Japanese scientists have grown the cells of frog eyes and ears in a test tube, in the latest step towards grow-your-own organs.

The technique uses special cells taken from frog embryos called stem cells. These have the potential to develop into any specific type of cell in the frog's body - heart, skin, muscle and so on.

The key technique used by the team at Tokyo University is the use of a chemical to direct the development of the stem cells into particular tissue.

This is the process that some medical scientists believe will eventually allow whole human organs to be grown in the laboratory for transplant. Such transplants would be created by cloning from a patient's own cells and would therefore have no problems of rejection.

Donor alternative

Makoto Asashima, who led the team, said the technology could provide an alternative to donor transplants.

His team cultivated thousands of embryo cells in a retinoic acid solution for five days to produce the organs.

Varying the concentration of the retinoic acid led to the stem cells developing into different kinds of cell. Dr Asashima said a lower concentration activates a set of genes producing bundles of cells like eyes, while a higher concentration activates genes producing ears.

Dr Asashima claimed this is the first time the eyes or ears of an animal have been produced in a test tube.

Liver transplants

In a similar but simpler procedure, the researcher said he had previously grown frog kidneys and transplanted them into other frogs.

The recipient animals lived for more than a month but the scientists did not run a control experiment, testing the survival rate of frogs with transplanted natural kidneys.

Neither has the research been published yet in a scientific journal, although Dr Asashima said this would happen soon.

Hideyuki Okano, a professor of neural development at Osaka University's School of Medicine, said Dr Asashima's work was "extremely striking" and could eventually help scientists reduce the reliance on donors for rare organs.

Scientists are already able to grow human skin from patients' own cells, and such transplants are carried out in the United States, Europe and Japan. Stem cell transplants have also been used to reverse partially the paralysis caused by strokes.