Researchers say they have discovered how to transform human embryonic stem cells into germ cells, the embryonic cells that ultimately give rise to sperm and eggs, an advance that will allow researchers to observe previously inaccessible human germ cells in laboratory dishes.

"This achievement opens a new window into what was only recently a hidden stage of human development," said Susan B. Shurin, M.D., acting director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), the NIH Institute that provided funding for the study. "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."

Studying this process in human beings has been impossible in the past, because it takes place so early in development—before the embryo is two weeks old. 

Dr. Renee A. Reijo Pera of Stanford University explained that the ability to observe germ cells in laboratory cultures opens up several promising new avenues of research. Although infertility is apparent only after sexual maturity, she said, many forms of unexplained infertility are thought to have their origins in errors that occur in the cells of the early embryo. The ability to observe embryonic germ cells as they develop may allow researchers to pinpoint potential genetic changes underlying infertility. According to the Centers for Disease Control and Prevention, infertility is a major health problem that costs the United States more than $5 billion annually.

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 to see if those genes played a role in the development of stem cells into immature germ cells. The three genes they tested are named DAZ, DAZL and BOULE, and all come from the same family of genes. Dr. Reijo Pera discovered the first of them, DAZ (Deleted in AZoospermia), in the mid-1990s, when she showed that infertile men who lack germ cells often lack the gene.

In the new study, she and her colleagues showed that DAZL was necessary to transform embryonic stem cells into germ cells. When DAZL was turned off, just 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. They did not stimulate female germ cells to progress through meiosis because female human germ cells pause partway through meiosis and remain in that state for many years, through sexual maturity.

Reijo Pera next plans to try the same techniques with so-called induced pluripotent stem cells. Induced pluripotent cells are 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.