Analysis of factors controlling primary germ layer formation and early hematopoiesis using embryonic stem cell in...

Differentiation and subsequent development are intricately interwoven processes operating as an integrated whole to form the organism. As an approach to examine these interactions in early mammalian development, we used embryonic stem (ES) cell in vitro differentiation. ES cells can, depending upon the environment differentiated to neuroectoderm, mesoderm and hematopoietic cells. We developed a serum-free, chemically defined medium (CDM) in which ES cells survive and differentiate. In CDM, in the absence of exogenous factors, ES cells form neuroectoderm, upregulating the early neural marker Pax-6. This is consistent with the view that neuroectoderm development can represent a default state, where the absence or sequestration of mesoderm inducing factors permits neuroectoderm formation. In contrast, if CDM is supplemented with bone morphogenetic protein (BMP) 2 or 4 a process resembling primitive streak formation, least at the molecular level occurs, with the formation of mesoderm and subsequently endothelial and hematopoietic cells. If used with care, ES cell in vitro differentiation can act as a guide in understanding the environment which controls early differentiation events in mammals.