If someone were to ask you what an antelope or a coffee cup or a bulldozer looks like, you could give them an answer. Now think back to high school biology if you are able. What if someone asked you “What does a cell look like”? Anyone could answer that. Because, of course, you either remember the cell membrane, the nucleus, the mitochondria, and so forth --- or you look it up on the internet. Either way, you could give an answer to the question “What does a cell look like?” The question you could not give an answer to is “What does a stem cell look like”? It may seem odd that there is no answer to that question. After all, tens of thousands of scientists have work millions of lab hours studying stem cells. So why is there no answer?
First of all there are many different types of stem cells. (That is a topic for another day) For the moment, let’s talk about stem cells that keep making the necessary blood cells all your life. They are found in bone marrow of adults. For newborns they are also found in umbilical cord blood, liver, and spleen. These are the cells needed when someone gets a bone marrow (or cord blood) transplant. It’s actually just the stem cells and not all of the marrow that is needed.
Regarding cells in general, there are different cells in the body doing different jobs. We describe them by their location, by the job they do, and by their physical features. For example, a skeletal muscle cells (myocytes) are in certain places in the body. They have certain features like myosin proteins and actin filaments (don’t worry, this will not be on the quiz). Other types of cells like lymphocytes (a type of white blood cell) we identify differently from other blood cells by the molecules they have on the cell surface. For example, some lymphocytes have a certain molecule called CD4 on the outside of the cell. Oddly enough, we call these CD4 cells. Other white blood cells have a molecule called CD8. We ID them as CD8 cells. But try as we might, the scientific community has never quite gotten the exact molecular ID for stem cells. We know that many of the blood forming stem cells that live in the bone marrow have a molecule called CD34. But not all the stem cells have CD34 (slightly troublesome). Furthermore, some cells that have CD34 are not stem cells (also troublesome). The only way to define a stem cell is that it does the job we expect a stem cell to do. We call this a “functional” definition.
First off, if an individual is given a lethal dose of radiation (enough to kill the stem cells in the bone marrow), a stem cell is defined as the cell that will allow this individual to survive. Secondly, a stem cell has to be able to divide and make a copy of itself. Lastly, a stem cell must be able to change (differentiate) into more than one type of cell. As we can see, the only way to tell whether or not you have a stem cell is to put the cell into a place where stem cells normally go and see if it acts like a stem cell. So as simple as the question “What does a stem cell look like” is – there is not a simple answer.