Until recently I didn’t know much about stem cells except that they produce other kinds of cells and that the medical research on them was controversial. But in the context of the history of life, it turns out, their importance is as fundamental as you can get.
It took more than a billion years for the first cell with a nucleus to come together. Since then, the only reliable source for a new cell has been another cell. Every cell is an offspring—true for plants as well as animals.
But while cells are specialized for one task or another, they are not always very good at dividing and reproducing. Muscle cells, blood cells, and nerve cells don’t reproduce at all. Other cells in the body divide only under some circumstances or only a limited number of times.
But reproduction is the stem cell’s specialty. When it divides, it produces another stem cell, ready for the next round, along with a muscle cell or blood cell or nerve cell or a cell of another organ. It looks the part for such flexibility—blob-like, unstructured, not committed until needed.
Stem cells are stationed throughout the body, small groups of them in each organ, like local hospitals on call to repair the sick and damaged. They are a profound piece of bodily engineering, a design for the long-term, like a futuristic dream-car that carries little 3-D printers throughout the engine and chassis to create new parts and replace the old parts automatically and on-board.
In human embryos, in contrast to adults, stem cells literally build the body. When an embryo is only a few days old, its stem cells begin to form all—all—of the specialized cells needed in a body, some two hundred of them.
Plants have stem cells too. Located near the tips of the roots and stems in a layer called the meristem, plant stem cells divide into both specialized cells for the plant and additional stem cells. In short, stem cells are the place where a plant grows.
One of the wonders of any living thing is the sheer variety of its parts, the inventory of its tubes, organs, fluids, surfaces, protrusions, electric circuits and rigid pieces. As we pause to appreciate this profusion, sing the praises of the smudgy cell that creates and repairs them all.