Stem Cells Fast Facts

(CNN)Here is some background information about stem cells.

Scientists believe that stem cell research can be used to treat medical conditions including Parkinson's disease, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis.
About Stem Cells:
Sources: National Institutes of Health, Mayo Clinic
Stem cell research focuses on embryonic stem cells and adult stem cells.
    Stem cells have two characteristics that differentiate them from other types of cells:
    - They are unspecialized cells that can replicate themselves through cell division over long periods of time.
    - Stem cells can be manipulated, under certain conditions, to become mature cells with special functions, such as the beating cells of the heart muscle or insulin-producing cells of the pancreas.
    There are four classes of stem cells: totipotent, multipotent, pluripotent and unipotent.
    - Totipotent stem cells that develop into cells that make up all the cells in an embryo and fetus. (Ex: The zygote/fertilized egg and the cells at the very early stages following fertilization are considered totipotent.)
    - Multipotent stem cells can give rise to multiple types of cells, but all within a particular tissue, organ or physiological system. (Ex: blood-forming stem cells/bone marrow cells, most often referred to as adult stem cells)
    - Pluripotent stem cells (ex: embryonic stem cells) can give rise to any type of cell in the body. These cells are like blank slates, and they have the potential to turn into any type of cell.
    - Unipotent stem cells can self-renew as well as give rise to a single mature cell type. (Ex: sperm producing cells)
    Embryonic stem cells are harvested from four to six-day-old embryos. These embryos are either leftover embryos in fertility clinics or embryos created specifically for harvesting stem cells by therapeutic cloning. Only South Korean scientists claim to have successfully created human embryos via therapeutic cloning and have harvested stem cells from them.
    Adult stem cells are already designated for a certain organ or tissue. Some adult stem cells can be coaxed into or be reprogrammed into turning into a different type of specialized cell within the tissue type - for example, a heart stem cell can give rise to a functional heart muscle cell, but it is still unclear whether they can give rise to all different cell types of the body.
    The primary role of adult stem cells is