National Institutes of Health

Biology of Stem Cells

Stem cells have a remarkable capability to develop into many different cell types. Stem cells can divide infinitely to develop into other cells in the body. When stem cells divide into daughter cells, the daughter cell has the ability to either remain a stem cell or become a cell with more specialized cell functions, such as being a red blood cell or a brain cell. Three classes of stem cells exist: totipo—tent, multipotent, and pluripotent.

A fertilized egg can be considered toti—potent; the potential of the mass is total and it can give rise to a multitude of cells within the body. Totipotency is the ability of a cell to divide and produce all of the undifferentiated cells within an organism. The growth and development of a living being is said to begin when a sperm fertilizes an egg and creates a single totipotent cell, the zygote. After fertilization, the cell begins to divide [Page 365]and produce other totipotent cells; these totipotent cells begin to specialize within a few days after fertilization. The totipotent cells specialize into plu—ripotent cells, which they develop into the tissues of the developing body. Pluripotent cells can further divide and specialize into multipotent cells, which produce cells of a particular function.

Multipotent cells, in contrast, can only give rise to a small number of cell types. For example, a hematopoietic cell, or a blood stem cell, can develop into several types of blood cells but cannot develop into liver cells or other types of cells; the differentiation of the cell is limited in scope. A multipotent blood cell can produce red and white blood cells, for example. At the end of cellular divisions during differentiation, the cells are terminally differentiated, meaning that they are considered to be devoted to that spécifie cellular function.

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