Stem cells are a part of each person's individual biology.
Stem cells are rapidly dividing cells that give rise to specialized cells. Totipotent stem cells contain genetic coding capable of forming a complete organism. For example, a fertilized human egg is a totipotent stem cell. These totipotent stem cells, after several days of cell division, form a hollow sphere called a blastocyst. The outer layer of cells of the blastocyst forms the placenta while the inner layer of cells can form almost any type of tissue.