Embryonic stem cells. Coloured scanning electron micrograph (SEM) of cultured mammalian embryonic stem cells. Embryonic stem cells are derived from blastocysts, hollow balls of cells that form after fertilisation and implant on the uterine wall. In culture, stem cells randomly differentiate into the cells that form the main embryonic tissue layers: the endoderm, ectoderm and mesoderm. These cells then undergo further differentiation to create all of the body's specialised cells. It is hoped that human embryonic stem cell development can be directed so that tissues, such as those needed for transplantation, can be produced. Magnification: x1,370 at 5x7cm size. x4850 at 6.5x9.5"~LANDSCAPE"