Original sentence: Adult stem cells are often present in only minute quantities and can therefore be difficult to isolate and purify.There is also limited evidence that they may not have the same capacity to multiply as embryonic stem cells do
Edited sentence: Adult stem cells are often present in only minute quantities (it is estimated that there are only about 10,000 primitive cells in the average adult’s bloodstream) and can therefore be difficult to isolate and purify.There is also limited evidence that they may not have the same capacity to multiply as embryonic stem cells do (only about 500 of the primitive cells found in the adult bloodstream might have the potential to replace embryonic stem cells