A stem cell is the most basic cell form in that it is undifferentiated and can be manipulated to become any type of cell in the body. It is essentially a “blank” cell.
The ability for stem cells to become any other type of cell is known as pluripotency. Apart from this unique ability, stem cells also have another special characteristic in that it has an unlimited potential to divide. Normal body cells (somatic cells) can only divide a certain amount of times after which replication will stop. Stem cells are not restricted by this finite ability to replicate.
Types of Stem Cells
There are two types of stems cells – embryonic and adult (somatic) stem cells. In 2007, the ability to “reprogram” an adult stem cell has given rise to a third category, known as induced pluripotent stem cell or iPSC.
Embryonic Stem Cells
These are the stem cells found in embryos. After an egg cell (ovum) is fertilized by a sperm, it forms a ball of cells (blastocyte) which is the embryo from where stem cells are sourced.
Due to the increased demand for in-vitro fertilization (IVF) as a means of treating infertility, many women around the world have their egg cells (ova) fertilized in a laboratory and the embryo is “frozen” (cryogenic storage). However, not all these embryos are used, either because pregnancy is successful or the woman no longer wishes to fall pregnant. With informed consent, the donors release these embryos for medical research and related therapies.
Embryonic stem cells are the preferred type of stem cell because it is totally undifferentiated. These cells can then be :
- “grown” in a laboratory by stimulating the embryonic stem cells to multiply as undifferentiated cells.
- “manipulated” by hormonal and genetic factors to differentiate into any type of cells (multipotency).
There has been much controversy surrounding the use of embryonic stem cells for medical purposes due to ethical issue that these embryos had the potential to become human beings.
Adult (Somatic) Stem Cells
Adult or somatic stem cells are found in many areas in the body as immature cells which can be triggered to differentiate into certain types of cells.
The advantage of adult stem cells was that there were no ethical issues as they were sourced directly from the patient and the chances of any rejection (histocompatibility) would have been nullified.
The disadvantage was that adult stem cells were found in limited quantities in tissues. The other drawback of these types of stem cells is that they could only differentiate into specific types of cells based on the area where it is found (multipotency or tissue-specific). For example : hematopoietic stem cells found in bone marrow could only differentiate into blood cells.
Cord Blood Stem Cells
Somatic stem cells can also be sourced from the blood within the umbilical cord and placenta at the time of childbirth. This has grown in popularity globally as parents opt to harvest these stem cells at the time of delivery and store it in private banks at a very affordable rate. The advancing field of stem cell therapy may mean that these stem cells could be used to treat degenerative conditions in the child at a later stage.
Induced Pluripotent Stem Cells (iPSC)
Recently it was discovered that adult stem cells could be “reprogrammed” to revert back to a state similar to embryonic stems cells where they could then differentiate into any other type of cell. This “reprogrammed” stem cell is known as induced pluripotent stem cell (iPSC). The research is still in the early stages and while iPSC’s seem to resemble embryonic stem cells, it has yet to be ascertained if it will be as effective for future stem cell therapy options.
Article reviewed by Dr. Greg. Last updated on June 23, 2010