CONCEPTS IN BIOLOGY
PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS
11. Applications of Biotechnology
11.4. Stem Cells
Stem cells are cells that are self-renewing and have not yet completed determination or differentiation, so they have the potential to develop into many different cell types. Scientists can generate stem cells by nuclear transfer techniques; they also occur naturally throughout the body. They are involved in many activities including tissue regeneration, wound healing, and cancer treatment.
If scientists had the ability to control differentiation it may allow the manipulation of an organism’s cells or the insertion of cells into an organism to allow the regrowth of damaged tissues and organs in humans. This could aid in the cure or treatment of many medical problems, such as the repair of damaged knee cartilage, heart tissue from a heart attack, or damaged nerve tissue from spinal or head injuries. Some kinds of degenerative diseases occur because specific kinds of cells die or cease to function properly. Parkinson’s disease results from malfunctioning brain cells, and many forms of diabetes are caused by malfunctioning cells in the pancreas. If stem cells could be used to replace these malfunctioning cells, normal function could be restored and the diseases cured.
Embryonic and Adult Stem Cells
Because embryonic stem cells have not undergone determination and differentiation and have the ability to become any tissue in the body, they are of great interest to scientists. As an embryo develops, its stem cells go through the process of determination and differentiation to create all the necessary tissues. To study embryonic stem cells, scientists must remove them from embryos, destroying the embryos (figure 11.11).
FIGURE 11.11. The Culturing of Embryonic Stem Cells
After fertilization of an egg with sperm, the cell begins to divide and form a mass of cells. Each of these cells has the potential to become any cell in the embryo. Embryonic stem cells may be harvested at this point or at other points in the determination process.
Scientists have also explored other methods of obtaining stem cells. Embryonic stem cells reach an intermediary level of determination at which they are committed to becoming a particular tissue type, but not necessarily a particular cell type. An example of this intermediate determination occurs when stems cells become determined to be any one of several types of nerve cells but have not yet committed to becoming any one nerve cell. Scientists call these partially determined stem cells “tissue-specific.” These types of stem cells can be found in adults. One example is hematopoietic stem cells. These cells are able to become the many different types of cells found in blood—red blood cells, white blood cells, and platelets (figure 11.12). The disadvantage of using these types of stem cells is that they have already become partially determined and do not have the potential to become every cell type.
FIGURE 11.12. The Differentiation of Blood Cells
One type of adult stem cell gives rise to various forms of blood cells. These stem cells are found in the red bone marrow, where they divide. Some of the stem cells differentiate and change their gene expression to become a specific cell type. The differentiated blood cells are shown across the bottom of the image.
Personalized Stem Cell Lines
Scientists hope that eventually it will be possible to produce embryonic stem cells from somatic cells by using somatic cell nuclear transfer techniques similar to that used for cloning a sheep. This technique would involve transferring a nucleus from the patient’s cell to a human egg that has had its original nucleus removed. The human egg would be allowed to grow and develop to produce embryonic stem cells. If the process of determination and differentiation can be controlled, new tissues, or even new organs, could be developed through what is termed regenerative medicine.
Under normal circumstances, organ transplant patients must always worry about rejecting their transplant and take strong immunosuppressant drugs to avoid organ rejection. Tissue and organs grown from customized stem cells would have the benefit of being immunologically compatible with the patient; thus, organ rejection would not be a concern (figure 11.13).
FIGURE 11.13. Customized Stem Cell Lines
One potential use of biotechnology is the production of customized stem cell lines. In this application a somatic cell from a patient would be inserted into a human egg from which the nucleus has been removed. The egg would divide and generate stem cells. These cells could then be cultured and used for therapy. In this example the stem cells could be used to create pancreatic cells to treat a diabetic patient.
The potential therapeutic value of stem cells has resulted in the founding of many clinics around the world. These clinics offer stem cell-based therapies to patients with a variety of medical conditions. However the benefits of these therapies are as yet unproven and, it fact, have the potential for serious harm. “Stem cell tourism” is a phrase being used to describe this industry. Desperate patients travel to these clinics in hopes that such therapies will save their lives. This new industry is teeming with “medical tourist traps” offering unproven medical treatments to unsuspecting consumers. Unfortunately, the days of customized stem cells, stem cell therapies, and organ culture are still in the future.
11.4. CONCEPT REVIEW
12. Embryonic stem cells are found in embryos, and adult stem cells are found in adults. In what other ways are they different?
13. What benefits does stem cell research offer?
14. What are some of the concerns with research on stem cells?