potency
/ POH-ten-see /
Picture a fresh university student deciding on a major. A first-year can still choose almost any field; a final-year student locked into one program has far fewer options left; and a graduate already employed in one trade can really only do that one job. Potency is the cell's equivalent of how many career paths are still open to it — how many different kinds of specialized cell it could still become.
Precisely, potency is the range of cell types a cell is capable of giving rise to. Biologists rank it on a scale: totipotent cells can make every cell type including the placenta; pluripotent cells can make every cell type of the body proper but not the placenta; multipotent cells can make a limited family of related types; and unipotent cells can make only a single type, though they still self-renew. As development proceeds, cells generally move down this scale, narrowing their options.
Potency matters because it tells you what a stem cell can and cannot be used for. A common misconception is that a cell with broader potency is always 'better' — but high potency also brings risks, such as forming disorganized growths called teratomas if pluripotent cells are placed in the body untamed. The right potency is the one matched to the task, which is why much research focuses on guiding cells to exactly the type needed and no further.
A fertilized egg is totipotent — it can build the whole embryo and the placenta. By the time blood-forming cells appear in the marrow, those are merely multipotent: powerful within the blood family, but unable to become, say, a neuron or a liver cell.
Potency narrows over development: totipotent egg to multipotent blood stem cell.
Potency describes what a cell could become under the right conditions, not what it will become — a pluripotent cell sitting in a dish does nothing dramatic until the right signals push it down a path.