have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.
Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity. Second, under certain physiologic or experimental conditions, they can be induced to become tissue- or organ-specific cells with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions.
Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: embryonic stem cells and non-embryonic “somatic” or “adult” stem cells. The functions and characteristics of these cells will be explained in this document. Scientists discovered ways to derive embryonic stem cells from early mouse embryos more than 30 years ago, in 1981. The detailed study of the biology of mouse stem cells led to the discovery, in 1998, of a method to derive stem cells from human embryos and grow the cells in the laboratory. These cells are called human embryonic stem cells. The embryos used in these studies were created for reproductive purposes through in vitro fertilization procedures. When they were no longer needed for that purpose, they were donated for research with the informed consent of the donor. In 2006, researchers made another breakthrough by identifying conditions that would allow some specialized adult cells to be “reprogrammed” genetically to assume a stem cell-like state. This new type of stem cell, called induced pluripotent stem cells (iPSCs), will be discussed in a later section of this document.
Embryonic stem cells
Tissue-specific stem cells
Mesenchymal stem cells
Induced pluripotent stem cells
Embryonic stem cells
Embryonic stem cells are obtained from the inner cell mass of the blastocyst, a mainly hollow ball of cells that, in the human, forms three to five days after an egg cell is fertilized by a sperm. A human blastocyst is about the size of the dot above this “i.”
In normal development, the cells inside the inner cell mass will give rise to the more specialized cells that give rise to the entire body—all of our tissues and organs. However, when scientists extract the inner cell mass and grow these cells in special laboratory conditions, they retain the properties of embryonic stem cells.
Embryonic stem cells are pluripotent, meaning they can give rise to every cell type in the fully formed body, but not the placenta and umbilical cord. These cells are incredibly valuable because they provide a renewable resource for studying normal development and disease, and for testing drugs and other therapies. Human embryonic stem cells have been derived primarily from blastocysts created by in vitro fertilization (IVF) for assisted reproduction that were no longer needed.
Tissue-specific stem cells
Tissue-specific stem cells (also referred to as somatic or adult stem cells) are more specialized than embryonic stem cells. Typically, these stem cells can generate different cell types for the specific tissue or organ in which they live.
For example, blood-forming (or hematopoietic) stem cells in the bone marrow can give rise to red blood cells, white blood cells and platelets. However, blood-forming stem cells don’t generate liver or lung or brain cells, and stem cells in other tissues and organs don’t generate red or white blood cells or platelets.
Some tissues and organs within your body contain small caches of tissue-specific stem cells whose job it is to replace cells from that tissue that are lost in normal day-to-day living or in injury, such as those in your skin, blood, and the lining of your gut.
Tissue-specific stem cells can be difficult to find in the human body, and they don’t seem to self-renew in culture as easily as embryonic stem cells do. However, study of these cells has increased our general knowledge about normal development, what changes in aging, and what happens with injury and disease.
MESENCHYMAL STEM CELLS:
You may hear the term “mesenchymal stem cell” or MSC to refer to cells isolated from stroma, the connective tissue that surrounds other tissues and organs. Cells by this name are more accurately called “stromal cells” by many scientists. The first MSCs were discovered in the bone marrow and were shown to be capable of making bone, cartilage and fat cells. Since then, they have been grown from other tissues, such as fat and cord blood. Various MSCs are thought to have stem cell, and even immunomodulatory, properties and are being tested as treatments for a great many disorders, but there is little evidence to date that they are beneficial. Scientists do not fully understand whether these cells are actually stem cells or what types of cells they are capable of generating. They do agree that not all MSCs are the same, and that their characteristics depend on where in the body they come from and how they are isolated and grown.
Induced pluripotent stem cells
Induced pluripotent stem (iPS) cells are cells that have been engineered in the lab by converting tissue-specific cells, such as skin cells, into cells that behave like embryonic stem cells. IPS cells are critical tools to help scientists learn more about normal development and disease onset and progression, and they are also useful for developing and testing new drugs and therapies.
While iPS cells share many of the same characteristics of embryonic stem cells, including the ability to give rise to all the cell types in the body, they aren’t exactly the same. Scientists are exploring what these differences are and what they mean. For one thing, the first iPS cells were produced by using viruses to insert extra copies of genes into tissue-specific cells. Researchers are experimenting with many alternative ways to create iPS cells so that they can ultimately be used as a source of cells or tissues for medical treatments.
How can stem cells treat disease?
When most people think about about stem cells treating disease they think of a stem cell transplant.
In a stem cell transplant, embryonic stem cells are first specialized into the necessary adult cell type. Then, those mature cells replace tissue that is damaged by disease or injury. This type of treatment could be used to:
replace neurons damaged by spinal cord injury, stroke, Alzheimer’s disease, Parkinson’s disease or other neurological problems;
produce insulin that could treat people with diabetes and heart muscle cells that could repair damage after a heart attack; or
replace virtually any tissue or organ that is injured or diseased.
But embryonic stem cell-based therapies can do much more.
Studying how stem cells develop into heart muscle cells could provide clues about how we could induce heart muscle to repair itself after a heart attack.
The cells could be used to study disease, identify new drugs, or screen drugs for toxic side effects.
Any of these would have a significant impact on human health without transplanting a single cell.
What diseases could be treated by stem cell research?
In theory, there’s no limit to the types of diseases that could be treated with stem cell research. Given that researchers may be able to study all cell types via embryonic stem cells, they have the potential to make breakthroughs in any disease.
How can I learn more about CIRM-funded stem cell research in a particular disease?
CIRM has created disease pages for many of the major diseases being targeted by stem cell scientists. You can find those disease pages here.
You can also sort our complete list of CIRM awards to see what we’ve funded in different disease areas.
What cell therapies are available right now?
Many clinical trials for embryonic stem cell-based therapies have begun in recent years. Results from those won’t be available until the trials reveal that the therapies are safe and effective—which could take a few years.
While ten cell therapies have been approved around the world as of January 2016, the only widely used stem cell-based therapy is bone marrow transplantation. Blood-forming stem cells in the bone marrow were the first stem cells to be identified and were the first to be used in the clinic. This life-saving technique has helped thousands people worldwide who had been suffering from blood cancers, such as leukemia.
In addition to their current use in cancer treatments, research suggests that bone marrow transplants will be useful in treating autoimmune diseases and in helping people tolerate transplanted organs.
Other therapies based on adult stem cells are currently in clinical trials. Until those trials are complete we won’t know which type of stem cell is most effective in treating different diseases.
When will therapies based on embryonic stem cells become available?
There is no way to predict when the first human embryonic stem cell therapies will become widely available. Several applications with the FDA to begin human trials of embryonic stem cell-based therapies have been approved. In general, the path from the first human trial to widespread use is on the order of a decade. That long time frame is a result of the many steps a therapy must go through in order to show that it is both safe and effective. Only once those steps are complete will the FDA approve the therapy for general use.
If embryonic stem cells follow a normal path it could still be many years before therapies based on embryonic stem cells are widely available. However, if researchers gave up on therapies simply because the path towards FDA approval is long, we would not have any of the lifesaving technologies that are now commonplace: recombinant insulin, bone marrow transplantation or chemotherapy drugs.