Stem cells: Cells that are able to (1) self-renew (can create more stem cells indefinitely) and (2) differentiate into (become) specialized, mature cell types.

Embryonic stem cells: Stem cells that are harvested from a blastocyst. These cells are pluripotent, meaning they can differentiate into cells from all three germ layers.

Embryonic stem cells are isolated from cells in a blastocyst, a very early stage embryo. Once isolated from the blastocyst, these cells form colonies in culture (closely packed groups of cells) and can become cells of the three germ layers, which later make up the adult body.

Adult stem cells (or Somatic Stem Cell): Stem cells that are harvested from tissues in an adult body. These cells are usually multipotent, meaning they can differentiate into cells from some, but not all, of the three germ layers. They are thought to act to repair and regenerate the tissue in which they are found in, but usually they can differentiate into cells of completely different tissue types.

Adult stem cells can be found in a wide variety of tissues throughout the body; shown here are only a few examples.

The Three Germ Layers: These are three different tissue types that exist during development in the embryo and that, together, will later make up the body. These layers include the mesoderm, endoderm, and ectoderm.

The three germ layers form during the gastrula stage of development. The layers are determined by their physical position in the gastrula. This stage follows the zygote and blastocyst stages; the gastrula forms when the embryo is approximately 14-16 days old in humans.

Endoderm: One of the three germ layers. Specifically, this is the inner layer of cells in the embryo and it will develop into lungs, digestive organs, the liver, the pancreas, and other organs.

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