Forever Young - human embryonic stem cells

LAST YEAR it was cloning. This year's blockbuster in molecular sleight of hand was announced in November, and it opens the door to some of the most far-out scenarios of biology. Two labs--working on privately funded research--reported that they had isolated human embryonic stem cells, the primordial cells that give rise to the many different tissues in our bodies. The most far-reaching implication of the breakthrough is the possibility of growing customized tissue to replace cells, and eventually organs, lost to disease.

In the early stages of embryonic development, cells are undifferentiated, and over the course of development they are irreversibly cast into the roles of cells with specialized functions. What the two groups succeeded in doing was capturing undifferentiated human embryonic cells (such stem cells have already been isolated from several other animal species) and cultivating them in a state of perpetual infancy. The researchers also found that under various conditions the cells would grow into more mature tissues resembling gut, neuron, and cartilage.

One group, led by James Thomson of the University of Wisconsin, worked with embryos donated for research by couples undergoing in vitro fertilization. After fertilization, the egg cell divides, and over the course of several divisions, it forms a blastocyst, a hollow sphere of cells with a few cells clustered inside. The Wisconsin team removed cells from within the blastocyst and coaxed them to grow by using a technique they had developed while working on monkey blastocysts. The other group--led by John Gearhart at Johns Hopkins--isolated cells from fetuses farther down the developmental pathway. From five-to nine-week-old fetuses obtained from therapeutic abortions, they isolated cells resembling germ cells, which ultimately develop into eggs or sperm. The cells recovered by both groups met the criteria for embryonic stem cells--they could divide endlessly in an undifferentiated state, and they could be prompted to mature into many different kinds of cells.

But capturing and cultivating human embryonic stem cells is just the first step. The real medical benefits will have to wait until researchers can understand and accurately manipulate the signals that produce specialized cells that could replace diseased ones--say, neurons lost to Parkinson's disease or defective islet cells in diabetics--and then engineer versions that could be accepted by anyone.

By providing an endless supply of primitive cells, embryonic stem cell technology could eliminate the need for fetal tissue in experimental transplants. On the other hand, it raises questions about the status of embryonic tissue and whether it is right to appropriate such cells for medical experimentation or treatment. Currently, the government has banned federal funding for research on human embryos or fetal tissue. The success of these new techniques--and the medical benefits they might ultimately yield--has reignited that debate.

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