Stem cells are biological cells found in all multicellular organisms, that can divide (through mitosis) and differentiate into diverse specialized cell types and can self-renew to produce more stem cells. In mammals, there are two broad types of stem cells: embryonic stem cells and adult stem cells from various tissues.
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, which is also referred to as regenerative or reparative medicine.
Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity. Second, under certain physiologic or experimental conditions, they can be induced to become tissue- or organ-specific cells with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions.
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 as long as the person or animal is still alive. 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.
Like all scientific work involving human embryos, hES cell research raises profound questions about the status of the human embryo, the extent to which it is justifiable to use human embryos to expand knowledge and ameliorate human suffering, and the conditions under which these goals may be pursued.
The principle ethical and religious objection to hES cell research is that the derivation of hES cells involves the destruction of the blastocyst, which is regarded by some people as a human being. A second objection, which relates to blastocysts created for research purposes—whether through fertilization or NT—is that it is wrong to create a blastocyst with the intention of destroying it. A third objection is that some of the research depends on donor oocytes, which could result in the exploitation of women. In addition, some people are concerned about the mixing of human and nonhuman cells for research purposes. Finally, some object to the use of NT to derive hES cells because they fear that the use of NT for research purposes could lead to its use to produce a child.
Research on one kind of stem cell—human embryonic stem cells—has generated much interest and public debate. Pluripotent stem cells (cells that can develop into many different cell types of the body) are isolated from human embryos that are a few days old. Pluripotent stem cell lines have also been developed from fetal tissue (older than 8 weeks of development).
Perhaps the most important potential application of human stem cells is the generation of cells and tissues that could be used for cell-based therapies. Today, donated organs and tissues are often used to replace ailing or destroyed tissue, but the need for transplantable tissues and organs far outweighs the available supply. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.
There are many ways in which human stem cells can be used in research and the clinic. Studies of human embryonic stem cells will yield information about the complex events that occur during human development. A primary goal of this work is to identify how undifferentiated stem cells become the differentiated cells that form the tissues and organs. Scientists know that turning genes on and off is central to this process. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation. A more complete understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy.
Research on stem cells continues to advance knowledge about how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. Stem cell research is one of the most fascinating areas of contemporary biology, but, as with many expanding fields of scientific inquiry, research on stem cells raises scientific questions as rapidly as it generates new discoveries.
This procedure is known as nuclear transplantation, or somatic cell nuclear transfer (SCNT). It involves removing the nucleus, which contains a cell's DNA, from an egg cell and transplanting the DNA from an adult cell into the enucleated egg. Under certain conditions, the egg then begins to replicate as though it were a fertilized embryo.
After the egg divides for several days, it produces embryonic stem cells, which are primitive cells that can theoretically develop into virtually any type of cells in the organism, from blood cells to skin cells. Scientists believe that research on human stem cells could lead to new cures for many diseases. The use of nuclear transplantation to produce human stem cells is often referred to as "research cloning" or "therapeutic cloning."
If this entity is implanted into a uterus, it has the potential to develop into a full organism which would have the same DNA as the donor of the adult cell. In other words, the organism would be a "clone." This procedure is known as "reproductive cloning."
More recently several groups have explored the possibilities of disease modeling using patient derived iPSCs and directed differentiation technologies (7, 10-18). Since iPSCs resemble embryonic stem cells (ESCs) in their pluripotency, and offer potential solutions for histo-incompatibility issues that limit the use of ESC-based therapies, patient-specific iPSCs hold great potential as an unlimited cell source not only for generating disease models but also drug screening and cell replacement therapy for various diseases.