Stem cells

• Stem cells are undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced. They have the ability to divide to form an identical copy of themselves in a process called self-renewal.
• Cell specialisation is irreversible in most animal cells. This means that once the cells have matured and specialised, they can no longer develop into other cells.
• There are four different stem cell sources in mammals : embryonic stem cells, umbilical cord blood stem cells, placental stem cells and adult stem cells.
• Embryonic stem cells come from embryos in the early stages of development. They can differentiate into any type of cell in the initial stages of development.
• Umbilical cord blood stem cells are derived from the umbilical cord blood, and are similar to adult stem cells.
• Placental stem cells are found in the placenta and develop into specific types of cells.
• Adult stem cells are found in the body tissues of a fetus through to an adult. They are specific to a particular tissue or organ within which they produce the cells to maintain and repair tissues throughout an organisms life.
• There are four different types of stem cell, each classified based on their ability to divide : totipotent stem cells, pluripotent stem cells, multipotent stem cells and unipotent stem cells.
• Totipotent stem cells are found in the early embryo and can differentiate into any type of cells. For example, the zygote is totipotent.
• As the zygote divides and matures, its cells develop into slightly more specialised cells called pluripotent stem cells.
• Pluripotent stem cells are found embryos and can differentiate into almost any type of cell. Examples of pluripotent stem cells are embryonic and fetal stem cells.
• Multipotent stem cells are found in adults and can differentiate into a limited number of specialised cells. They usually develop into cells of a particular type (e.g. bone marrow stem cells can produce blood cells). Examples of multipotent stem cells include adult stem cells and umbilical cord blood stem cells.
• Unipotent stem cells can only differentiate into a single type of cell. They are derived from multipotent stem cells and are made in adult tissue. An example of unipotent stem cells are cardiomyocytes.