Stem cells and differentiation
Biology · Key Stage 3 · The School
What makes a cell a stem cell
A STEM CELL is not yet specialised. It can divide to make more cells. It can also DIFFERENTIATE, meaning it can turn into one or more specialised cell types. What defines a stem cell is what it CAN become, not any special structure of its own. Under a microscope, a stem cell often looks quite plain.
Differentiation is a one-way change
DIFFERENTIATION is how a plain cell becomes a specialised one. It gains the exact structure of, say, a nerve cell, a muscle cell or a red blood cell. In most human cells, this change cannot be undone. Once a cell has differentiated, it stays that type for the rest of its life.
Where stem cells are found
In animals, stem cells are mainly found in two places. Early embryos hold stem cells that can become almost any cell type. Some adult tissues, such as bone marrow, hold stem cells too, but these can only become a narrower set of cell types. In PLANTS, stem cells sit in growing tips called MERISTEMS. These keep the power to become any plant cell type for the whole life of the plant. That is one reason a small cutting from many plants can grow into a whole new plant.
An adult human's bone-marrow stem cells can only become a small set of blood-related cells. They have already lost the power to become, say, a nerve cell. A plant's meristem stem cells keep far more of their original power for the plant's whole life. That is exactly why a cutting placed in water or soil can grow entirely new roots, leaves and shoots. The stem cells at the cut end turn into whatever the plant now needs. This difference explains something real: growing a new plant from a cutting is routine. Growing a whole new organ from an adult human's own cells is still one of the hardest open problems in medicine.