Three to five days after fertilization an embryo becomes a blastocyst and contains around 150 cells. The interior cluster of cells (around thirty cells) in the blastocyst is referred to as the inner cell mass. These cells are the pluripotent stem cells. The blastocyst also has an outer layer of cells called the trophoblast, and the blastocel, a fluid-filled cavity forming the material between the inner and outer clusters of cells. In early stem cell research the removal of the pluripotent stem cells resulted in the death of the embryo but in 2006 Lanza developed a way of removing the stem cells whilst keeping the blastocyst alive thus removing some of the ethical constraints on stem cell research.
