Stem cells are a promising regenerative medicine tool and treatment. So much information on stem cell research exists that sorting through the facts and the hype can be daunting. This post will focus on explaining stem cell basics: What are stem cells, and what are the different types? Here, we will focus on adult stem cells, also known as somatic stem cells, since these are most relevant to medical research and clinical use today.
Nearly every cell in the body has a specific purpose. When a fetus begins to develop, cells become specialized (or differentiated), developing structure and characteristics specific to carrying out a special function in the body.1 For example, heart muscle cells generate movement (i.e., heartbeat), and red blood cells contain hemoglobin so that they can transport oxygen throughout the bloodstream. Stem cells, on the other hand, are unspecialized (undifferentiated); they have the potential to become any type of cell.
There are different categories of stem cells. Most current research focuses on these stem cell types:
- Totipotent—can form all cell types in the body. During embryonic development, totipotent stem cells differentiate into all of the different cell types in order to form a whole organism. Researchers have been able to take differentiated cells and return them to a state of totipotency. More work is needed to make totipotent cells clinically useful.
- Pluripotent—are descended from totipotent stem cells and can form into almost all cell types. Pluripotent stem cells can form any type of cell derived from the three germ layers: endoderm (forms the inside lining of stomach, digestive tract, and lungs), mesoderm (forms blood, muscle, and bone), and ectoderm (forms the skin and nervous system.) Pluripotent stem cells naturally exist in the embryo.
- Multipotent—more specialized than totipotent and pluripotent stem cells, multipotent cells can develop into several different somatic cell types. Multipotent stem cells give rise to various tissues and organs during development, and some of these stem cells remain in a state of dormancy (“quiescence”) within their specific tissue niches of the adult human body. Types of multipotent stem cells include:
- Hematopoietic stem cells—form the components of the blood.
- Mesenchymal stem cells—form bone, cartilage, muscle, and fat cells.
- Neural stem cells—form the nervous system.
- Epithelial stem cells—form cells in the digestive tract lining.
- Oligopotent—can form a few different cell types. They are derived from multipotent stem cells, descending from hematopoietic stem cells. Examples:
- Lymphoid cells—form several types of immune cells, such as B cells and T cells.
- Myeloid cells—form red blood cells and white blood cells, among others.
As mentioned above, stem cells are found in the adult body within tissues such as blood, bone, skin, heart, and gut, as multipotent cells. It’s believed that the cells are located in a stem cell niche, or a specific area of each tissue. The number of niche-dwelling stem cells is relatively small. These cells may remain quiescent (undifferentiated) for a long time, until disease, injury, or normal body processes cause the need for differentiation.
Three main sources of these adult somatic stem cells are bone marrow, blood, and adipose, tissue. Research is being conducted on ways to grow larger numbers of adult stem cells outside the body. When experimental conditions in the laboratory are right, stem cells can be induced to become specialized cells that perform specific functions. This has important implications for regenerative medicine.
Inducing cells from a patient’s own body to grow into the specific tissue they need to receive could lead to tremendous medical advances, someday eliminating the need for allografts and organ transplants. In the lab, researchers have been able to manipulate adult somatic stem cells to become induced pluripotent stem cells, or iPSCs. Through forced expression of specific genes, transcription factors, and epigenetic factors, differentiated somatic cells can be de-differentiated or induced to return to a pluripotent state. Advances have been made in scientific research on therapeutic implications of iPSCs.
Another source of pluripotent stem cells is umbilical cord blood. Stem cells from umbilical cord blood can be saved just after birth, cryopreserved for later use, and stored in an umbilical cord blood bank.2 There are now commercialized injectable therapies using umbilical cord blood stem cells.
There are many exciting possible applications of the stem cell research now being performed in laboratories worldwide. One example of this involves limbal stem cells, or LSCs. LSCs are found in the limbus, the part of the eye where the cornea meets the sclera. Normally, these cells are used to regenerate the cornea, which has a very fast turnover rate of 1-2 weeks in the human body. If a person’s LSCs deteriorate or they do not have enough LSCs, their corneas degenerate and they will go blind. However, research suggests that if LSCs were transplanted into such an eye, the entire cornea would regenerate.3 Researchers have studied LSCs using experiments with mice, and were able to restore the cornea of mice with experimental blindness. These results are promising for regenerative medicine in humans. For more information about the applications of stem cell research, check out Part 2.
Stem Cell Basics (2016). National Institutes of Health. Retrieved from https://stemcells.nih.gov/info/basics.htm.
Jawdat, Dunia (2016). Banking of Human Umbilical Cord Blood Stem Cells and Their Clinical Applications. In Abdelalim E. (ed.) Recent Advances in Stem Cells. Humana Press. Retrieved from https://link.springer.com/chapter/10.1007/978-3-319-33270-3_8.
Limbal Stem Cells for Treatment of Corneal Blindness (2017). Advanced Science News. Retrieved from http://www.advancedsciencenews.com/limbal-stem-cells-treatment-corneal-blindness/.