The prize in this hunt is an invisibly small translucent dot found on the inside of an early stage of the human embryo, known as the blastocyst. Several days following fertilization, the blastocyst, a hollow ball of about 140 cells, rolls out of the fallopian tube and into the uterus, to be implanted there. Clinging to the inside of this rolling sphere are a group of identical cells-the ES cells-which are the starting point of the fetus. Soon they will divide rapidly and their descendants will take on increasingly specialized roles, emerging as heart, muscle, blood, bone, hair, nerves and all the rest of the human apparatus. For now, though, they are pure potential: holding the capacity to become any part of the body. And therein lies their mystery and their biomedical significance.
Biologists, understandably, are fascinated. But before they can study this primordial cell, they need to capture it-and control its growth-in the laboratory, something that hasn’t proved easy to do. Like physicists studying particles present at the birth of the universe by recreating its initial conditions in high-energy colliders, biologists are attempting to isolate the ES cell with a concoction of powerful biological substances that mimic those present in the first days of life.
The science behind ES cells began in earnest in 1981, when researchers in Great Britain and California independently succeeded in isolating a curious kind of cell from the interior of the mouse blastocyst. These embryonic cells were identical but each had the potential to give rise to an enormous range of different cell types-a defining mark of a stem cell.