Introduction........................................................................................................................................................................................................................................................................................................................ 183 Origin of the visceral endoderm.................................................................................................................................................................................................................................................. 183 Migration and functional differentiation of visceral endoderm cells.......................................................................................................................... 184 Role of the visceral endoderm in nutrient uptake and delivery......................................................................................................................................... 184 Visceral endoderm and cavitation........................................................................................................................................................................................................................................... 185 Role of visceral endoderm as an anterior organizer............................................................................................................................................................................. 186 Visceral endoderm factors critical for early development........................................................................................................................................................... 186 A. Transcription factors........................................................................................................................................................................................................................................................................ 186 B. Growth factors, receptors and intracellular signal transducing molecules................................................................................. 193 C. Other factors.................................................................................................................................................................................................................................................................................................. 195 Factors normally not expressed in visceral endoderm which influence its function.................................................................. 196 Developmental abnormalities associated with defects in imprinting and X-chromosome inactivation............. 197 Conclusions and future directions........................................................................................................................................................................................................................................... 198 Summary................................................................................................................................................................................................................................................................................................................................ 199