Origin

• An embryonic stem cell is derived from a four- or five-day-old human embryo from a fertilized egg. At that stage, the embryo is a ball of cells called a blastocyst, which has typically been donated from an in vitro fertilization (IVF) clinic. An adult stem cell is found in already developed organs and tissues, among the mature cells that make up the tissue. They are found in brain, bone marrow, blood, teeth, skeletal muscle, heart, liver and more.
  
  

Characteristics

• An embryonic stem cell is the youngest type of cell known. It is undifferentiated, or unspecialized, and pluripotent, meaning it can turn into almost any type of cell in the body. An adult stem cell is also undifferentiated but is multipotent, meaning that it can still change its identity into more than one type of cell, but typically only into the cell types of the tissue or organ from which it was found. An adult stem cell is therefore not as versatile for use in research.
  
  

Culture

• An embryonic stem cell is easy to obtain and grow in culture in large quantities. In a laboratory environment, these cells grow and divide without problem. However, an adult stem cell is rare, as it is found in fully formed tissues and organs among fully mature cells that can no longer change identity. Therefore, adult stem cells are harder to obtain and isolate. Plus, outside the body, their division is severely limited, making it difficult to culture in large quantities, which is a problem because stem cell therapies require a large number of cells.
  
  

Cell Therapy

• According to the National Institute of Health (NIH), an embryonic and adult stem cell may differ in their likelihood of causing rejection in patients when used for stem cell therapy. As of 2009, it is unknown whether transplantation of tissues derived from hESC (human embryonic stem cells) will cause rejection because clinical trials have just been approved by the FDA. Tissues derived from adult stem cells however, are less likely to be rejected because cells will be obtained from the patient's own tissues, grown and differentiated into the desired type of cell, then reintroduced back into the patient's body. The patient's immune system should not reject its own cells, thus reducing the need for anti-rejection medication and the side effects it may cause.
  
  

Adult Stem Cell Controversy

• It has been generally accepted that embryonic stem cells may differentiate into virtually any type of cell, and adult stem cells can only give rise to the type of tissue from which it was derived. However, according to the NIH, recent experiments have reported that adult stem cells from one type of tissue have given rise to cells from a completely different tissue. This debate continues and research is ongoing to more fully understand these cells.