Mesenchymal-Epithelial Transition Regulates Initiation of Pluripotency Exit before Gastrulation

The pluripotent epiblast gives rise to all tissues and organs in an adult body. Its differentiation starts at gastrulation when the epiblast generates mesoderm and endoderm germ layers through a process called epithelial-mesenchymal transition (EMT). Although gastrulation EMT coincides with loss of epiblast pluripotency, pluripotent cells in development and in culture can adopt either mesenchymal or epithelial morphology. The relationship between epiblast pluripotency and epiblast morphology is not well understood. In this work, using chicken epiblast and human induced pluripotent stem cell (hiPSC) models, we show that pluripotent cells undergo an obligatory mesenchymal-epithelial transition (MET) prior to EMT-associated pluripotency loss. Epiblast MET and the subsequent EMT are two distinct processes. The former, a partial MET, is associated with reversible pluripotency exit; whereas the latter, a full EMT, is associated with complete and irreversible pluripotency loss. We provide evidence that integrin-mediated cell-matrix interaction, but not E-cadherin-mediated cell-cell interaction, is a key player in pluripotency exit regulation. We propose that epiblast partial MET is an evolutionarily conserved process among all amniotic vertebrates and its developmental function is to mediate planar symmetry-breaking within an epithelialized epiblast, taking place after epiblast MET but before gastrulation EMT. Using the chick embryonic model, we generated transcriptomic datasets of the avian epiblast and area pellucida during the first 14h of embryonic development, with a special focus on genes associated with epithelialization and pluripotent state of the epiblast. Total RNA from 0.5h, 5h, 9h and 14h incubated area pelucida (AP) epiblast samples (two sets of independent samples) were isolated and used for gene expression profiling.