Stem cells exist at various times and tissues during the human body development. Depending on when and where they have derived from as well as their diverse abilities to differentiate, stem cells can be categorized into the following types:

I. Embryonic stem cells
Embryonic stem cells are cells that have derived from the blastocyst of the embryo. This type of stem cells is pluripontent and gives rise during development to all derivatives of the three primary germ lines: ectoderm, endoderm and mesoderm. In other words, they can develop into each of the more than 200 cell types of the adult body when given sufficient and necessary stimulation for a specific cell type. However, their use either for research or transplantation gives rise to several moral issues.

II. Adult stem cells
Adult or somatic stem cells exist throughout the body after embryonic development and are found inside of different types of tissue. These stem cells have been found in tissues such as the brain, bone marrow, blood, blood vessels, skeletal muscles, skin, and the liver. It is generally thought that adult stem cells are limited in their ability to differentiate based on their tissue of origin, but there is some evidence to suggest that they can differentiate in order to take the form of other cell types.

III. Cord Blood Stem cells
Cord blood stem cells are cells that exist in the cord blood and the placenta. Until recently, cord blood and the placenta were thought to be biological waste and were thrown away. However, it was found that both cord blood and the placenta are rich sources of stem cells. Cord blood stem cells are multipotent stem cells that give rise to blood cell types including myeloid (monocytes and macrophages, neutrophils, eosinophils, erythrocytes, platlets) and lymphoid (T-cell, B-cell, NK-cells) lineages.

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