STEM CELLS AND ITS TYPES
STEM CELLS AND ITS TYPES
- Stem cells are undifferentiated, primitive cells that have the potential to develop into many different cell types such as muscles, kidney, liver, blood and even nerve cells, 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.
CHARACTERISTICS OF STEM CELLS DISTINGUISHING THEM FROM OTHER CELLS:
- They are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity.
- Under certain physiological 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.
- 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 to understand how to use these cells for cell-based therapies to treat disease, which is also referred to as regenerative or reparative medicine.
- 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.
TYPES OF STEM CELLS:
- Embryonic Stem Cells
- Non-embryonic/ somatic or adult stem cells
- Induced Pluripotent Stem Cells (iPSCs)
EMBRYONIC STEM CELLS
- Embryonic stem cells 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.
- Human embryonic stem cells (hESCs) are generated by transferring cells from a preimplantation-stage embryo into a plastic laboratory culture dish that contains a nutrient broth known as culture medium.
- The cells divide and spread over the surface of the dish. The inner surface of the culture dish is typically coated with mouse embryonic skin cells that have been treated so they will not divide. This coating layer of cells is called a feeder layer.
- The mouse cells in the bottom of the culture dish provide the cells a sticky surface to which they can attach. Also, the feeder cells release nutrients into the culture medium.
- Embryonic stem cells that proliferate in cell culture for a prolonged period of time without differentiating, are pluripotent, and don’t develop genetic abnormalities are referred to as an embryonic stem cell line.
NON EMBRYONIC STEM CELLS OR ADULT STEM CELLS
- An adult stem cell or somatic stem cell is thought to be an undifferentiated cell, found among differentiated cells in a tissue or organ that can renew itself and can differentiate to yield some or all of the major specialized cell types of the 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. Unlike embryonic stem cells, which are defined by their origin (cells from the preimplantation-stage embryo), the origin of adult stem cells in some mature tissues is still under investigation.
- Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis.
- They are thought to reside in a specific area of each tissue (called a “stem cell niche”). In many tissues, current evidence suggests that some types of stem cells are pericytes, cells that compose the outermost layer of small blood vessels.
- Stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain tissues, or by disease or tissue injury.
- Adult hematopoietic, or blood-forming, stem cells from bone marrow have been used in transplants for 40 years. If the differentiation of adult stem cells can be controlled in the laboratory, these cells may become the basis of transplantation-based therapies.
- Typically, there is a very small number of stem cells in each tissue, and once removed from the body, their capacity to divide is limited, making generation of large quantities of stem cells difficult.
- Scientists in many laboratories are trying to find better ways to grow large quantities of adult stem cells in cell culture and to manipulate them to generate specific cell types so they can be used to treat injury or disease.
- Some examples of potential treatments include regenerating bone using cells derived from bone marrow stroma, developing insulin-producing cells for type 1 diabetes, and repairing damaged heart muscle following a heart attack with cardiac muscle cells.
DIFFERENCE BETWEEN EMBRYONIC AND NON-EMBRYONIC STEM CELLS
- Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for cell-based regenerative therapies. One major difference between adult and embryonic stem cells is their different abilities in the number and type of differentiated cell types they can become.
- Embryonic stem cells can become all cell types of the body because they are pluripotent. Adult stem cells are thought to be limited to differentiating into different cell types of their tissue of origin.
- Embryonic stem cells can be grown relatively easily in culture. Adult stem cells are rare in mature tissues, so isolating these cells from an adult tissue is challenging, and methods to expand their numbers in cell culture have not yet been worked out.
- It is not yet known whether tissues derived from embryonic stem cells would cause transplant rejection. On the other hand, adult stem cells, and tissues derived from them, are currently believed less likely to initiate rejection after transplantation.
- This is because a patient’s own cells could be expanded in culture, coaxed into assuming a specific cell type (differentiation), and then reintroduced into the patient. The use of adult stem cells and tissues derived from the patient’s own adult stem cells would mean that the cells are less likely to be rejected by the immune system.
- This represents a significant advantage, as immune rejection can be circumvented only by continuous administration of immunosuppressive drugs, and the drugs themselves may cause deleterious side effects.
- Besides that there are no ethical questions involved in the research related to somatic stem cells as they are harvested from the patient. Whereas research involving embryonic stem cells, require destruction of human embryo which according to Pro-life group tantamount to destruction of human being.
INDUCED PLURIPOTENT STEM CELLS
- Induced pluripotent stem cells (iPSCs) are adult cells that have been genetically reprogrammed to an embryonic stem cell—like state by being forced to express genes and factors important for maintaining the defining properties of embryonic stem cells.
- Mouse iPSCs were first reported in 2006, and human iPSCs were first reported in late 2007. Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumours containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very early stage in development.
- Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all three germ layers. iPSCs are useful tools for drug development and modelling of diseases, and scientists hope to use them in transplantation medicine.
- Viruses are currently used to introduce the reprogramming factors into adult cells, and this process must be carefully controlled and tested before the technique can lead to useful treatments for humans.
- In animal studies, the virus used to introduce the stem cell factors sometimes causes cancers. Researchers are currently investigating non-viral delivery strategies. In any case, this breakthrough discovery has created a powerful new way to “de-differentiate” cells whose developmental fates had been previously assumed to be determined.
- In addition, tissues derived from iPSCs will be a nearly identical match to the cell donor and thus probably avoid rejection by the immune system. The iPSC strategy creates pluripotent stem cells that, together with studies of other types of pluripotent stem cells, will help researchers learn how to reprogram cells to repair damaged tissues in the human body.
SOURCES OF STEM CELLS
- Embryonic stem cells are harvested from the core of 5-7 days human embryos. Adult stem cells are derived from the bone marrow. Wharton’s Jelly in umbilical matrix is rich source of readily available Stem Cells.
- It is believed that wharton’s jelly may be the reservoir which contains primitive stem cells that are formed within few days after the fertilization of egg and remained preserved in that state.
POTENTIAL USES OF STEM CELLS
- 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. Predictably controlling cell proliferation and differentiation requires additional basic research on the molecular and genetic signals that regulate cell division and specialization.
- While recent developments with iPS cells suggest some of the specific factors that may be involved, techniques must be devised to introduce these factors safely into the cells and control the processes that are induced by these factors.
- Human stem cells could also be used to test new drugs. For example, new medications could be tested for safety on differentiated cells generated from human pluripotent cell lines. Other kinds of cell lines are already used in this way.
- Cancer cell lines, for example, are used to screen potential anti-tumor drugs. The availability of pluripotent stem cells would allow drug testing in a wider range of cell types. However, to screen drugs effectively, the conditions must be identical when comparing different drugs. Therefore, scientists will have to be able to precisely control the differentiation of stem cells into the specific cell type on which drugs will be tested.
- 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.
STEM CELL RESEARCH IN INDIA
- Stem cell research in India is carried out by various centres such as, National bioethics committee, Reliance Life Group, Mumbai and National Centre for biological science, Bangalore.
- National Task force on Stem Cell Research under the chairmanship of Dr D. Balasubramanian has proposed a priority fund to finance research activities in stem cell research.
- AIIMS became the first institute in the country to use stem cell for treating end-stage cardiac patients in the country, which has been authenticated by the Indian Council for Medical Research (ICMR).