Types of Stem Cells
Stem cells are characterized by their ability for self-renewal (i.e., maintaining their undifferentiated state during several rounds of cell division), and their potency (i.e., the ability to differentiate into specialized cell types).
The two main stem cell types are embryonic stem cells (ES) cells and adult stem cells (i.e., somatic stem cells). Other types, such as induced pluripotent stem cells (iPSCs), are produced in the lab by reprogramming adult cells to express ES characteristics.
(ES) cells are isolated from the inner cell mass of blastocysts of preimplantation-stage embryos. These cells require specific signals to differentiate to the desired cell type; if simply injected directly, they will differentiate into many different types of cells, resulting in a tumor derived from this abnormal pluripotent cell development (a teratoma). The directed differentiation of ES cells and avoidance of transplant rejection are just two of the hurdles that ES cell researchers still face.
With their potential for unlimited expansion and pluripotency, ES cells are a potential source for regenerative medicine and tissue replacement after injury or disease. ES cell therapy is at an early stage, but human trials were first carried out in 2010 on spinal injury victims.
The use of adult stem cells in research and therapy is less controversial than ES cells, because their production does not require the destruction of an embryo. Additionally, when adult stem cells are obtained from the intended recipient (an autograft) there is no risk of immune rejection. Adult stem cell treatments have been successfully used for many years to treat leukemia and related bone/blood cancers through bone marrow transplants.
iPSCs are somatic cells that have been genetically reprogrammed to an embryonic stem cell–like state by being forced to express genes important for maintaining the defining properties of embryonic stem cells. Although additional research is needed, iPSCs are already useful tools for drug development and modeling of diseases, and scientists hope to use them in transplantation medicine.
In addition, tissues derived from iPSCs will be a nearly identical match to the cell donor and thus probably avoid rejection by the immune system. By studying iPSCs and other types of pluripotent stem cells, researchers may learn how to reprogram cells to repair damaged tissues in the human body.