"On the one hand, it's been very difficult to grow sufficient numbers of ASCs to regenerate tissues or create replacement organs," explains Technical Insights Research Analyst Katherine Austin. "On the other hand, ESCs provoke immune rejection much like any other transplant, and it's also been difficult to grow them in culture without mouse or other animal cells. These obstacles should be overcome in the near future, however."
It is possible to use a procedure called nuclear transplantation to replace the DNA of an embryonic stem cell with DNA from one of the patient's own cells, eliminating the problem of immune rejection. In another approach, differentiating small numbers of ESCs into blood cells and injecting them into the patient creates immune tolerance for other cells types derived from the ESCs, thereby reducing the intensity of rejection by the body.
ASCs, on the other hand, are patient derived and do not face the problem of immune rejection. However, for the most part they have not been as versatile as ESCs, exhibiting only those properties specific to their point of origin. For example, brain and hematopoietic stem cells give rise only to neural tissue and blood cells, respectively.
ASCs' presence in several mature tissues, including bone marrow, blood, adipose tissue, and the brain offsets their inability to differentiate into numerous types of tissues. Recent research has also detected rare ASCs that are 'pluripotent' and hence, capable of forming numerous cell types of the body.