Stem cells are undifferentiated cells that can be induced to form more specialised cells by cellular differentiation. Stem cells can stay non-dividing and non-specific for years. To be a stem cell requires that it possesses two properties, it can undergo self-renewal, and it has potency - the capacity to differentiate into specialised cell types.
In mammals, there are two types of stem cells, adult stem cells and embryonic stem cells. Adult stem cells, also called tissue-specific or somatic stem cells, are found in fully developed mammals. They can divide or self-renew indefinitely and maintain and repair the tissue in which they are found. Embryonic stem cells are isolated from the inner cell mass of blastocysts in early embryonic development. They are pluripotent and have the ability to divide indefinitely, and can develop into each of the more than 200 cell types of the adult body. Potency specifies the potential to differentiate into different cell types, and they can be totipotent, pluripotent, multipotent, oligopotent or unipotent.
Stem cells offer the potential to treat many diseases including cancer and neurodegenerative conditions. Stem cells can now be artificially grown and differentiated into specialised cell types of various tissues such as muscles or nerves.