Regression of trophectodermal protrusions in the mouse blastocyst correlates with primitive endoderm formation

Abstract This study examined trophectoderm (TE) protrusions covering the inner cell mass (ICM) during blastocyst development. Using fluorescein-labelled latex beads in E3.5 and E4.5 mouse embryos, we assessed the presence of TE protrusions (TP) and their relationship to primitive endoderm (PrE) formation. Beads were endocytosed by outer embryonic cells, enabling visualization of protrusions by confocal microscopy. We observed that TE protrusion coverage of the ICM decreased as blastocyst cell numbers increased, regressing entirely by E4.5. At this stage, no protrusions remained and the ICM exhibited a distinct PrE layer following cell sorting.