Researchers at Stanford University have transformed human embryonic stem cells into germ cells that they believe are so perfect that they could be grown into fully-functioning sperm and eggs.
While it's not the first time that these cells have been produced in the lab, the quality has been so poor that they they were reckoned to be unviable.
"This achievement opens a new window into what was only recently a hidden stage of human development," said Susan B Shurin, acting director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). "Laboratory observation of human germ cells has the potential to yield important clues to the origins of unexplained infertility and to the genesis of many birth defects and chromosomal disorders."
The researchers began with human embryonic stem cells, to which they added a gene that makes a protein which flashes green when a gene found only in germ cells is turned on. After the embryonic stem cells grew and changed for two weeks, the researchers isolated the cells that flashed green.
The researchers next conducted a variety of tests to confirm that the green fluorescing cells behaved like germ cells. Once convinced that their cells were in fact germ cells, the researchers turned on and off several candidate genes - named DAZ, DAZL and BOULE - to see if those genes played a role in the development of stem cells into immature germ cells.
Lead author Dr Reijo Pera and her colleagues showed that DAZL was necessary to transform embryonic stem cells into germ cells. When DAZL was turned off, half as many germ cells formed. DAZ and BOULE, by contrast, acted later in the germ cells' maturation, nudging the cells into a phase called meiosis, during which cells reduce their number of chromosomes by half.
The researchers even observed that some male germ cells went all the way through the process of meiosis, to the point where they had half as much genetic material as they had begun with.
Dr Pera next plans to try the same techniques with so-called induced pluripotent stem cells - adult cells that have been reprogrammed to behave like embryonic cells. If it works, she hopes to take cells from an adult with infertility, transform them into germ cells and study them for clues to the cause of the adult's infertility.
The results are published online in Nature.