Totipotency and lineage segregation in the human embryo

Abstract

During human preimplantation development the totipotent zygote divides and undergoes a number of changes that lead to the first lineage differentiation in the blastocyst displaying trophectoderm (TE) and inner cell mass (ICM) onDay5. TheTEis a differentiated epithelium needed for implantation and the ICMforms the embryo proper and serves as a source for pluripotent embryonic stem cells (ESCs). The blastocyst implants around Day7. The second lineage differentiation occurs in theICMafter implantation resulting in specification of primitive endoderm and epiblast. Knowledge on human preimplantation development is limited due to ethical and legal restrictions on embryo research and scarcity of materials. Studies in the human are mainly descriptive and lack functional evidence. Most information on embryo development is obtained from animal models and ESC cultures and should be extrapolated with caution. This paper reviews totipotency and the molecular determinants and pathways involved in lineage segregation in the human embryo, as well as the role of embryonic genome activation, cell cycle features and epigenetic modifications. © The Author 2014. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.