As patients start to investigate the many different types of stem cells, it can become overwhelming. Often stem cells are organized into groups by tissue source, i.e., by the type of tissue stem were derived. While it can be important to know whether stem cells come from adipose tissue (fat cells) or the umbilical cord, for example, it is equally important to understand the types of stem cells by their capacity to differentiate. In other words, what are the types of stem cells organized by their ability to become different or more mature cells?
One of the most intriguing features of stem cells is their ability to become virtually any other type of cell. We all start out as a sperm and an egg but end up as an exquisitely organized collection of heart cells and brain cells and kidney cells, etc. At each step of the way—from early cells to the adult human body—stem cells become more differentiated and less capable of becoming any other cell. Thinking about them this way, stem cells are grouped into four categories:
- Omnipotent stem cells
- Pluripotent stem cells
- Multipotent stem cells
- Unipotent stem cells
Omnipotent stem cells
Omnipotent stem cells, also called totipotent stem cells, truly have the capacity to become any other cell. Omnipotent stem cells can become embryonic cells or even the cells that support the embryo, like the placenta. A fertilized egg is an example of an omnipotent cell. While omnipotent/totipotent stem cells are potentially incredibly useful, their use is highly restricted and controversial. As such, they are not usually used for therapeutic purposes, or even for research.
Pluripotent stem cells
Pluripotent stem cells are the next best thing to omnipotent stem cells. Pluripotent stem cells can become nearly any tissue in the body. Like omnipotent stem cells, the use of naturally occurring pluripotent stem cells is also controversial because they require the use of embryos. However, researchers have devised a rather ingenious way to take regular cells and turn them into pluripotent stem cells. These stem cells are referred to as induced pluripotent stem cells or iPSCs. Induced pluripotent stem cells are particularly exciting because of their potential as research tools and clinical therapeutics. Most importantly, iPSCs sidestep controversy because they are made from mature cells.
Multipotent stem cells
Multipotent stem cells can become any number of mature cells. For example, a mesenchymal stem cell can become a muscle cell or fat cell, a bone cell or cartilage cell. Another powerful feature of multipotent stem cells is that they can divide and form other multipotent stem cells. Thus, their ability to differentiate and self-renew makes them incredibly versatile for research and clinical purposes.
Another example of multipotent stem cells is the type of cells that give rise to blood cells, namely hematopoietic stem cells. Hematopoietic stem cells can differentiate into myeloid type or lymphoid type, but once they do, they are restricted to become their corresponding type of blood cell. For example, myeloid cells can become red blood cells or certain white blood cells, while lymphoid cells are more or less destined to become lymphocytes.
Unipotent stem cells
Unipotent stem cells are technically stem cells but have very little potential become anything other than the one cell they were destined to become. For example, a cartilage stem cell is destined to become a cartilage cell, while a bone stem cell is destined to become a bone cell. A mesenchymal stem cell could become either one, but by the time it has differentiated into a cartilage or bone stem cell, its fate is essentially predetermined. As you may expect, unipotent stem cells have limited clinical usefulness.
In summary, as you think about different types of stem cells, it can be important to think about where the stem cell came from (e.g. fat tissue), but also consider its potential for self-renewal and differentiation.