The stem cells are cells found in most of multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types.

The two broad types of mammalian stem cells are embryonic stem cells that are isolated from the inner cell mas of blastocysts, and the other, adult stem cells that are found in adult tissues.

The potential to differentiate into different cell types of the stem cell. Totipotent stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable, organism. These cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent. Pluripotent stem cells are the descendants of totipotent cells and can differentiate into nearly all cells, i.e. cells derived from any of the three germ layers. Multipotent stem cells can differentiate into a number of cells, but only those of a closely related family of cells e.g hematopoietic stem cells differentiate into red blood cells, white blood cells, platelets, etc. Oligopotent stem cells can differentiate into only a few cells, such as lymphoid or myeloid stem cells. The other , unipotent cells can produce only one cell type, their own, but have the property of self-renewal which distinguishes them from non-stem cells e.g. muscle stem cells.

In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of regenerative organs, such as skin, blood, and intestinal tissues.

Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow are routinely used in medical therapies.

Most adult stem cells are lineage-restricted or multipotent and are generally referred to by their tissue origin such mesenchymal stem cell, adipose derived stem cell, endothelial stem cell, etc.

Adult stem cell treatments have been successfully used for many years to treat leukemia and related bone or blood cancers through bone marrow transplants.

The use of adult stem cells in research and therapy is not as controversial as embryonic stem cells, because the production of adult stem cells does not require the destruction of an embryo. Additionally, because in some instances adult stem cells can be obtained from the intended recipient, (an autograft) the risk of rejection is essentially non-existent in these situations.

Some researchers believe that stem cell therapy has the potential to dramatically 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 to treat a wider variety of diseases including Parkinson's disease, cancer, spinal cord injuries, etc.