embryonic stem cell

Embryonic stem cells are pluripotent cells taken from a very early embryo, before it has begun forming any particular tissue. At around five days old, an embryo is a hollow ball of cells called a blastocyst, and tucked inside it is a small cluster — the inner cell mass — that would normally go on to build the entire body. Embryonic stem cells are these inner-cell-mass cells, captured and grown in the lab while still blank and undecided.

What makes them scientifically prized is that they are pluripotent by nature and can be kept dividing almost indefinitely in culture without losing that flexibility. From a single starting batch, researchers can grow vast, renewable supplies of cells that retain the full potential to become any tissue type. For decades they were the gold standard against which all other pluripotent cells were measured, and the workhorse for understanding how a blank cell chooses its fate.

Their power, however, comes with serious controversy, because obtaining the inner cell mass from a blastocyst destroys that early embryo. People hold deeply different views about the moral status of an embryo, and that disagreement makes this research ethically contested and tightly regulated in many places. Partly to sidestep this dilemma, scientists later developed ways to make embryo-free pluripotent cells — but embryonic stem cells remain a key scientific reference point.