Stem Cells 101
What are stem cells?
Stem cells are master or parent cells of the body — cells from which all other cells with specialized functions are created. Stem cells are unspecialized cells that are generally defined by two functional properties: the ability to self-renew and the ability to differentiate into other more mature cells. In this respect, stem cells are considered unique compared to mature cells that lack either or both capabilities.
Stem cells 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. 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.
Under the right conditions in the body or a laboratory, stem cells divide to form more cells, or daughter cells. These daughter cells either become new stem cells (self-renewal) or become specialized cells (differentiation) with a more specific function, such as blood cells, brain cells, heart muscle or bone.
Types of Stem Cells
Until recently, scientists primarily worked with two kinds of stem cells derived from both animals and humans: embryonic stem cells and "adult" or "somatic" (non-embryonic) stem cells.
In 2007, a new type of stem cell called an iPS (induced pluripotent stem cell) was produced. Induced Pluripotent Stem Cells are similar to natural pluripotent stem cells, such as embryonic stem (ES) cells, in many respects, but the full extent of their relation to natural pluripotent stem cells is still being assessed. For scientific purposes, it is important to study and compare all types of stem cells so that the full potential and possible limitations of the varying types can be realized.
Difference between human embryonic and adult stem cells
Not all stem cells are the same. Differences in stem cells can be defined by their potency. For example, a bone marrow derived stem cell is not the same as an embryonic stem cell. The different stem cell potencies are defined as the following:
Totipotent - the ability to differentiate into all possible cell types. Examples are the zygote formed at egg fertilization and the first few cells that result from the division of the zygote.
Pluripotent - the ability to differentiate into almost all cell types. Examples include embryonic stem cells and cells that are derived from the mesoderm, endoderm, and ectoderm germ layers that are formed in the beginning stages of embryonic stem cell differentiation.
Multipotent - the ability to differentiate into a closely related family of cells. Examples include hematopoietic (adult) stem cells that can become red and white blood cells or platelets.
Oligopotent - the ability to differentiate into a few cells. Examples include (adult) lymphoid or myeloid stem cells.
Unipotent - the ability to only produce cells of their own type, but have the property of self-renewal required to be labeled a stem cell. Examples include (adult) muscle stem cells.
Embryonic stem cells
Embryonic stem cells are considered pluripotent instead of totipotent because they do not have the ability to become part of the extra-embryonic membranes or the placenta.
Embryonic stem cells are generated from residual fertilized, frozen eggs commonly found in in-vitro fertilization clinics. The cells are not derived from eggs that have been fertilized in a woman’s body nor embryos once implanted in a woman’s uterus. Donors willingly donate early stage blastocysts to research because 1) the in vitro fertilization clinic has deemed them unusable (i.e. not “implantable”) or 2) because the donor no longer wishes to continue storing the frozen cell aggregates. In either case, the pluripotent cell masses would be discarded as “biological waste” and not used for reproductive purposes.
One of the unique advantages of using embryonic stem cells is that they have been definitively proven to generate all cell types of the body, and are therefore much more versatile than any single adult stem cell.
Why is stem cell research important?
Some day, doctors anticipate being able to use stem cells either directly as a regenerative therapy or apply insights uniquely gained from stem cell research to combat a wide range of congenital and genetic diseases, traumatic tissue injuries, and degenerative ailments including: ALS (aka Lou Gehrig’s disease), Alzheimer’s disease, Parkinson’s disease, Spinal cord injury, Stroke, Diabetes, Heart disease (myocardial infarction), Poor circulation, Hemophilia, Muscular dystrophy, Sickle cell disease and Fanconi anemia.
How can stem cells be used to treat disease?
Given their unique regenerative abilities, stem cells offer new potentials for treating diseases such as diabetes and heart disease. However, much work remains to be done in the laboratory and the clinic to understand how to use these cells for cell-based therapies to treat disease in the context of regenerative medicine. Either cell transplantation as a therapy or use the cells to develop new drug therapies.