Sex differentiation in the brain occurs earlier than previously thought and is not solely determined by sex hormones like testosterone. This has been shown for the first time by researchers at Örebro University.

Every cell in our body has a sex. But most research so far has focused on our gonads – ovaries or testes – and not on the early development of the brain.
Per-Erik Olsson, professor of biology at Örebro University, has studied rat embryonic brains before gonads have formed or hormones have been produced. The results show that the gene Sry on the Y chromosome, known to regulate the development of the testes, is also active in the brain. It functions as a start signal for male brain development already on day 12 in the rat embryo’s development – before the formation of the testes on day 13 and before the start of hormone production on day 14.

“We could see that the signals in the brain appear before we have started producing hormones. That indicates that sex-specific brain development starts earlier than previously show,” says Per-Erik Olsson.

Separate signalling pathways for sex differentiation in the brain

Even though both sex differentiation in the brain and the development of testes start with the same gene, Sry, this research shows that the signalling pathways differ. In the brain, the signalling pathways activated for sex-specific development differ from those that regulate testis development.

“The traditional theory was that first the testes were formed, releasing testosterone, which along with other hormones formed the brain. But our findings show that even if genes regulate the development in both testes and brain, there are no direct parallels between the development of sex organs and sex differentiation in the brain,” says Per-Erik Olsson.

Important clue in developmental biology

According to Per-Erik Olsson, the findings of this study may be of great significance in the future, not only for the understanding of the biological mechanisms behind sex differentiation but also for various diseases and disorders. Many conditions are sex-specific, such as autism, where around 70 per cent of those affected are men.

“Understanding how the brain develops differently in males and females can provide us with important keys as to why certain diseases affect the sexes differently,” he says.