compared to their male counterparts.
These physical features established before birth carry on into adulthood and could account for the differences in male and female behavior. For instance, there are many sex- and gender-related differences in levels of certain proteins found in the brain, some of which are connected to language acquisition and development. Females in some species tend to have higher expression of these proteins, and this difference could be associated with higher levels of communication among females. Researchers believed that these differences were established in a narrow, prenatal developmental time frame; however, a recent study may prove otherwise.
Scientists from the University of Maryland School of Medicine were able to shed some light on the process of prenatal gender differentiation by successfully transforming a female rat’s brain into a male rat’s brain 10 days after birth, long after the time period that scientists originally believed gender differentiation occurred.
The process involved DNA methyltransferases (Dnmts), a group of enzymes that can inhibit the expression of certain genes. The inhibitors mimic effects of a drug commonly used to treat effects of menopause, estradiol. When expressed, this hormone produces masculinizing effects. A significant amount of it is found in the brains of male rats during prenatal development.
When the researchers injected Dnmts into the female rats’ preoptic areas, a specific part of the brain that regulates male sexual behavior, the female rats’ brains developed some of the structural characteristics of male rat brains. While their anatomies did not change, the female rats displayed sexual behavior that was more typical of males. When the animals were recorded, the researchers could not distinguish the gender of the rats by observing their behaviors, showing that gender differentiation is possible after birth.
But what does this mean for humans? By showing that gender determination can be flexible, the researchers suggest that gender differences can be epigenetically controlled. Gender and sexual orientations are not necessarily ingrained in our DNA — they are also developed through the environment in which we grow up.