Alejandro Aguilera Castrejón

When Alejandro Aguilera Castrejón, 32, was growing up in the Ecatepec, a low-income suburb of Mexico City, he dreamed of a better place. “I didn’t even know you could be a scientist,” says Castrejón. “But I liked animals, and in university I got interested in molecular biology, and then stem cells.” 

He landed at a cutting-edge lab in Israel studying embryology, or how animals develop. In 2021, Castrejón proved he could grow mouse embryos ex utero, that is, outside the uterus, inside a rotating bottle under just the right gas pressure and bathed in human blood. The mouse embryos survived about a third of their 19-day gestation period, long enough to develop brains and fast-beating hearts. 

No mammal had ever developed so extensively outside of its mother, excluding oddballs like the platypus, which lays eggs. 

The New York Times hailed the team’s feat as a “mechanical womb.” And, this year, Castrejón opened his own lab at the top-flight Janelia Research Campus in Loudoun County, Virginia, to take the system even further. “My dream, or aim, is to see if a mouse can be born this way,” he says. Collaborators, meanwhile, are clamoring for his help to nurture and grow human tissues as well, such as brain organoids, or ovaries that could be used to treat infertility. 

Long-term, there is the unnerving prospect of growing a human baby outside the body. But that’s “like 100 years away,” says Castrejón. For now, he says, the mechanical womb offers a new way to observe fetal animals as they grow and change. “I think we are creating a general system that could work for many species,” he says. “For scientists, it’s a window to perform experiments in a really easy way.”