The Source: Stem Cells
All organisms—their organs and tissues—are made of cells. Throughout the body, organisms have a store of special cells called stem cells.
Stem cells are cells with the potential to become many different types of cells and tissues.
Video about stem cells.
Using stem cell therapies, we can treat diseases and injuries that we couldn’t treat before.
One day, scientists hope, we might even be able to use them to regrow limbs, like a salamander does when it loses its leg. Stem cells are able to:
1. Self-renew
2. Multiply without accumulating a lot of genetic damage, and without differentiating
3. Differentiate into any type of cell in the organism (when the right signals are provided)
In other words, researchers can tell stem cells when to differentiate as well as what kind of cells to become.
Where Stem Cells Come From
Stem cells from different sources can do different things. The main sources of stem cells are:
1. Embryonic
2. Umbilical cord blood
3. Bone marrow
NOTE: Even more sources of adult stem cells are being found now, including the brain! It was previously thought that the brain wasn’t able to regenerate.
How Stem Cells Can Heal
Doctors use stem cells to heal damaged tissue. To do this, they can either put the stem cells on a scaffold or inject the cells directly into the body.
Eventually, researchers want to be able to get stem cells from the body of the person with an injury, multiply them in the lab and then place them back in the body.
Stem cells have the amazing ability to not only find the site of an injury (by reading inflammatory cues), but also read the chemical composition of the environment (growth factors) and the texture of the damaged tissue to know what kind of cells to become.
Cells alone won’t do the trick for regenerative medicine, though - they need a shape. To give them that shape, researchers use scaffolds!
Differentiation: Making a PersonStem cells can differentiate - that is, become any kind of cell in the body. This ability is necessary, because we all come from a ball of stem cells in the womb! Some stem cells are still present throughout our bodies as children, teens, and adults, helping us heal throughout our lives.
Stem cells can be taken from either of these processes - development or healing – and they can be characterized differently depending on how potent they are. Potency means how many types of cells they can become, or how easy a time they’ll have differentiating.
The different categories of stem cell potency are:
- Totipotent Cells: After a sperm and egg join together (fertilization), the SINGLE CELL they create is totipotent, meaning it can create every cell of the body. That single cell will become the whole body. That first cell divides into other totipotent cells for about five days after fertilization.
- Pluripotent Cells (commonly referred to as “Embryonic Stem Cells”): The next stage in the development of an embryo is a blastocyst, a ball of cells. These cells are pluripotent, meaning they each have the potential to create every cell of the body except the placenta.
- Multipotent Cells: Multipotent cells are found throughout a person’s development—from fetus to adult. Such cells can become only certain other kinds of cells. For example, hematopoietic (blood) cells in the bone marrow are multipotent: they become different types of blood cells. Multipotent cells aren’t as plentiful as pluripotent cells. Also, as a person grows older, there are fewer and fewer of these stem cells.
In general, the earlier in development the cells are taken, the easier it is for them to differentiate.
Issues of Embryonic vs. Adult Stem CellsEmbryonic stem cells
One of the main uses of embryonic stem cells is as transplants into patients with tissue injuries. Any transplant carries a risk of rejection because the body sees the transplant as foreign.
The advantage of embryonic stem cells is that they have fewer markers on their surface that indicate they come from a different genetic material stock (the major histocompatability complex), so they are less likely to be rejected by the body.
However, one of the hurdles of embryonic stem cells is that if they’re injected directly into the body, they may differentiate uncontrollably into many different types of cells and could cause a tumor.
Differentiating embryonic stem cells into usable cells and avoiding rejection are just a few of the hurdles that embryonic stem cell researchers still face. Scientists are trying to determine which growth factors are needed to control this differentiation.
Because of their combined abilities of unlimited expansion and pluripotency, embryonic stem cells remain a theoretically potential source for regenerative medicine and tissue replacement after injury or disease.
Umbilical cord bloodStem cells can also be taken from the umbilical cord of a newborn baby. This is an accessible source of stem cells, compared to adult tissues like the brain and bone marrow.
Getting stem cells from cord blood has the advantages of being non-invasive and plentiful. (Lots of babies are born every day!) Also, they are taken earlier in the developmental process, so these stem cells are more proliferative than adult stem cells.
Adult stem cellsNew sources of stem cells?
Stem cells are being found in many tissues in the body, giving a promise that all tissues may have stem cells somewhere!
Recently, scientists have discovered that stem cells exist in baby teeth, as well as in amniotic fluid—the cushioning fluid around a fetus in the womb. These cells may be pluripotent. Research to characterize and study these cells is very promising, but it’s still early.
However, adult stem cells can’t turn into as many types of cells as embryonic stem cells can.
For example, bone marrow cells can easily become bone and blood cells, but not new neurons!
Some tissues, like the bone and skin, have a lot more stem cells. This is because they naturally regenerate, so they’re prepared to constantly make more. In some tissues like the brain, although stem cells exist, they are not very active, and thus do not readily respond to cell injury or damage.
Adult stem cell treatments have been used for many years to treat leukemia and other bone and blood cancers through bone marrow transplants.
More on treating cancer using stem cells >>
NEAT-O PAGE on sources of stem cells >>
Researchers have induced pluripotent stem cells from skin cells.
However, the approach is not ready to test in people. The researchers use viruses called retroviruses to carry in four genes that transform the skin calls back into their primitive and malleable state.
And when injected into mice, the human iPS cells often formed tumors. The researchers are afraid this method might cause cancer or other unforeseen problems in human beings.
“Researchers get embryonic stem cells from skin," Reuters, 12/23/07
One day, you could be able to walk into the doctor’s office and have stem cells taken from your body. These cells could be reproduced and differentiated into whatever tissue was needed, and put back into your body.
Bone marrow and umbilical cord blood stem cells have the most potential to be reengineered into pluripotent cells.
Researchers are investigating how treating mice cells with “anti-differentiation signals” can reverse differentiation and may allow adult cells to become pluripotent. Though this research is still in the preliminary stages, results are encouraging! (See box.)
Cells alone won’t do the trick for regenerative medicine, though—they need a shape. To give them that shape, researchers use scaffolds!
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