Adult stem cells,like all stem cells share at least two charecteristics. First,they make identical copies of themselves for long periods of time, this ability to proliferate is referred to as long-term self-renewal. Second,they can give rise to mature cell types that have charecteristic morphologies (shapes) and speliazed functions. Stem cells generate an intermediate cell type before they differantiate.The intermediate cells are called precursor or progenitor cell.Progenitor or precursor cells in fetal or adult tissues are partly differantiated cells that divide and give rise to differantiated cells.

Adult stem cells are rare. Their primary functions are to maintain the steady state functioning of a cell called homeostasis-and,with limitations,to replace cells that die because of injury or disease. Adult stem cells behave very differently depending on their local envirement.For example hemotopoetic stem cells are constantly being generated in the bone marrow where they differantiate into mature types of blood cells. The primary role of HSC is to replace blood cells.In contrast stem cells in small intestine are stationary and are physically seperated from the mature cell types they generate. The list of adult tissues reported to contain stem cells is growing and includes bone marrow, peripheral blood, brain, spinal cord,dental pulp,blood vessels,skeletal muscle,epithelia of the skin,digestive system,cornea,retina,liver and pancreas.

In order to be classified as an adult stem cell,the cell should be capable of self-renewal for the lifetime of the organizm. In addition, adult stem cells should also be clonogenic, in an other word, single adult stem cells should be able to generate a line of genetically identical cells, which give rise to all the appropriate, differantiated cell types of the tissues An adult stem cell should also be able to give rise to fully differantiated cells that have mature phenotypes,are fully integrated into the tissue , and are capable of specialized functions that are appropriate for the tissue.The term phenotype refers to all the observable charecteristics of a cell,its shape,interactions with other cells and the non cellular environment,proteins that appear on the cell surface and the cell’s behaviour etc.