A self-renewing, unspecialized cell that is capabile of becoming any one of a number of more specialized cells.
A stem cell is a primitive, unspecialized cell that has the capacity to self-regenerate, but develops a highly specialized function when grown in the appropriate environment and treated with specific proteins or survival factors. In other words, it's a cell that "hasn't decided what it wants to be when it grows up." Stem cells in the human retina have been found in the ciliary body, a specialized structure that sits on either side of the iris, near the retina. These ciliary stem cells usually lie dormant in the adult. Stem cells have now been isolated by many scientific groups and are being evaluated as a potential source for replacement of RPE and photoreceptor cells. There are many different types of stem cells, including adult and embryonic stem cells. Adult stem cells are from mature tissue that can renew itself but has a limited ability to transform into specialized cell types (i.e., fewer cell options). Embryonic stem cells, being from early stages of development, have the theoretical potential to produce any kind of cell in the body.