What Are Stem Cells?

The life of a human begins at conception. At that moment, a single cell, the egg, is fertilized. Soon after, it begins to divide — into two, four, eight, 16 cells and so on, with each division doubling the number of cells. After just 20 divisions, there are over a million cells. These first cells are all the same. Gradually, with successive divisions, the daughter cells become more specialized. The process of increasing specialization eventually leads to the highly specific tissues of the body. The early, not yet specialized cells are called stem cells — they have the capacity to produce cells that become specialized into any kind of tissue. Stem cells are remarkable. They can divide many times to create a large number. After receiving certain chemical signals, they can produce directed specialized cells, such as a heart cell or nerve cell. If these cells could be produced and transformed at will, they could create new heart and nerve tissue. Actor Christopher Reeve, who suffered neckdown paralysis after a horseback riding accident, had hoped before his untimely death that stemcell therapy could eventually restore his spinal cord. With new tissues from stem cells, organ replacements would no longer be necessary. Each person could replace any damaged tissue — endlessly. The potential of stem cells can barely be imagined. During the 20th century, the discovery and development of antibiotics transformed medicine. In the 21st century, I am convinced that stem cells will do the same.

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Types of Stem Cells
Totipotent stem cells can differentiate into any cell type in the body plus the placenta, which nourishes the embryo. A fertilized egg is a type of totipotent stem cell, as are the first few divisions. Pluripotent stem cells are descendants of the totipotent stem cells of the embryo, developing about four days after fertilization. They can differentiate into any cell type, except for totipotent stem cells and the cells of the placenta. Multipotent stem cells are descendents of pluripotent stem cells. They are partly specialized. The hematopoietic (blood) stem cells, for example, can differentiate into the dozens of different types of blood cells. Neural stem cells can differentiate into all the different nerve cells. Unipotent or progenitor stem cells can produce only one cell type. But they can reproduce these cells endlessly.

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