Stem cells are cells that can divide into multiple kinds of other cells. Unlike other cells, stem cells are not limited to dividing into copies of themselves. Stem cells are formed and differentiate during early embryonic development, and are also a natural part of the tissue renewal processes throughout life.
Stem cells roles in early development and tissue renewal make them particularly interesting from a therapeutic standpoint, potentially useful for healing otherwise incurable conditions and injuries.
Types of Stem Cells
Stem cells are categorized based on their potency, or the diversity of cell types they can become as they differentiate.
- Totipotent stem cells can divide into all cell types in an organism. A totipotent cell has the potential to divide until it creates an entire, complete organism.
- Pluripotent stem cells can divide into most, or all, cell types in an organism, but cannot develop into an entire organism on their own.
- Multipotent stem cells can differentiate into the various cell types in a family of related cells, such as blood cells. Sometimes, they divide into oligopotent cells as an intermediate step. Oligopotent cells are similar to multipotent cells, but become less able to differentiate.
In most vertebrates, totipotent and pluripotent stem cells are very rare, occurring in abundance only during embryonic development. Therefor, these cells are often called “embryonic stem cells,” in contrast to “adult stem cells” such as the multipotent stem cells found in bone marrow.
Totipotent and pluripotent stem cells are a critical part of embryonic development and show how the single cell of a zygote can develop into every distinct tissue and cell type found in an organism. Many healing processes instead rely on multipotent stem cells, or basic cell division.
Stem Cells in Medicine
Stem cells are of interest in medicine because many organs have a very limited capacity to heal.
For example, when a nerve or heart-muscle cell dies, the body often has no way to replace these highly specialized cells. These cells do not divide in the same way as skin cells and other highly replaceable cells. There is also no supply of pluripotent cells ready to take on this role. Therfore, injuries to some organs or tissues are permanent. Even in tissues that can heal, the healing process often creates scar tissue that impedes long-term function.
Similarly, some diseases such as type 2 diabetes and AIDS, target specific cells types, limiting their ability to replenish themselves by cell division.
Stem cells provide the possibility of replenishing cells that have been damaged or lost, thereby restoring functions that the body cannot restore on its own. This is in part how some vertebrates manage limb regeneration.
Stem Cell Therapy
Stem cell-based medical procedures are not as new as their controversial status makes them seem.
Bone marrow transplant is a form of stem cell therapy.
Bone marrow is home to multipotent stem cells which produce blood cells, and a bone marrow transplant gives the recipient an entirely new selection of blood cells over time.
This is lifesaving for patients whose original blood cells were destroyed, such as in the aftermath of leukemia treatment. Future stem cell therapies make similar promises for other tissues.
Stem Cell Research
Some of the most promising research into stem cells is based on “induced” stem cells.
This concept hinges on the fact that stem and most other cells have the same genetic code. The code is utilized differently in different cell types. Epigenetic modifications are used to dictate which genes are active in each unique cell type. These modifications are changeable over the course of a cell’s life.
Returning those epigenetic markers from those of a final, differentiated cell type to those of a pluripotent or even totipotent cell is known as dedifferentiation. If successful, this would cause the cell to behave as a stem cell thereafter. Not only that, but unlike cells used in transplants or derived from embryos, these “induced” stem cells could be produced from and returned to the same person, thereby effectively bypassing the immune response to foreign stem cells.
Researchers continue to explore the possibility that embryonic, adult and “induced” stem cells can offer a new avenue of treatment for a variety of illnesses. The cells offer hope for a new frontier in regenerative medicine.