It is unknown if human embryonic stem cells are capable of establishing colonies in all tissues in the human embryo, but from their other properties, they are assumed to really be pluripotent cells, and so are considered as a possible source of differentiated cells for cellular therapies–the substitution of the patients diseased cell type for a healthier one.
Adult stem cells may be capable of self-renewal within the body, making identical copies of themselves over an organisms lifespan, or becoming specialized to produce cell types from their source tissues. Research on adult stem cells has been fueled by their ability to split or self-renew endlessly, and generate all of the cell types of the organs they come from–potentially rebuilding an entire organ from just a handful of cells.
There is a shortage of donor organs, but by directing stem cells to differentiate in certain ways, scientists can harness them to grow a particular type of tissue or organ. Cells to transplant were formerly available only from sources with a very limited supply, such as beta pancreatic cells obtained from cadavers of human organ donors.
Stem cells are also instrumental in expanding our understanding of embryonic and fetal development, helping us to identify cells and molecules that are responsible for driving normal (and aberrant) patterns in tissue and organ formation. In a three-to-five-day-old embryo called a blastocyst, inner cells produce the whole body of the three-to-five-day-old embryo, including all of the many types of specialized cells and organs, such as the heart, lungs, skin, eggs, and other tissues.