Amniotic ectoderm expansion occurs via distinct modes and requires SMAD5-mediated signalling

Abstract

Upon gastrulation, the mammalian conceptus transforms rapidly from a simple bilayer into a multilayered embryo enveloped by its extra-embryonic membranes. Impaired development of the amnion, the innermost membrane, causes major malformations. To clarify the origin of the mouse amnion, we used single-cell labelling and clonal analysis. We identified four clone types with distinct clonal growth patterns in amniotic ectoderm. Two main types have progenitors in extreme proximal-anterior epiblast. Early descendants initiate and expand amniotic ectoderm posteriorly, while descendants of cells remaining anteriorly later expand amniotic ectoderm from its anterior side. Amniogenesis is abnormal in embryos deficient in the bone morphogenetic protein (BMP) signalling effector SMAD5, with delayed closure of the proamniotic canal, and aberrant amnion and folding morphogenesis. Transcriptomics of individual mutant amnions isolated before visible malformations and tetraploid chimera analysis revealed two amnion defect sets. We attribute them to impairment of progenitors of the two main cell populations in amniotic ectoderm and to compromised cuboidal-to-squamous transition of anterior amniotic ectoderm. In both cases, SMAD5 is crucial for expanding amniotic ectoderm rapidly into a stretchable squamous sheet to accommodate exocoelom expansion, axial growth and folding morphogenesis.

Opening the black box: Stem cell–based modeling of human post-implantation development

“…Moreover, even though the mouse does not develop an amniotic sac structure at all and begins amnion formation after gastrulation begins, BMP signaling is critical for amniogenesis in that model as well. In mice, BMP ligands are expressed at the embryonic/extraembryonic border of the cup-shaped cylinder, and Smad5 mutants show defects in that border, as well as aberrant amnion and chorion development (Pereira et al, 2011; Dobreva et al, 2018). A recent detailed clonal analysis of amniotic ectoderm progenitors in the mouse shows that these cells arise from the epiblast cells at this border and expand both anteriorly and posteriorly (Dobreva et al, 2018).…”

“…In mice, BMP ligands are expressed at the embryonic/extraembryonic border of the cup-shaped cylinder, and Smad5 mutants show defects in that border, as well as aberrant amnion and chorion development (Pereira et al, 2011; Dobreva et al, 2018). A recent detailed clonal analysis of amniotic ectoderm progenitors in the mouse shows that these cells arise from the epiblast cells at this border and expand both anteriorly and posteriorly (Dobreva et al, 2018). Loss of Smad5 compromises both clonal expansion of these progenitors and squamous morphogenesis of their progeny.…”

“…Apart from the apparent conservation of a requirement for BMP signaling during amniogenesis, our current understanding of the mechanisms underlying human amniotic ectoderm formation and maturation is rudimentary at best. The possibility that a mechanical signal could initiate amniogenesis in mouse (Pereira et al, 2011; Dobreva et al, 2018) and human (Shao et al, 2017a) seems likely, but it is not clear how that signal is generated or how it activates downstream BMP signaling. Moreover, though RNaseq studies suggest that BMP activates de novo expression of several transcription factors (Shao et al, 2017a), the specific roles that these factors play in amniogenesis remain to be elucidated.…”

Understanding the Mechanobiology of Early Mammalian Development through Bioengineered Models

“…While important insights have been obtained by extracting and manipulating embryos ex vivo, key contributions might come from the very mechanical and geometrical constraints imposed by the uterus itself (Hiramatsu et al, 2013;Matsuo and Hiramatsu, 2017). Indeed, it has been found that the mechanics (i.e., the mechanical properties, forces, and strains) of the extraembryonic environment or of the embryo proper directly underpin key developmental events such as AP (Hiramatsu et al, 2013) or LR axis establishment (Chien et al, 2018), notochord emergence (Imuta et al, 2014), or amnion development (Dobreva et al, 2018;Shao et al, 2017) in multiple embryonic systems. Moreover, direct links between mechanics and developmental events have been drawn and shown to be evolutionarily conserved (Brunet et al, 2013;Pukhlyakova et al, 2018).…”

“…Deflation experiments by Imuta et al (2014) have shown for example that the anisotropic forces exerted on the EPI by the expansion of the amniotic cavity drives the convergent-extension of cells during notochord morphogenesis ( Figure 1C). Additionally, mechanotransducing pathways have been posited to feed into the oscillatory networks driving somitogenesis (Hubaud et al, 2017) and mechanical cues may even guide the development of the amnion, in mouse as in human (Dobreva et al, 2018;Shao et al, 2017;Taniguchi et al, 2018). Even more interestingly, mechanosensitive gene expression has been documented in various developmental models (Brunet et al, 2013) as well as in sea anemones where prolonged embryo compression licenses ectopic expression of the gastrulation marker T/Brachyury (T/Bra) (Pukhlyakova et al, 2018).…”