Keeping the beat. (Stem Cells).

Pacemakers have done wonders for heart patients, but some doctors envision replacing these gadgets with cells that can keep hearts beating. Toward that end, researchers have reported that muscle cells taken from embryonic rats and put into an adult rat's heart can transmit the electric signals that govern the heartbeat.

"The cells have survived in rats for more than a year, and they appear to have made connections with cardiac [heart] cells," says lead researcher Douglas B. Cowan of Children's Hospital in Boston.

Cowan's team used immature skeletal-muscle cells with collagen--a protein in connective tissue--to form a matrix where the cells grew into strips of tissue. Within 10 weeks after being implanted into the rats' hearts, the cells began sending electric signals between the upper and lower chambers of the heart. The scientists also found that the implanted cells made proteins called connexins, which physically and electrically connect heart cells and aren't found in mature skeletal muscle.

Such implants could be especially important for babies born without an electric connection between the heart's upper chambers and lower ones, a defect that occurs once in every 22,000 births, Cowan says. These defective hearts don't beat effectively on their own, but pacemakers are awkward to implant in small children and may need to be replaced as the children grow. Tissue-based pacemakers would be better, he says, while cautioning that they may take more than a decade to reach the clinic.--D.C.