What are embryonic stem cells?

What is the period when embryonic stem cells can produce any type of cell?

• Reworded: As in, when is there embryonic stem cells in an embryo and when can they produce any type of cell?

• Answer:

  Totipotent (totally potent) embryonic stem cells are capable of forming every type of body cell. Each totipotent cell could replicate and differentiate and become a human being. All cells within the early embryo are totipotent up until the 16 cell stage or so. Next are the pluripotent embryonic stem cells which can develop into any of the three major tissue types: endoderm (interior gut lining), mesoderm (muscle, bone, blood), and ectoderm (epidermal tissues and nervous system). Pluripotent stem cells can eventually specialize in any bodily tissue, but they cannot themselves develop into a human being.