Researchers in the U.S. created monkey embryos asexually, without the use of sperm, in order to harvest stem cells from them.

The company behind the research, Advanced Cell Technology, says the technique could provide a new source of human stem cells in the future.

They say it could provide an alternative to harvesting stem cells from cloned human embryos.

The researchers used chemicals to trick monkey egg cells into developing into embryos without being fertilized by sperm cells.

"The chemicals cause the egg to believe it has contacted a sperm," said Michael West of Advanced Cell Technology, who led the research team.

The scientists then extracted stem cells from the embryos at an early stage of development, called the blastocyst, and created specialized cells from them.

"These were fully developed cells that could have been used medically," said West.

West said the new study used only monkey eggs, but it shows that human stem cells could be created in the same way, and without the ethical objections raised by therapeutic cloning.

The process used to create the embryos is called parthenogenesis, Greek for "virgin birth," and occurs naturally in some animals, such as insects, to produce offspring without fertilizing eggs. Female worker bees, for example, are created by parthenogenesis.

It also occurs in humans, although the result is a benign growth in the ovary called a teratoma, made up of several different kinds of cells, which usually have to be removed surgically.

Parthenogenic embryos have been created artificially in many different kinds of animals, but they usually develop abnormally and incompletely.

Of the 77 monkey eggs exposed to the chemicals in this study, only four reached the blastocyst stage, a ball of 50 to 200 cells with an inner cell mass. Of these four embryos, only one produced stem cells.

The researchers then extracted stem cells from the blastocyst and treated them with other chemicals to cause them to develop into specialized cells.

The stem cells were then injected into mice, where they developed into teratomas containing several different types of cells, including, West said, functioning nerve cells that make the neurotransmitter dopamine.

Advanced Cell Technology cause a stir in November when it announced that it had clone a human embryo and allowed it to grow to the six-cell stage.

There were also unconfirmed reports this week that the company has created partially functional mini-kidneys from cow stem cells.

The monkey stem cell study appears in Friday's edition of the journal Science.