Mechanism of cell polarisation and first lineage segregation in the human embryo

Abstract

The formation of differential cell lineages in the mammalian blastocyst from the totipotent zygote is crucial for implantation and the success of the whole pregnancy. The first lineage segregation generates the polarised trophectoderm (TE) tissue, which forms the placenta, and the apolar inner cell mass (ICM), which mainly gives rise to all foetal tissues and also the yolk sac1-3. The mechanism underlying this cell fate segregation has been extensively studied in the mouse embryo4,5. However, when and how it takes place in the human embryo remains unclear. Here, using time-lapse imaging and 325 surplus human embryos, we provide a detailed characterisation of morphological events and transcription factor expression and localisation to understand how they lead to the first lineage segregation in human embryogenesis. We show that the first lineage segregation of the human embryo is triggered by cell polarisation that occurs at the 8-cell stage in two sequential steps. In the first step, F-actin becomes apically polarised concomitantly with embryo compaction. In the second step, the Par complex becomes polarised to form the apical cellular domain. Mechanistically, we show that activation of Phospholipase C (PLC) triggers actin polarisation and is therefore essential for apical domain formation, as is the case in mouse embryos6. Finally, we show that, in contrast to the mouse embryo, the key extra-embryonic determinant GATA37,8 is expressed not only in extra-embryonic lineage precursors upon blastocyst formation. However, the cell polarity machinery enhances the expression and nuclear accumulation of GATA3. In summary, our results demonstrate for the first time that cell polarisation reinforces the first lineage segregation in the human embryo.

Reconstructing human early embryogenesis in vitro with pluripotent stem cells

“…2f-g, Extended Data 3c) . In accord with recent findings in the human embryo 36 , our results further support the role of the acquisition of apicobasal polarity in promoting the expression and nuclear localisation of GATA3 to drive TE specification in early human development.…”

“…3b). Finally, we confirmed the relationship between polarisation and outer cell commitment by using siRNA transfection to knock-down (KD) PLCB1 in order to deplete PLC activity 36 in cells during 3D aggregation (Fig. 2g) .…”

“…Previous studies have demonstrated that the first lineage segregation event begins with compaction and cell polarisation in the mouse embryo at the 8-cell stage 32–34 . This process remains unclear in human early embryogenesis and only recently have studies begun to shed light on these events 35,36 . Hence, we utilised our platform to analyse the establishment and dynamics of compaction and cell polarisation at the early time-points of multicellular aggregate formation.…”

“…2e). We have recently shown that the PLC-Protein Kinase C (PKC) pathway controls cellular polarisation at early stages of human development 36 . Of note, when U73122 (a selective PLC inhibitor) was applied at 2 μM and 3 μM in hEPSC 3D cultures, there was significant reduction of the nuclear GATA3 signal intensity (Fig.…”

Morphogenesis of the human preimplantation embryo: bringing mechanics to the clinics

“…Apical markers such as PRKCz, PARD6B or AMOT are observed at the apical domain of human embryos (Toolbox 2). The accumulation of PARD6B also seems to occur about 90 h post fertilization [114]. The human embryo then consists of at least 16 cells and compaction has completed [10,12,103].…”