Stem Cells ... What Are They?

Medical Author: Michael Lill, M.D.
Medical Editor: Frederick Hecht, MD, FAAP


Over the last few months, there has been a lot of publicity on the subject of "stem cells".There have been reports from the National Academy of Sciences and comments have been made by the President. Regulations have been promulgated and there has been discussion of 60 stem cell lines being licensed. Speeches have been made in the US Senate and the House of Representatives and there has been much discussion about the use of stem cells to cure diseases as diverse as diabetes, Alzheimer's disease, cancer, heart attacks, and strokes .

But what does this all mean? What is a stem cell and why has there been so much fuss made about it?

Part of the difficulty with this topic is terminology. Different people are using the same term, "stem cells", to refer to vastly different things. Hopefully, by the end of this article, we will have a good understanding of what a stem cell is and what it is not.

Fertilized eggs

The best and most readily understood example of a stem cell is that of the fertilized egg, or zygote. A zygote is a single cell that is formed by the union of a sperm and ovum. The sperm and the ovum each carry half of the genetic material required to form a new individual. Once that single cell or zygote starts dividing, it is known as an embryo. One cell becomes two, two become four, four become eight, eight to sixteen, and so on, doubling rapidly until it ultimately creates the entire sophisticated organism reading this article - meaning you. That organism, a person, is an immensely complicated structure consisting of many, many billions of cells with functions as diverse as those of your eyes, your heart, your immune system, the color of your skin, your brain, etc.