"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.

Secondly, ASCs may be unsuitable for regeneration of damaged tissues in the case of elderly patients or due to presence of cancer in the bone marrow.

In addition, ASCs are restricted to the individual patient, making it difficult to spread the cost of isolation and expansion among multiple recipients. Some groups have found, however, that certain ASCs do not cause immune rejection, allowing cells to be pooled from many patients, which would solve this problem.

Though biological hurdles are common ground for both ASCs and ESCs, the latter face political barriers because they originate from discarded human embryos or aborted fetuses stored at in vitro fertilization clinics. Another method that has raised political hackles is 'therapeutic cloning,' which involves the creation of a cloned embryo using the patient's DNA.

Fuelling such controversies are stringent regulatory practices specifically in the United States such as President Bush's decree in August 2001 forbidding federal funding for research on ESCs, with the exception of those derived from the approved twelve existing stem cell lines.

Following the Bush decree, of the $370 million in grants awarded by the National Institute of Health (NIH) during 2002 for animal and human stem cells, only $10 million went to research involving human ESCs. The ESCs are also expensive at $5,000 per vial, forcing NIH to subsidize cell culturing or offer supplements to its grants in order to cover the extra cost.

Quality control is another critical issue in stem cell therapies, where assessing products containing combinations of active substances and live cells become extremely complex. Addressing this issue during the early stages of design will avoid future delays in product approval and post-approval recalls.

"Contamination is a constant threat, hence living cells grown in vitro for organ transplants and regenerating damaged tissues must be cultured under GMP (good manufacturing practice) conditions in order to meet the FDA quality standards," concludes Austin.

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