Eyeballs just don’t become toenails — even though the same genome sits in the nucleus of every cell. The difference is in the parts of the genome that are expressed — a cell’s identity is determined by the specific genes that are active within that cell. The differentiation of a cell, and a cell’s commitment to become a particular type, have been long considered irreversible processes.
Stem cells (embryonic ES or induced iPS cells) have not yet committed to become any particular cell type, and they exist in an entirely undifferentiated stage. Only stem cells can become any type of cell in a particular organism –- the property of pluripotency. Pluripotent stem cells become the slightly more differentiated, slightly more distinctive, multipotent adult stem or progenitor cells –- neural stem cells can become any type of cell in the nervous system; hematopoietic stem cells can become any type of cell in the blood.