- Stem cells are a class of undifferentiated cells that are able to differentiate into specialized cell types and divide to produce more cells. Although unable to serve any function, their significance lies in their ability to differentiate (develop) into specialized mature cells such as skin, bone, muscle cells etc.
- Two main sources:
- Embryonic stem cells that are derived from a four- or five-day-old human embryo that is in the blastocyst phase of development. The embryos are usually extras that have been created in IVF (in vitro fertilization) clinics where several eggs are fertilized in a test tube, but only one is implanted into a woman.
- Adult tissue (adult stem cells) exist throughout the body and are found in tissues such as brain, bone marrow, liver and blood. They remain in a non-dividing state for long until activated by disease or tissue injury.
- Placenta and umbilical cord are also rich sources of stem cells and are collected after the birth of a baby. They are a type of non-embryonic stem cells. Banking stem cells from the placenta in addition to those found in cord blood significantly increases the number of prenatal stem cells that are collected and preserved, this is Placental banking.
- Potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell
- Totipotent (a omnipotent) stem cells can differentiate into embryonic and extra embryonic cell types. Such cells can in theory construct a complete, viable organism. These cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent.
- Pluripotent stem cells are the descendants of totipotent cells and can differentiate into nearly all cells, i.e. cells derived from any of the three germ layers that are present in an organism.
- Multipotent stem cells can differentiate into a number of cell types, but only those of a closely related family of cells.
- Oligopotent stem cells can differentiate into only a few cell types, such as lymphoid or myeloid stem cells.
- Unipotent cells can only produce their own type, but have the property of self-renewal, which distinguishes them from non-stem cells (e.g.progenitor cells, muscle stem cells).
- Stem cells are extracted from either of the above two sources, and allowed to grow in a controlled medium which prevents their differentiation, but promotes further division.
- Out of this culture, the collection of healthy, dividing, undifferentiated stem cells are called as stem cell lines. The researcher can then stimulate them to differentiate into a particular type of cell.
- Stem cell research, particularly that of embryonic stem cells is surrounded by controversy as it requires the extraction of a blastocyst (mass of cells formed after fertilization of male and female gametes) not allowing it to develop into an embryo, therefore likening it to abortion.