Investigating cellular heterogeneities and SOX2 expression during preimplantation development

Abstract

In contrast to invertebrate models of embryonic development, where symmetry breaking is spatially and temporally well-defined, early mammalian embryos lack maternally inherited or fertilization-induced asymmetries that would instruct subsequent cell fate decisions. Instead, cell fates are thought to gradually emerge during mammalian embryo development. A key question in the field is whether cell fates emerge in a stochastic fashion, or whether systematically emerging heterogeneities among cells can influence fate decisions. Previous work has shown that SOX2 expression, a key transcription factor of the inner cell mass lineage, is initiated in a heterogenous manner among inner cell mass cells. Moreover, this heterogeneity was linked to the initiation of subsequent differentiation events. This study investigates whether early morphological and temporal features of cells —blastomere volume, surface exposure, and division timing—correlate with the earliest induction of SOX2. Using live imaging of endogenously tagged SOX2 in preimplantation mouse embryos, lineage trajectories were reconstructed from the 8-cell to 32-cell stages. Morphological parameters including blastomere volume, relative exposed surface area, and division timing were quantified and compared between SOX2-positive and SOX2-negative inner cell mass cells. No consistent associations were found between early SOX2 induction and either volume or relative surface exposure at the 16-cell stage. However, it was qualitatively observed that SOX2-positive ICM cells frequently divided later than their SOX2-negative counterparts, suggesting that delayed division may facilitate transcriptional priming.