Last reviewed and revised on May 20, 2013
By Anthony L. Komaroff, M.D.
Brigham and Women's Hospital
To understand what a stem cell is, it's helpful to know what cells are, in general, and how they work.
Cells are the basic unit of any living organism. Every animal and human being is composed of cells. Each human being is made of trillions of cells.
Cells are grouped together into organs, such as the eye, the brain, the heart and the pancreas. Each organ contains different types of specialized cells. Each type looks and acts different: each has a different role. These different cells support each other and even "talk" to each other using chemical signals.
The Three Miracles of Cells
The first miracle of how cells develop is that the trillions of cells in each of our bodies came from just one original cell: the fertilized egg.
The second miracle is that all of these cells are unique and have a specialized purpose. Certain cells in the back of our eyes detect light, allowing us to see. Certain cells lining our stomach make acid that helps to digest food. Certain cells in the pancreas make insulin, which drives a source of energy (sugar) inside our cells. That's why cells that live in different organs are called specialized cells.
The third miracle is that all of the specialized cells form in the right place (the light-sensing cells in the eye, the acid-producing cells in the stomach), and in the right amount: not too many and not too few. Starting inside the mother's womb and then after birth, some miraculous process in each of us has directed one cell to both multiply and to change into very different cells, in a highly controlled way. What is that miraculous process?
What Controls How Cells Grow and Develop?
Inside every human cell is a set of about 23,000 genes — the same 23,000 genes. The genes inside a light-sensing cell in the eye are the same as the genes in an acid-making cell in the stomach. So if both cells have the same genes, why are the two different types of specialized cells so different from each other?
It doesn't matter which genes are inside a cell. All cells have the same genes. What matters is which genes inside a cell are turned on. Genes work only when they are turned on. Chemical signals in the immediate environment — usually chemicals produced by the cells next door — turn certain genes on or off to make one specialized cell different from another specialized cell. For example, different genes are turned on and off in the light-sensing cells of the eye than in the acid-producing cells of the stomach.
Recently, scientists have begun to identify which genes are turned on or off in each type of specialized cell, a particular type of cell, and which chemical signals influence these genes.
One final concept is important. Specialized cells cannot replace themselves when they die and generally cannot turn into any other type of cell. With a few exceptions, once a cell has become specialized, it will stay that way until it dies.
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Anthony L. Komaroff, M.D., is professor of medicine and editor-in-chief of Harvard Health Publications at Harvard Medical School. Dr. Komaroff also is senior physician and was formerly director of the Division of General Medicine at Brigham and Women's Hospital. Dr. Komaroff has served on various advisory committees to the federal government, and is an elected Fellow of the American Association for the Advancement of Science.