Embryonic stem cells typically are derived from 4- to 5-day old human embryos formed via in vitro fertilization or cloning in a lab. Since embryonic stem cells are naturally “pluripotent” — able to transform themselves into any cell in the body — researchers have regarded them as the most useful. The use of embryonic stem cells has been condemned by the Catholic Church because embryos must be destroyed in the process of acquiring them.
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Adult stem cells, meanwhile, are found in a variety of tissues, such as skin, bone marrow, fat and umbilical cord blood. Their job is to maintain the body’s tissues. For instance, each day adult stem cells within bone marrow replace billions of red blood cells that the spleen has removed because they’ve been damaged by wear and tear. Adult stem cells are less versatile than pluripotent embryonic stem cells; they are apparently limited to forming only the specialized cells of their tissue of origin.
The third type is a new scientific creation known as the induced pluripotent stem cell (iPS cell). The iPS cell is a differentiated cell, such as a skin cell, that scientists recently have learned how to genetically engineer back to embryonic stem-cell-like pluripotency. Scientists are excited by their promise because these cells potentially can develop into any specialized cell. Since these cells can be generated from a patient’s own adult cell, the body will not reject tissue derived from an iPS cell.