Microbiologist Karim Nayernia of the North East England Stem Cell Institute just took a step toward rendering men obsolete. By immersing stem cells harvested from men’s bone marrow in a cocktail of chemicals that mimic the environment of the testes, Nayernia and his team turned the stem cells into immature sperm. The experiment marks the first time that any nonreproductive human tissue has been transformed into gametes. If the cells can be grown into mature sperm, the technique would allow men without functioning sperm, or even testes, to father children.

Editor's Note: This 2007 study was retracted in 2009. We followed up on the topic in 2020 to see where the research had gone. Read the new story: How Close Are We To Making Babies from Bone Marrow?

Nayernia says the technique could enable women to have a biological child with two mothers and no father. Their offspring would always be daughters, though, because sperm made from a female cell would always carry an X instead of a Y chromosome. Weirder still, a woman could conceivably use sperm made from her bone marrow to inseminate her own eggs.

Nayernia’s work has already raised a few ethicists’ eyebrows. And some scientists doubt that Nayernia’s engineered sperm could ever be functional enough to inseminate an egg successfully. Renee Reijo Pera, a biologist at Stanford’s Institute for Stem Cell Biology and Regenerative Medicine, points out that earlier attempts to create offspring with sperm from embryonic stem cells resulted in short-lived mouse pups that were either giants or midgets. Nayernia says it’s possible that transplanting his immature sperm cells into human testes could make them functional — but he’s awaiting permission for that experiment from his institute’s ethics board.