I get great questions from readers of The Niche including, “Are there multiple ways to get stem cells?”
The answer is “yes.”
Why is this important?
The different sources of stem cells can impact both their use in basic research and their potential clinically. My goal in today’s post is to give an overview of the main ways that stem cell scientists like me obtain stem cells. There are some important technical and bigger-picture considerations in thinking about the sources of stem cells.
Of the multiple ways to get stem cells, the most common is from tissue
The most common source of stem cells is tissue that is either biopsied or removed on a larger scale from a patient.
While biopsied or surgically removed tissues are generally analyzed for some other reason such as to look for a possible disease, sometimes part of such tissue is used for research. The patient gives consent first for that process. Portions of those tissues can have cells extracted and put into culture. Researchers then hope that some of the cells will grow enough to be used in research. It doesn’t always work.
Typically such newly isolated cells are mortal, meaning they can only be grown for a matter of weeks before they undergo senescence. For that reason, sometimes researchers make genetic changes to the cells to make them immortal. Such cells that grow forever are generally more useful for research. It’s challenging to do studies on mortal cells, what we sometimes call primary cells, given their short proliferative life in the lab. On the other hand, immortal cells generally have mutations, which can complicate the results gained from experiments using them.
Blood or hematopoietic stem cells
Some of the cells isolated from tissues can be stem cells, but most adult tissues have relatively few stem cells.
Bone marrow has hematopoietic stem cells, which can be very useful for research and clinically.
Many research labs study these blood stem cells without genetically modifying them first, although that is also possible.
Umbilical cord blood is another source of hematopoietic stem cells. Researchers have also been isolating cells from the actual cords themselves. Inside the umbilical cord wall there are populations of mesenchymal cells. These are sometimes called MSCs. Additional sources of MSCs include marrow and fat tissue.
Stem cell purity
Unfortunately, some folks still think of MSCs as just stem cells, but only a small subset of these MSCs is true stem cells. Most are fibroblasts and other diverse cells.
More generally, within tissues the actual stem cells are generally intermixed with many other cells that aren’t stem cells. How do we get out just the stem cells? It’s not always easy. One major method is called fluorescence-activated cell sorting or FACS, by which researchers can isolate only certain kinds of cells.
FACS can be done for measurement of cellular properties without purifying the cells. Alternatively, we can use this approach to sort or isolate only specific cells, such as stem cells, for culture. We usually just call this “sorting” of cells. It’s pretty accurate, but not perfect.
Stem cells are sometimes teased out of tissues by simply culturing them in media that in theory only supports stem cell growth. Also, in certain contexts only stem cells will grow in suspension or floating. These are less optimal ways of getting relatively pure stem cell populations but they can work too.
However one isolates stem cells, you need to validate what you have carefully. This can be done by FACS, staining, gene-expression analysis or functional analysis. Or some combination of these methods. The functional part is key because sometimes the more molecular methods aren’t perfect. For instance, some non-stem cells might at times express markers that make them look like stem cells.
Reproductive sources of stem cells: tissues, embryo or fetus, reprogramming
Stem cells can also be isolated from reproductive tissues and early stage embryos. In the case of aborted tissues, cells can sometimes be isolated. Not all such cells will be actual stem cells though. HEK 293 cells are frequently used in research and were isolated from a fetus, but aren’t stem cells. See image above of these human embryonic kidney cells.
The culture of early human embryos called blastocysts is the standard way to make embryonic stem cell lines. Blastocysts are a ball of about 100 cells.
Alternatively, we can make powerful stem cells called iPS cells, which resemble embryonic stem cells in just about every way, through a process called reprogramming. Interestingly, we can make iPS cells from just about any other kind of cell. Skin cells are commonly used as a starting point.
Mouse and human iPS cell production were first reported in 2006 and 2007, respectively. So these are relatively newer lab-based sources of stem cells. There are probably a couple of dozen ways of making iPS cells. So when asked, “are there multiple ways to get stem cells?”, the answer goes well beyond “yes” because there are even many ways just to make iPS cells.
Cancer stem cells
Another common way to get stem cells is from cancers. One of the advantages here is that cancer cells are immortal. They tend to grow very well and deal effectively with stress. That latter point is important because isolating cells and trying to grow them in the lab is very harsh. Many normal primary cells don’t handle being removed from the body and placed on a plastic dish in a lab in artificial growth media very well.
Some of the first cell cultures were from tumors, enabling a vast amount of research. A key caveat with that kind of work is that tumors have numerous mutations and we don’t always know what those mutations are and their functions. Cancer cells tend to accumulate even more mutations as we grow them in the lab too. While it might seem logical to use cancer cells in the lab to study cancer, thousands of experiments are done on cancer cell lines even though the focus of that particular research has nothing to do with cancer. The cancer cells are just useful tools, but in that context researchers need to be careful about data interpretation.
It’s also important to stress that most cancer cells are not stem cells. However, it is thought that many or even most tumors have a stem cell subpopulation. These cells, sometimes called cancer stem cells, are often going to be the most important cells clinically as to therapeutic resistance and tumor recurrence.
Cells from another lab or vendor like ATCC
Finally, it is, of course, a common thing to obtain already existing stem cells from somewhere else. The source is often another lab that already has a stock of the cells frozen down. We also often order cells from vendors like ATCC or other suppliers.
Then ideally you want to validate that the cells are the correct ones. It’s not as easy as it may sound.
I’ve never heard of ATCC cell lines being the wrong thing or being contaminated, but cells from other labs are less certain.
Researchers are increasingly aware that cell lines can sometimes be contaminated with other different cells. Here’s an interesting piece on cell line contamination.
Consent and respect for donors: the case of HeLa cells
No matter which of the multiple ways to get stem cells that a researcher uses, it requires some broader consideration too.
All human cells that are used in research or therapies had to have come from a person. In that sense we should at least sometimes consider the human source and in a sense be grateful to them.
Henrietta Lacks comes to mind. Before she passed away from cancer, the tumor was used to generate the now frequently-used HeLa cell line, but it wasn’t done ethically. The researcher didn’t get proper consent. She and her family did not get the respect they deserved. There is some discussion over whether HeLa cells should no longer be used in research because of how they were produced. I’m not sure. What do you think? Research on HeLa cells probably saves lives. It also provides insights into cell biology and observations like the mitosis time in HeLa cells can be useful more generally.
Overall, if you’re using human cells, give some thought to their source and the potential story behind it.