US scientists say they have isolated from human bone marrow the parent cells that give rise to various "mesenchymal" tissues, such as bone, cartilage, fat, and muscle.

It opens the door to new experiments that will improve our understanding of these important stem cells and their potential for the treatment of damaged tissues.

Stem cells have been a hot topic since last year, when scientists managed to cultivate human embryonic stem cells.

Embryonic stem cells are "totipotent;" that is, they can differentiate into all types of tissue.

Mark Pittenger and colleagues from Osiris Therapeutics report in the journal Science that they have isolated stem cells that are multipotent; that is, the cells are capable of differentiating into a more limited number of specialised roles - in this case, mesenchymal tissues.

Researchers had previously managed to isolate from human bone marrow a different type of stem cell, one that renews circulating blood elements such as red and white blood cells.

Now mesenchymal stem cells can be studied with similar ease with the goal of learning how to control and guide their specialisation in the lab.

Important technology

Researchers have high hopes for the emerging cell technologies. A combination of stem cell and cloning techniques may make it possible to grow from scratch human tissues for use in transplants.

At a cloning conference in London this week, a leading UK embryologist predicted the technologies would enable scientists to grow life-saving bone marrow in the lab for children with leukaemia.

Dame Anne McLaren, who advises the UK Government on fertility and cloning issues, said the treatment could be available within three years.

It would involve extracting the nucleus of a cell taken from a child, and transferring it to an unfertilised egg which had had its own nucleus removed.

The egg, programmed by the child's DNA inside the nucleus, would then start to divide and form an embryo. At an early stage, the embryo would be dismantled to obtain stem cells. These would then be treated with the right chemical growth factors to trigger their development into bone marrow cells.

And because the original nuclear material came from the child, the new bone marrow cells would be a perfect match.