An embryonic stem cell is a cell derived from the early stages of an embryo which is capable of differentiating into any type of body cell. Embryonic stem cells are capable of differentiating into any cell type because in the embryo that is what they are used for.
As the embryo grows and divides, cells which are generalized must become more and more specific as they divide. This eventually creates the different organs, tissues, and systems of an organism. Embryonic stem cells are totipotent, meaning they can divide into any other cell type within an animal. This is a process known as cellular differentiation.
After the sperm reaches an egg (oocyte), fertilization occurs and the DNA from the two cells merge into a single nucleus, in a single cell. This is the zygote and is technically an embryonic stem cell because as it divides it will differentiate into all of the cells of the body.
This cell and the first few divisions of this cell have the ability to become any type of tissue. This means that they have the ability to become an entire organism. Identical twins, for example, develop from the same zygote which accidentally separates when it begins to divide.
In medicine and research, scientists use pluripotent embryonic stem cells. These cells do not have the ability to become an entire organism. Rather, they are directed by signals from the early embryo which tell them which cell type to differentiate into. Scientists prefer these cells for many reasons.
First, they can be stored and maintained more easily. Totipotent cells have a tendency to differentiate quickly, and immediately try to become an organism. Pluripotent cells are waiting for a signal to divide and can be maintained for longer periods. Further, because pluripotent cells are simply waiting for the proper signals to tell them which cell type to become, they can easily be integrated into medical applications in which new tissue must be grown.
The use of embryonic stem cells is a very new form of medicine. For decades, the cause of many degenerative diseases and physical injuries has been understood. Tissue damage is the root cause of many of these ailments, and scientists have long been searching for a method of growing tissues which do not easily repair themselves. Because an embryonic stem cell is pluripotent and can become almost any cell in the body, these cells have long been studied for their possible use in medicine.
Since the late 1950’s scientists have been trying to test various methods of growing tissue with an embryonic stem cell. The first clinical trials were in the late 1960s, but not much progress has been made. President Bush put a moratorium on using Federal funds for stem cell research, which was finally lifted by the Obama Administration in 2009. European countries have also faced an uphill battle in funding stem cell research. However, with advances in the science came new discoveries which allowed for more ethical harvesting of an embryonic stem cell. The first treatments with medicinal stem cells were in 2010.
Medically, the embryonic stem cell is limited in its current uses, though many novel applications are in the works. Current treatments focus on the replacement of damaged tissue from injury or disease. Of these, the first treatment approved by the FDA to undergo trials was replacing damaged tissue in spinal injuries.
Because nerve cells rarely regenerate, an embryonic stem cell can be used to replace the damaged nerve and restore function. In someone with a spinal injury, this means being able to walk again. For a blind person, this might mean being able to see again. While the treatment is still new and success is limited, it has shown some positive results.
Still, other medical advances are made with the embryonic stem cell, although these don’t come as direct medical treatments but rather as the knowledge that stem cells give us. As an embryonic stem cell differentiates into its target tissue, scientists can study the chemicals and methods it uses to do so. Scientists can also alter the genome of these cells, and study the effects different mutations have on a cell’s functioning.
Between these two paths of discovery, scientists have assembled much information about how and why cells differentiate and divide. Using these tools, scientists are closing in on methods which would allow them to turn regular cell back into a pluripotent stem cell. These are known as induced pluripotent stem cells. They are not embryonic stem cells, because they are not derived from an embryo. This process could not only fix injuries and ailments but could potentially reverse aging and prevent death.
On a less dramatic and grand scale, these methods are also being used to cure common diseases, such as diabetes. By learning how embryonic stem cells become pancreas cells and secrete insulin, scientists are learning the methods of converting other tissues to insulin-secreting tissues. This could help cure diabetes, often caused by the destruction of insulin-producing cells. If these were replaced with stem cells, or other cells were induced to become pancreas cells, the disease could be cured.
Other diseases, like cystic fibrosis, fragile x syndrome, and other genetic disorders are studied in embryonic stem cells. Not only can many cells be created, but they can be differentiated into different cell types. In this way, a scientist can build a picture of the disease from snapshots of each cell type, and understand exactly how the disease is affecting a person.
While there was once a concern that embryonic stem cells were being harvested without consent from unknowing women, the vast majority are now ethically harvested an in vitro fertilization clinics. In these clinics, in order to get a successful pregnancy, many eggs must be fertilized. Only one is implanted, and with the woman’s consent, the rest can be used to harvest embryonic stem cells. To do this, scientists extract some embryonic stem cells from an embryo when it is only a small ball of cells. This can be seen in the image below.
A harvested embryonic stem cell is placed in a petri dish with nutrients and is allowed to divide. Without any signals from the embryo, the cells remain pluripotent. They continue dividing, fill one dish, and they are transferred to many more dishes and continue to grow. After 6 months of this, they are considered a successful pluripotent embryonic stem cell line. They can then be used to study disease, be used in treatments, or be manipulated genetically to provide models for how cells work.
To test that these cells are indeed pluripotent stem cells, they are injected into mice with depressed immune systems. The mice must have depressed immune systems, or their bodies would naturally reject the human tissue. Once implanted into the mouse, successful pluripotent cells will form a small tumor called a teratoma. This small tumor has different tissue types and proves that the cell line is still pluripotent and can differentiate into different cell types.
There are also other types of stem cells, not to be confused with an embryonic stem cell. Embryonic stem cells are derived from embryos. There are also adult stem cells, umbilical cord stem cells, and fetal stem cells. Not only are these stem cells sometimes more ethically challenging, they are only multipotent, meaning they can only become a small range of cell types.
One example is umbilical cord blood stem cells, which have been used in medical treatments to treat various blood diseases and suppressed immune systems. The stem cells in the blood of the umbilical cord can differentiate into almost any type of blood or immune cell, making them multipotent. However, this limits their use in other areas of medicine.
There are also adult stem cells, which survive in various organs throughout the body. These cells are also multipotent, and can only differentiate into the kinds of tissue in which they are found. A common use of adult stem cells is the bone marrow transplant. In this procedure, a healthy donor must have their marrow extracted from their bones. The marrow is a blood-like substance on the inside of large bones which creates blood cells and immune cells.
Cancer patients, having undergone radiation and chemotherapy, lose most of their immune cells and become immunocompromised. Often a bone marrow transplant is needed to replace these tissues. The new stem cells begin producing new immune cells, which help the patient recover and fight off infection and disease.
