Stem cells are immature tissue precursor cells that have the capacity to self-renew and differentiate into multiple different cell types. They can be found in the embryo, as well as in several fetal and postnatal (adult) organs and tissues. Characteristics of stem cells Stem cells have two important characteristics that distinguish them from other types of cells. Self-renewal - the ability to go through numerous cycles of cell division while maintaining the undifferentiated state. Plasticity - under specific physiologic or experimental conditions, they can be induced to become specialized cells such as the beating cells of the heart muscle or insulin-producing cells of the pancreas.

The two main categories of stem cells are embryonic and adult stem cells. Embryonic stem cells are derived from the developing embryo. These cells are described as pluripotent, as they have the ability to give rise to cells of every organ system of the adult. ESC also have unlimited self-renewal capacity, due to which they can spontaneously give rise to tumours. ESC therefore pose several risks for use in cell therapy, and areused only for research. Adult stem cells are of the following types: Hematopoietic stem cells (HSC): They are found in bone marrow and cord blood. These stem cells give rise to the different types of blood cells. Tissue specific stem cells: They are found in adult tissues (Eg. Cardiac stem cells, Neural stem cells, pancreatic stem cells etc). They are committed precursor cells which help in repair and maintenance of the respective organs. Mesenchymal stromal cells (MSC): They are found in almost all postnatal organs such as bone marrow stroma, fat tissue, umbilical cord and placenta. MSC are described as multipotent as they are capable of giving rise to multiple cell types, and they do not have unlimited self-renewal capacity like ESC. MSC are therefore non-tumorigenic and safe for use in cell therapy.

Regenerative medicine is a promising area of science that encompasses cell-based therapy and tissue engineering to repair and rebuild diseased or damaged tissues. Stem cell research is continually advancing knowledge on how an organism develops from a single cell and how healthy cells are derived from precursor cells to replace damaged tissue. These fundamental principles can be exploited for designing strategies for cell therapy and tied in with tissue engineering practices for developing treatments for diseases which do not yet have a cure..

Adult stem cells are safe, non-tumorigenic, and have no ethical issues for use in cell-based therapies. Mesenchymal stromal cells or MSC have received wide attention in regenerative medicine as they have multiple benefits in repair of diseased and damaged tissues. These cells are best known for their ability to regenerate bone, cartilage, blood vessels and muscle; they are also capable of giving rise to other cells such as nerve cells and liver cells under specialized conditions. Therefore, MSC have therapeutic potential in a number of medical conditions such as bone repair, osteoarthritis, repair of heart muscle affected by myocardial infarction, wound healing in case of burns or diabetic ulcers, vascularization of ischemic limb etc. MSC also have potent anti-inflammatory property which contributes to their therapeutic activity, and makes them candidates for cell therapy in immune-related conditions such as Crohn's disease, inflammatory bowel disease and multiple sclerosis. MSC have practical advantages as they can be easily isolated from bone marrow or fat tissue, and expanded in large scale to obtain sufficient cells for clinical use. They are also stable under long term storage, and do not require tissue matching for transplantation. MSC can therefore be received from any unrelated donor. Hence they are ideal candidates for off-the-shelf cell based therapy required for managing acute and chronic conditions, with single or multiple dose regimens.

Studies of human embryonic stem cells may yield information about the complex events that occur during human development. Researchers believe that stem cell therapy has the potential to radically change the treatment of human disease. A number of adult stem cell therapies already exist, particularly bone marrow transplants that are used to treat leukemia. In the future, medical researchers anticipate being able to use technologies derived from stem cell research for 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 Parkinson's and Alzheimer's diseases, spinal cord injury, Cerebral stroke, burns, Multiple Sclerosis, Motor Neuron Disease, Duchenne's muscular dystrophy, ischemic heart disease, diabetes mellitus, Critical Limb Ischemia, vision and hearing loss, osteoarthritis, rheumatoid arthritis and many more. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation. A better understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy. 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 stem cell lines. Cancer cell lines, for example, are used to screen potential anti-tumor drugs. But, the availability of pluripotent stem cells would allow drug testing in a wider range of cell types.