In 2006 Professor Karim Nayernia and his group persuaded mouse embryonic stem cells to become sperm producing cells and then used the sperm to create healthy baby mice. They have recently managed a similar feat of biological engineering, using stem cells from human adult bone marrow. These are not normal bone marrow stem cells but what are known as mesenchymal stem cells, which come from a slightly different population.
These mesenchymal stem cells are the most malleable type of stem cells that we know of in adults; they can go on to form many other body tissues including muscles. The group, based at Newcastle University, grew these cells in a laboratory and treated them so they developed into male reproductive cells called germ cells. They found some partly developed sperm cells known as spermatogonial stem cells, one of the early steps to sperm production. The next step for them is to see if they can grow mature sperm from these stem cells.
These experiments are really exciting because they use adult stem cells. There has been a lot in the news on embryonic stem cells in recent years as they can turn into many different tissues, but there are ethical questions on using them, especially those from human embryos. So finding really malleable cells from adult stem cell populations could be really exciting for the future; it could eventually be a route to infertile men being able to father their own children.