Stem cells: what they are and what they can do
Stem cells are undifferentiated biological cells found in multicellular organisms that can differentiate into specialized cells and can divide (through mitosis) to produce more stem cells. According to the Canadian stem cell foundation, they start in the embryo as unprogrammed cells and then become specialized to create bone, muscle, skin, the heart, the brain, and over 250 other types of specialized cells. Stem cells can repair and replace tissue in the human body. In other words, stem cells have the power to heal. That is, 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.
In adult organisms, stem cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm. For example, the tissues of the skin are constantly renewed by stem cells and damaged tissues in the muscles due to injuries are repaired by stem cells.
It all started around 3 decades ago in 1981 when scientists discovered ways to derive embryonic stem cells from early mouse embryos. The detailed study of the biology of mouse stem cells led to the discovery, in 1998, of a method to derive stem cells from human embryos and grow the cells in the laboratory. These cells are called human embryonic stem cells. The embryos used in these studies were created for reproductive purposes through in vitro fertilization procedures. When they were no longer needed for that purpose, they were donated for research with the informed consent of the donor. In 2006, researchers made another breakthrough by identifying conditions that would allow some specialized adult cells to be “reprogrammed” genetically to assume a stem cell-like state called induced pluripotent stem cells (iPSCs).
Embryonic stem cells
Embryonic stem cells, as their name suggests, are derived from embryos. Most embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro—in an in vitro fertilization clinic—and then donated for research purposes with informed consent of the donors. They are not derived from eggs fertilized in a woman’s body.
Adult stem cells
An adult stem cell or somatic stem cells is an undifferentiated cell, found among differentiated cells in a tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. In fact, adult hematopoietic, or blood-forming, stem cells from bone marrow have been used in transplants for more than 40 years. Scientists now have evidence that stem cells exist in the brain and the heart, two locations where adult stem cells were not at first expected to reside. If the differentiation of adult stem cells can be controlled in the laboratory, these cells may become the basis of transplantation-based therapies.
Sources of adult stem cells
There are three known accessible sources of autologous adult stem cells in humans:
- Bone marrow, which requires extraction by harvesting, that is, drilling into bone (typically the femur or iliac crest).
- Adipose tissue (lipid cells), which requires extraction by liposuction.
- Blood, which requires extraction through apheresis, wherein blood is drawn from the donor (similar to a blood donation), and passed through a machine that extracts the stem cells and returns other portions of the blood to the donor.
Stem cells can also be taken from umbilical cord blood just after birth. Of all stem cell types, autologous harvesting involves the least risk. By definition, autologous cells are obtained from one’s own body, just as one may bank his or her own blood for elective surgical procedures.
All stem cells—regardless of their source—have three general properties which are:
- Self renewal – Ability to divide and renew themselves for long periods. This is called long-term renewal. A starting population of stem cells that proliferates for many months in the laboratory can yield millions of cells.
- Potency – This specifies the ability of stem cells to differentiate into different cell types. Base on this, stem cells can be classified as follows:
- Totipotent (a.k.a. omnipotent): these are stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism
- Pluripotent: are the descendants of totipotent cells and can differentiate into nearly all cells,e. cells derived from any of the three germ layers.
- Multipotent: can differentiate into a number of cell types, but only those of a closely related family of cells
- Oligopotent: can differentiate into only a few cell types, such as lymphoid or myeloid stem cells
- Unipotent cells can produce only one cell type, their own
Generation of cells and tissues that could be used for cell-based therapies is considered perhaps the most important application of the stem cell technology. Today, donated organs and tissues are often used to replace ailing or destroyed tissues, but the need for transplantable tissues and organs far outweighs the available supply thus constituting a serious challenge. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases such as macular degeneration, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis and even type 1-diabetes.
Challenges to the application of the stem cell technology
To realize the promise of novel cell-based therapies for pervasive and debilitating diseases, scientists must be able to manipulate stem cells so that they possess the necessary characteristics for successful differentiation, transplantation, and engraftment. The following is a list of steps in successful cell-based treatments that scientists will have to learn to control to bring such treatments to the clinic. To be useful for transplant purposes, stem cells must be reproducibly made to:
- Proliferate extensively and generate sufficient quantities of cells for making tissue.
- Differentiate into the desired cell type(s).
- Survive in the recipient after transplant.
- Integrate into the surrounding tissue after transplant.
- Function appropriately for the duration of the recipient’s life.
- Avoid harming the recipient in any way.
Stem cell treatment
Details of stem cell therapy will be discussed on this platform soon but the following are diseases and conditions where stem cell treatment is being investigated include:
- Rheumatoid arthritis
- Parkinson’s disease
- Alzheimer’s disease
- Stroke and traumatic brain injury repair
- Learning disability due to congenital disorder
- Spinal cord injury repair
- Heart infarction
- Anti-cancer treatments
- Baldness reversal
- Replace missing teeth
- Repair hearing
- Restore vision and repair damage to the cornea
- Amyotrophic lateral sclerosis
- Crohn’s disease
- Wound healing
- Male infertility due to absence of spermatogonial stem cells
Disadvantages of stem cell treatments
Stem cell treatments sometimes require suppression of the immune system because of a requirement for radiation before the transplant to remove the person’s previous cells, or because the patient’s immune system may target the stem cells. One approach to avoid the second possibility is to use stem cells from the same patient who is being treated.
Pluripotency in certain stem cells could also make it difficult to obtain a specific cell type. It is also difficult to obtain the exact cell type needed, because not all cells in a population differentiate uniformly. Undifferentiated cells can create tissues other than desired types.
Some stem cells form tumors after transplantation; pluripotency is linked to tumor formation especially in embryonic stem cells, fetal proper stem cells, induced pluripotent stem cells.
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