Male embryos grow faster: Breakthrough study reveals genetic reasons why
In a quiet lab at Cornell University, scientists have made a discovery that could reshape how we think about life’s earliest stages. Researchers found that male and female embryos begin developing differently just seven to eight days after fertilization. This difference appears before sex hormones like testosterone or estrogen are even present.
For decades, scientists knew that male embryos in many mammal species—such as cows, mice, and even humans—tend to grow faster than female ones. But no one really knew why. The new study, published in the journal, Cell & Bioscience, dives deep into the genetics of this mystery and uncovers clear molecular reasons.
The team studied cow embryos in petri dishes. By examining each embryo’s RNA—tiny molecules that help turn genes into action—the researchers were able to track how genes were being expressed at this early stage. The results were striking.
Male embryos prioritized genes related to energy metabolism. This allowed them to grow more quickly. Female embryos, however, focused on genes related to sex differentiation, the development of reproductive organs, and inflammation, which plays a key role in immune system development.
This wasn’t just a tiny difference. The two sexes followed very different genetic “blueprints” from the very beginning. According to Jingyue “Ellie” Duan, assistant professor of functional genomics at Cornell’s College of Agriculture and Life Sciences and co-author of the study, the discovery proves that sex matters at the cellular level much earlier than most people realize.
“Sex difference has been a factor ignored in a lot of studies and clinical trials,” said Duan. “Until very recently, most mouse studies have used exclusively male mice. And most drug discovery is done with male mice.”
That oversight could be leading to gaps in understanding how diseases affect men and women differently. Conditions like Alzheimer’s, autoimmune diseases, and even heart disease often show up differently depending on biological sex. These new findings suggest those differences may begin long before birth.
Author: Joshua Shavit
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