Keratin dynamics govern the establishment of the maternal-fetal interface

Abstract

After implantation, the mouse embryo undergoes gastrulation and forms mesoderm and endoderm. Mesoderm participates in embryonic and extra-embryonic tissues including the amnion, yolk sac, chorion and allantois, the umbilical cord precursor. Extra-embryonic mesoderm is rich in intermediate filaments. Two-photon live imaging of keratin 8-eYFP knock-in embryos allowed recording nucleation and elongation of keratin filaments, which formed apical cables coordinated across multiple cells in amnion, allantois, and blood islands. Embryos lacking all keratins displayed a deflated exocoelomic cavity, a narrow thick amnion, and a short allantois, indicating a hitherto unknown role for keratin filaments in post-implantation extra-embryonic membranes morphogenesis. Single-cell RNA sequencing of mesoderm cells, microdissected amnion, chorion, and allantois provided an interactive atlas of transcriptomes with germ layer and regional information. Keratin 8high mesenchymal cells in contact with the exocoelom shared a cytoskeleton and adhesion expression profile that might explain the adaptation of extra-embryonic structures to the increasing mechanical pressure.

The extended analogy of extraembryonic development in insects and amniotes

“…In both insects and amniotes, by the onset of gastrulation there are multiple early genetic markers for the presumptive serosa/chorion. These include Tc-zen1 , Tc-zen2 and Tc-hnt for Tribolium [52] and Cdx2 , Elf5 and Esrrb in Mus [53,54] (table 2). Upstream regulation of the presumptive insect serosa requires a subset of axial patterning determinants for anterior and dorsal regions of the blastoderm (figure 2, e.g.…”

“…In amniotes, where the amnion is an EE ectodermal–mesodermal bilayer, the mesoderm is critical for these properties. The amniotic mesoderm in Homo and Mus expresses high levels of NRP1/Nrp1, POSTN/Postn, COL1A1/Col1a1, TAGLN/Tagln, ACTA2/Acta2 and FN1/Fn1 [44,54,75], which are responsible for conferring both elasticity and strength. In Mus embryos defective for Fn1, gastrulation initiated and the knockout embryos formed EE mesoderm, showed a ‘closed’ amnion and chorion, and have an allantois, but the exocoelomic and amniotic cavities appeared to have defective pressure and distended shape [93].…”

“…In other cases, insect orthologues hold promise as a novel line of evidence in selecting new candidate genes for research into the amniote amnion (box 1). Transcriptomic datasets for the amnion have been generated for Mus [53,54] and Homo [74]. Moreover, single-cell RNA-seq datasets for gastrulating embryos of mouse [78,79] and human [75] are available.…”

The development of the amnion in mice and other amniotes

“…The cluster of PGCs (cluster 9) showed expression of Pou5f1 and Ifitm3 and specific high levels of Ptn , encoding a heparin-binding growth factor not previously associated with mouse PGCs, but shown to have a mitogenic effect in PGCs in pig [37]. From the five clusters of extraembryonic mesoderm (clusters 3, 5, 6, 8 and 10), cluster 5 and cluster 6 expressed many different Hox genes, suggestive of the base of the allantois, whereas cluster 10 showed high-specific levels of Pitx1 and Twist2 present in the tip of the extending allantois [38].…”

“…Exploring the expression of the few known markers of amniotic ectoderm, such as Wnt6 and Lrp2 [38] (figure 4 c ), suggested that the population of amniotic ectoderm may be contained within cluster 2 (surface ectoderm). This subpopulation of cluster 2 was also positive for Krt7 .…”