Embryonic vs. Adult

All stem cells are characterized by the ability to self-replicate (make themselves) and the ability to turn into other cell types. However, there are two main categories of stem cells embryonic and adult .

Embryonic stem cells are undifferentiated , pluripotent and derived from morula or blastocysts from fertilized eggs left over from in vitro fertilization (IVF) and scheduled otherwise for destruction. Embryonic stem cells are the youngest cells we know of. They can turn into almost any cell type in the body; this is called pluripotency .

As ES cells grow and mature, they begin to differentiate into specific cell type. At that point cells have more limited potential, and therefore those are called multipotent. These adult stem cells are undifferentiated cells, multipotent and found in tissue or organs with differentiated cells. These are not as versatile for research purposes.

The oldest cells are ones that are fully differentiated and mature. These cells are no longer able to make copies of themselves, and they will not change their identity.

Human embryonic stem cells (hESCs) were first derived in 1998 by James Thompson at the University of Wisconsin. Large numbers of embryonic stem cells can be easily grown in culture, while adult stem cells are rare in mature tissues and methods for isolating those cells and expanding their numbers in cell culture have not yet been worked out. This is very important, as large numbers of cells are needed for stem cell replacement therapies. One advantage of using the adult stem cell is that patient's own cells could be expanded in culture and then reintroduced into the patient and would not be rejected by the immune system. However, through the process of somatic cell nuclear transfer (SCNT), which is the process of transferring a somatic nucleus into an enucleated cell, embryonic stem cells introduced into a patient would not cause transplant rejection as they would be recognized as self cells. SCNT is also know as therapeutic cloning.