What is a Stem Cell?

Can anyone give me a really basic, dumbed down (and I’m talking VERY dumbed down) definition of what a human stem cell is and what it actually does? I’ve searched for this but all that turned up were a lot of medical definitions that either (a) went completely over my head or (b) made reference to things in their explanation that I was ignorant of, thus rendering the explanation useless.

All help is greatly appreciated. Thanks


A stem cell has the potential to give rise to all types of offspring and descendant cells. The ultimate stem cell is the fertilized egg. It gives rise to a complete human being.

A good example of a stem cells is found in the bone marrow where there are cells which give rise to the red cells, the white cells, and the platelets. In other words, one type of marrow stem cell gives rise to very different types of descendants.

Typically, stem cells have potential. They give rise to specialized cells which fulfil that potential.

Is this a start?

Thanks Karl. That’s very helpful.

Unlike a fertilized egg, however, stem cells are self-replenishing. When they divide, they produce more stem cells just like themselves in addition to the differentiated cells that will go on to realize some other potential.

Embryonic stem cells, so much in the news lately, have more complete potential than mature types of stem cells such as those in the bone marrow.

There are 3 kinds of stem cells: plenipotential, which are the stem cells in the very early stage of an embryo, immediately upon fertilization. I believe when it has just 6 cells or less. I forget what it’s called, a blastosphere, or something like that. After that stage, it’s potential is somewhat more limited: pluripotential. Stem cells from an adult, taken from the bone marrow usually, but could be from other cells, have limited potential, and they are not capable of developing into any other organ.

A stem cell primer.

OK, so my “plenipotential” should have been “totipotent.” Sue me.