Stem cells are the building blocks of life, capable of developing into almost any kind of cell in the body. These cells have the power to regenerate damaged tissue and organs, making them a critical area of study for medical applications. When it comes to stem cell research, there are multiple types of stem cells to consider, and one critical aspect of their functionality is their potency. This term refers to the range of cells that stem cells can differentiate into, and it is a key factor in their potential therapeutic use.
Stem cells are a unique cell type in that they can divide and differentiate into many different cell types, even beyond the cells specific to their tissue types. They are often categorized based on their potency, or their ability to differentiate into these different cell types. Potency can vary between stem cells in an individual and across species, and the extent of this ability will inform the potential applications of stem cells as a tool in the treatment of disease.
There are four different potency levels of stem cells—totipotent, pluripotent, multipotent, and unipotent. These are often conveyed on a scale that reflects the extent to which cells can differentiate into different cell types, with totipotent being the most versatile.
Totipotent cells are the most powerful stem cells as they can differentiate into any cell type in the body, including placental cells. These cells can create an entire organism from a single cell, as they have the ability to divide and specialize into any cell type, thanks to the genetic makeup of the zygote. However, totipotent cells are only present in the earliest stages of embryonic development, primarily within the first 4 days after fertilization. For instance, zygotes, cells formed during the fertilization of an egg by a sperm, are considered totipotent.
After the formation of the blastocyst, in which the zygote starts forming into an embryo with specialization of cells beginning and the formation of the inner cell mass (ICM), the cells in totipotent form are no longer present. The ICM will eventually turn into an embryo and the placenta, and by dividing and differentiating into all of the different cell types in the human body, the embryo will eventually form into a fetus.
Pluripotent cells have an equally powerful ability to differentiate into any cell type in the body, with the exception of the placental tissue, seen in totipotent cells. Like totipotent cells, pluripotent stem cells have enough genetic material to divide and differentiate into most of the cells of the body; however, they cannot become placental tissue. Pluripotent cells are typically found in the inner cell mass of the early embryo, and they can be derived from embryonic stem cells or induced pluripotent stem cells (iPSCs).
Mouse and human embryonic stem cells are prime examples of pluripotent stem cells that have been used extensively in clinical research. Embryonic stem cells are derived from embryos that are less than a week old. They are carefully extracted and cultured until they differentiate into one of the 3 primary germ layers that form the body. These germ layers—mesoderm, ectoderm, and endoderm—give rise to the vast array of differentiated cells seen in the adult human body.
Induced pluripotent stem cells, or iPSCs, are another type of pluripotent stem cells that have gained popularity in recent years as they can be generated from a patient’s own cells (usually skin cells). These cells can be developed into almost any cell type of the body, cutting the risk of the body rejecting the transplant. Also, because these are patient-specific cells, the ethical concerns about embryonic stem cells and their derivation are eliminated.
The next level of potency, multipotent stem cells, is more limited in its capacity and is only capable of producing cells that are the same or similar to those found in the tissue that the stem cell originated from. They are slightly more specialized than pluripotent cells, and therefore, their usefulness lies in medical treatments specific to the origin of the stem cells. For example, bone marrow stem cells are able to differentiate into any of the various blood cell types, but not cells from other organs. Even within the mesoderm germ layer, which gives rise to bone, muscle, and cartilage, among others, multipotent stem cells can only create cells of that specific tissue type, such as bone marrow cells.
Finally, we have unipotent cells, the least potent stem cell population. These cells can only differentiate into a single cell type. Typically, these cells are found in adult tissues and have limited regenerative properties. These cells are often seen as the last resort of self-repair as they can only differentiate into a simple type of cell(s), providing a limited ability to regenerate and maintain the tissue of which they are part. Examples of these cells include satellite cells that can differentiate into muscle cells and basal cells in the skin that can only create skin cells.
Understanding stem cell potency is critical when considering therapeutic interventions using stem cells, as the potential uses for different cells are completely dependent on this level of differentiation for their effectiveness. To leverage the full potential of stem cells, scientists must understand all of the potential effects of these cells on the tissues and diseases they may be used to treat.
Stem cells can be developed into specific cell types in the laboratory and delivered to patients. These techniques include using several factors, including signaling molecules to instruct cells on what to become and/or partially manipulating their genetic material. Scientists have used pluripotent stem cells in clinical research with promising outcomes as they have therapeutic potential to cure incurable diseases and regrow tissue.
Stem cells, particularly pluripotent stem cells, have gained a lot of attention in the field of regenerative medicine due to their unique ability to differentiate into any cell type. It is a matter of ethical concerns (embryonic stem cells versus iPSCs), potency, and efficiency of differentiation. As research continues to advance and we gain a better understanding of stem cell potency, we will continue to see new and innovative therapies that could transform the world of medicine.