SUPPLY - Stem cells are difficult to locate among the billions of cells in a human body. Even when isolated in tissues, scientists can only manage to acquire a small number of stem cells. Because of the scarcity of stem cells, often times the cells must be obtained through many different sources. The integration of the different stem cells creates another problem for scientists, as different cells require different conditions for growth. Creating the perfect environment with the specific signals to stimulate stem cell differentiation has also proven to be a challenge. 

 

TRANSPLANTATION - Scientists may have difficulty transporting the stem cells into a patient's body, especially if the target tissue/organ is deep inside the body. The stem cells also have to acclimate to their new environment and work together with other tissues and organs. 

 

REJECTION - Even after the stem cells have been transplanted into a patient's body, there is a chance that the body's immune system will reject the cells. While the risk is lowered if the stem cells originated from the patient's body, it still is an area for concern. In order to prevent the immune system from attacking the new cells, most patients have to take immunosuppressive drugs that make them more vulnerable to outside pathogens. 

 

TUMOR FORMATION - Without regulation, stem cells can express self-renewal and divide incessantly. This results in a cancerous tumor in the targeted area. Scientists need to find a balance between beneficial and harmful stem cell growth.