From where do we get embryonic stem cells?
Embryonic stem cells, are derived from embryos before to the 2nd week of development long before the developing embryo has transitioned to becoming a Fetus. During these first two weeks, essentially all of the cells of the embryo are stem cells, in that they have not differentiated into cells with specialized functions. Typically embryonic stem cells are derived from embryos that are created in laboratory conditions, not harvested directly from a human mother. In other words, a human egg has been harvested from a woman and fertilized with a human sperm in vitro (in a laboratory). Thus usually takes place in an in vitro fertilization clinic—and then donated for research purposes.
How are embryonic stem cells grown in the laboratory?
The technique of growing cells in the laboratory is referred to as cell culture. Human embryonic stem cells (hESCs) are grown by harvesting the cells derived from an early stage preimplantation embryo (a very young embryo that if present in a human mother would not yet be implanted in her uterus). These cells are grown in a special laboratory dish that contains a nutrient broth known as culture medium.
Once the cells have taken hold and are surviving they can be removed and placed into several additional culture dishes. The process is called sub-culturing the cells and can be repeated many times over many weeks and months. Each cycle of sub-culturing the cells is referred to as a passage, and is a way that a few original stem cells can be “expanded” into many generations and millions of stem cells and are referred to as an embryonic stem cell line.
How do we identify embryonic stem cells?
During the process of generating lines of embryonic stem cells in laboratory conditions, it is important to test the cells to see if they exhibit the basic properties or “characteristics” of stems cells. This process is called “characterization”.
Though this process has not been standardized throughout the cell-biology industry, the following are some of the tests that are commonly performed:
- Testing the cells’ ability for Self-Renewal. This is accomplished simply by growing and sub-culturing the cells for extended periods of time and evaluating their rate of growth. At each stage, the cells can also be inspected through a microscope to ensure that they look healthy and remain undifferentiated.
- Testing for the presence of specific Gene Transcription factors. Let me explain. Every cell is constantly activating and deactivating specific genes which are the templates for the creation of different proteins. For instance, when we eat sugar, cells in our pancreas will create the protein insulin which is essential in regulating our blood sugar levels. To do this, certain transcription factors that turn-on the transcription of the insulin gene will be made. Conversely, when our sugar levels are low and we don’t need insulin any more, transcription factors will be made that inhibit the transcription of the insulin gene. Basically transcription factors are what turn-on or turn-off the transcription of genes. When stem cells are in an undifferentiated state, they produce certain transcription factors; specifically the “Oct-4” and “Nanog” transcription factors. Apparently, without these factors, the cells will start going down the path of differentiation. Therefore to establish that the cells we are growing are true undifferentiated stem cells, the transcription factors “Oct-4” and “Nanog” must be present.
- All cells have specific membrane cell-surface-markers that are characteristic for that kind of cell. Thus we can test for the presence of cell-surface-markers that are typical of Stem-cells.
- Another important determination is to test for the cells durability. Can the cells be frozen and thawed and still remain viable.
- Testing to see if the stem cells are truly pluripotent… meaning that they can differentiate into any kind of adult cell. This can be accomplished by allowing the cells to naturally and spontaneously differentiate, or by stimulating them by inducing them into differentiation. Some labs even inject the cells into laboratory mice to see what kind of tissues might grow in the injected area. As you recall from the previous article, truly pluripotent stem cells can differentiate into any one of three fundamental cell types, (germ layers), endoderm, mesoderm and ectoderm cells.
How are embryonic stem cells stimulated or “induced” to differentiate?
Perhaps an even better question to ask is “how do we induce stem cells to differentiate into the exact tissue or organ we need”?
Let me explain. Obviously, the holy grail of regenerative medicine and stem cell therapy would be to grow a new organ – let’s say a liver – for a patient who has a diseased liver. In such a world, any damaged or diseased organ could simply be replaced by a new young organ generated right from the patient’s own stem cells.
The hope is that by changing the composition of the nutrient base in which the cells are cultivated, or by adding certain transcription factors, or by using any number of chemical, biochemical and electronic elements, we might find the correct “recipe” for inducing a stem cell to differentiate into the cells we need or want. Though we have discovered some basic protocols for limited induction of stem cells into specific organ tissues, we are far from growing a complete and viable human organ.
To date our best hope is focusing on developing a specific cell type and not the entire organ. For example the cells that produce insulin within a pancreas, but not the entire pancreas.