Cell division in a human embryo as seen through a microscope. The precursor cells need the nutrients and environment to turn into sperm or egg cells. REUTERS

In a first, researchers from the Chinese Academy of Sciences have successfully created healthy and fertile mouse offspring using sperm cells made in the lab. This goes a step ahead form earlier work that produced the precursor germ cells of the mouse sperm.

Expected to help treat male infertility, the technique will have to be assessed for risks before being tried in humans. A third of all infertility cases are believed to be due to male infertility.

To start with, mouse embryonic stem cells (ESCs) were exposed to a chemical cocktail that helped them turn into the primordial germ cells, or precursor sperm cells. The cells were placed in an environment similar to that found in the testes and exposed to testicular cells and relevant hormones, including testosterone. Earlier studies relied on injecting the precursor into the testes of mice.

"We established a robust, stepwise approach that recapitulates the formation of functional sperm-like cells in a dish," co-senior study author Jiahao Sha of Nanjing Medical University said in a statement. "So we think that it holds tremendous promise for treating male infertility."

A major cause of male infertility is the failure of precursor germ cells in the testes to undergo a type of cell division called meiosis to form functional sperm cells.

Experts hope that the technique can be adapted in the future to use induced pluripotent stem cells (iPSCs) which can be derived from many parts of the body, removing the need to use embryonic stem cells given ethical concerns. Research has today progressed into genetic editing of the sperm cells and embryo, raising concerns of designer babies and unintended mutations of the human DNA.

Stem cells are the body's growth and maintenance units that differentiate in the growing embryo into the various cell types that go on to build organs. At later stages, they are used by the body to replace damaged tissues. These stem cells are found in most tissue types as well as in the embryo.