Biology has long maintained that when the cells of a developing embryo assume their intended specialization -- as a skin cell, for example, or a blood cell -- they lose their ability to change.

Scientists at the Stanford University Medical Center have discovered that it's not necessarily so. Through laboratory manipulations, they have turned fully differentiated liver cells into muscle cells.

Biologists have long known that all cells of an organism contain the same set of genes with which the parents endowed their offspring at conception. As a fertilized egg develops into a complex organism, different genes are switched on and off in individual cells in sequences appropriate to the ultimate fate of the cell and its descendants after repeated divisions.

Until now, biologists believed that once the last stages of development were completed, each cell was left with all its genes switched off except those needed to carry out the cell's specialized function.

Helen Blau, who led the Stanford research team, reported in the journal Science that the further transformation was achieved by fusing a human liver cell with a mouse muscle cell. This allowed substances in the mouse cell to mingle with those of the human cell. These chemicals are thought to have entered the liver cell's nucleus and somehow triggered the dormant genes normally active only in muscle cells. The liver cell then began manufacturing various muscle proteins.

The method is expected to enable scientists to study how genes are regulated and how embryonic development is controlled. Aberrations in these processes are the cause of a wide variety of diseases, especially cancer.