Stem cells are unspecialized cells that have two important characteristics that distinguish them from other cells in the body. First, they can replenish their numbers for long periods through cell division. Second, after receiving certain chemical signals, they
can differentiate, or transform into specialized cells with specific functions, such as a heart cell or nerve cell.
Stem cells can be classified by the extent to which they can differentiate into different cell types:
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. Cells produced in the first few divisions of the fertilized egg are also totipotent.
Pluripotent stem cells are descendants of the totipotent stem cells of the embryo. These cells, which develop about four days after fertilization, 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 and antecedents of specialized cells in particular tissues. For example, hematopoietic stem cells, which are found primarily in the bone marrow, give rise to all of the cells found in the blood, including red blood cells, white blood cells, and platelets. Another example is neural stem cells, which can differentiate into nerve cells and neural Progenitor cells (or unipotent stem cells) can produce only one cell type. For example, erythroid progenitor cells differentiate into only red blood cells.
At the end of the long chain of cell divisions are “terminally differentiated” cells, such as a liver cell or lung cell, which are permanently committed to specific functions. These cells stay committed to their functions for the life of the organism or until a tumor develops. In the case of a tumor, the cells dedifferentiate, or return to a less mature state.
