During embryonic development, primitive ectoderm forms three primary germ layers, the mesoderm, the ectoderm, and the endoderm. These germ layers interact forming all the tissues and organs of the developing embryo. The influences controlling the transition of ectoderm to visceral and parietal endoderm in the blastocyst, followed by the formation of mesoderm at gastrulation, are only beginning to be defined. In the mouse, this process occurs between d 3 and 7 post-fertilization, and as such, it is both difficult to monitor and to experimentally influence. With this in mind, many groups have used mouse embryonic stem (ES) cells, and more recently human ES cells, to study the control of germ layer formation and their subsequent differentiation.
