are adult stemcells or embryonic stem cells better for research?

Adult stem cells are "multipotent", meaning they can differentiate into several - but not all - cell types found in the adult body. For example, haematpopoetic stem cells can become any blood cell type (erythrocyte, lymphocyte, macrophage, platelet, neutrophil, basophil, monocyte, etc.) but never nerves or muscle; or mesenchymal stem cells can become bone, cartilage, fat, or muscle, but not liver or pancreas, etc.

Embryonic stem cells, OTOH, are "pluripotent", and so can theoretically become *any* cell type in the adult body (including ones we don't have an adult stem cell population for).

Though work is ongoing to try and "revert" adult cells to being pluripotent (called "induced pluripotent stem cells"), this is at an early stage, and we still need ESCs to compare them to.

Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for cell-based regenerative therapies. Of course, adult and embryonic stem cells differ in the number and type of differentiated cells types they can become. Embryonic stem cells can become all cell types of the body because they are pluripotent. Adult stem cells are generally limited to differentiating into different cell types of their tissue of origin. However, some evidence suggests that adult stem cell plasticity may exist, increasing the number of cell types a given adult stem cell can become.

Large numbers of embryonic stem cells can be relatively easily grown in culture, while adult stem cells are rare in mature tissues and methods for expanding their numbers in cell culture have not yet been worked out. This is an important distinction, as large numbers of cells are needed for stem cell replacement therapies.

A potential advantage of using stem cells from an adult is that the patient's own cells could be expanded in culture and then reintroduced into the patient. The use of the patient's own adult stem cells would mean that the cells would not be rejected by the immune system. This represents a significant advantage as immune rejection is a difficult problem that can only be circumvented with immunosuppressive drugs.

Embryonic stem cells from a donor introduced into a patient could cause transplant rejection. However, whether the recipient would reject donor embryonic stem cells has not been determined in human experiments.