Stem cells: The undifferentiated mass of cells that hold the differentiation ability to continuously divide and develop into other kinds of cells/tissues.
Two main sources of stem cells:
Umbilical cord: The cord links the developing fetus with the placenta when the fetus is in the mother's uterus, the stem cells found in the umbilical cord are multipotent (able to form multiple types of cells).
Bone marrow: The lymphoid tissue is found in the bones of the skin and thigh, the stem cells found in the bone marrow can divide and differentiate into blood cells.