Stem Cell Definitions
The term “stem cell” by itself can be misleading. There are many different types of stem cells, each with very different potential to treat disease. The so-called adult stem cells come from any organ, from the fetus through the adult. These are also called tissue stem cells. The so-called pluripotent stem cells, which have the ability to form all cells in the body, are mostly embryonic or induced pluripotent stem (iPS) cells.
All stem cells, whether they are tissue stem cells or pluripotent stem cells, have the ability to divide and create an identical copy of themselves. This process is called self-renewal. The cells can also divide to form cells that go on to develop into mature tissue types such as liver, lungs, brain, or skin.
- What is an embryonic stem cell?
- What is a pluripotent stem cell?
- What is an adult stem cell?
- What is an iPS cell?
- What is a cancer stem cell?
Pluripotent cells, from which embryonic stem cells are derived, exist only at the earliest stages of embryonic development and go on to form all the cells of the adult body. In humans, these cells no longer exist after about five days of development.
When removed and grown in a lab dish these stem cells can continue dividing indefinitely, retaining the ability to form the more than 200 adult cell types. Because the cells have the potential to form so many different adult tissues they are also called pluripotent ("pluri" = many, "potent" = potentials) stem cells.
James Thomson, a professor of Anatomy at the University of Wisconsin, isolated the first human embryonic stem cells in 1998. He now shares a joint appointment at the University of California, Santa Barbara.
Pluripotent means many (pluri) potentials (potent). In other words, these cells have the potential of taking on many fates in the body, including all of the more than 200 different cell types. Embryonic stem cells are pluripotent, as are iPS cells that are reprogrammed from adult tissues. When scientists talk about pluripotent stem cells they mostly mean either embryonic or iPS cells.
What people commonly call adult stem cells are more accurately called tissue-specific stem cells. These are specialized cells found in tissues of adults, children and fetuses. They are thought to exist in most of the body’s tissues such as the blood, brain, liver, intestine or skin. These cells are committed to becoming a cell from their tissue of origin, but they still have the broad ability to become any one of these cells. Stem cells of the bone marrow, for example, can give rise to any of the red or white cells of the blood system. Stem cells in the brain can form all the neurons and support cells of the brain, but can’t form non-brain tissues. Unlike embryonic stem cells, researchers have not been able to grow adult stem cells indefinitely in the lab.
In recent years, scientists have found stem cells in the placenta and in the umbilical cord of newborn infants. Although these cells come from a newborn they are like adult stem cells in that they are already committed to becoming a particular type of cell and can’t go on to form all tissues of the body. The cord blood cells that some people bank after the birth of a child are a form of adult blood-forming stem cells.
Irving Weissman of Stanford University School of Medicine isolated the first blood-forming adult stem cell from bone marrow in 1988 in mice and later in humans.
An induced pluripotent stem (iPS) cell is a cell taken from any tissue from a child or adult that has been genetically modified to behave like an embryonic stem cell. As the name implies, these cells are pluripotent, which means that they have the ability to form all adult cell types.
Shinya Yamanaka at the Kyoto University in Japan created the first iPS cell from a mouse in 2006. Yamanaka now has an appointment at the David J. Gladstone Institutes in San Francisco. In 2007 several groups of researchers including Yamanaka and James Thomson from the University of Wisconsin and University of California, Santa Barbara developed human iPS cells.
Cancer stem cells are a subpopulation of cancer cells that can self-renew, can propagate the cancer, and differentiate into the many types of cells that are found in a tumor. Cancer stem cells are a relatively new concept, but they have generated a lot of excitement among cancer researchers because they could lead to the design of more effective cancer therapies. However, researchers are still debating which types of tumors contain these cells. For those cancers that are propelled by cancer stem cells, those cells are thought to be the source of all cells that make up the cancer. Conventional cancer therapy, such as chemotherapy, may only destroy those cells that form the bulk of the tumor, leaving the cancer stem cells intact and ready to give rise to a recurring tumor. Based on this hypothesis, researchers are trying to find therapies that destroy the cancer stem cells in the hopes that it will improve the chances of treating a patient’s cancer.
John Dick at the University of Toronto identified the first cancer stem cell in 1997. Michael Clarke, then at the University of Michigan, later found the first cancer stem cell in a solid tumor, in this case breast cancer. Now at Stanford University School of Medicine, Clarke and his group have now found cancer stem cells in colon cancer and head and neck cancers.
UCLA Publication: Stem cells and the origin of cancer
Stanford Publication: The true seeds of cancer