"Adult stem cells are also problematic, as they are difficult to identify, purify and grow, and simply may not exist for certain diseased tissues that need to be replaced"

This quote by Eliot Engel is describing the troubles associated with adult stem cells. Adult stem cells are cells that are discovered in the body of an adult organism and can be utilized to replace damaged or unhealthy tissue. However, these cells are tough to determine, purify, and grow in a laboratory setting. This makes them difficult to use for medical treatments. Additionally, there may not even be adult stem cells available for certain infected tissues that require to be changed. This further makes complex using adult stem cells for medical treatments. Therefore, adult stem cells are problematic due to the troubles connected with determining, purifying, and growing them, in addition to the possibility that they may not even exist for certain diseased tissues.