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\b[Mouse <epiblast> formation involves two lineage separations, followed by a coordinated change in pluripotent state and epithelial architecture. Subsequent <gastrulation> then converts a patterned <epiblast> into embryonic germ-layer derivatives.] The <primitive endoderm> is an earlier extraembryonic lineage and must not be conflated with <definitive endoderm> generated during <gastrulation>.
During cleavage and compaction, differences in position and polarity help separate outer <trophectoderm> from inner <cells>. Polarized outer <cells> have relatively active nuclear <YAP>–<TEAD4> and promote a trophoblast programme including <CDX2>. In inner <cells>, contact-associated <Hippo signalling> and <LATS kinase> activity favour <YAP> phosphorylation and exclusion from the nucleus, supporting inner-<cell>-mass identity rather than the outer programme. Experimental manipulation of these factors changes lineage-associated expression, linking position-dependent signalling to fate.<The Hippo signaling pathway components Lats and Yap pattern Tead4 activity to distinguish mouse trophectoderm from inner cell mass by Noriyuki Nishioka and others>[2009, pp. 398–410, YAP localization and YAP/TEAD4 perturbation]{ref} Position is therefore interpreted through regulatory machinery, rather than being an immutable label attached at the first cleavage.
Within the <inner cell mass>, initially overlapping expression of <NANOG> and <GATA6> resolves into an interspersed population of <epiblast>-biased and primitive-<endoderm>-biased <cells>. <FGF4> from <epiblast>-biased <cells> acts through <FGF receptors> and <ERK> signalling to promote primitive-<endoderm> maturation, including <SOX17> and <GATA4>. Lower FGF/<ERK> activity favours the <NANOG>-positive <epiblast> programme. This is a combination of <cell>-intrinsic differences and communication that adjusts the relative lineage allocation. <FGF4>-null analysis shows that initial primitive-<endoderm>-associated expression can occur without normal later restriction and maintenance; <FGF4> should not simply be described as creating all early <GATA6> expression.<FGF4 is required for lineage restriction and salt-and-pepper distribution of primitive endoderm factors but not their initial expression in the mouse by Minjung Kang and others>[2013, pp. 267–279, early coexpression versus later lineage restriction]{ref} <Grb2>/MAPK-pathway <mutants> and pathway manipulation independently support its role in the mouse segregation process.<Early lineage segregation between epiblast and primitive endoderm in mouse blastocysts through the Grb2-MAPK pathway by Claire Chazaud and others>[2006, pp. 615–624, lineage markers and Grb2-dependent signalling]{ref}
Subsequent <cell> sorting and positional stabilization then place <primitive endoderm> on the <blastocyst>-cavity-facing surface, with <epiblast> deeper inside and <trophectoderm> outside. Active <cell> movement, changes in adhesion and removal of some misplaced <cells> contribute to the final arrangement; the initial salt-and-pepper molecular pattern is not already a complete layered tissue. The preimplantation <epiblast> is a pluripotent population with <OCT4>, <Sox2> and <NANOG>-associated circuitry. Its normal descendants supply the <embryo> proper and some extraembryonic <mesoderm>, whereas <trophectoderm> and <primitive endoderm> establish distinct supporting tissues.
\b[Around <implantation>, roughly E4.5–E5.5, <epiblast> <cells> acquire an organized apical–basal axis and a central lumen.] Matrix supplied around the <epiblast> provides a basal cue through <integrins>. The <cells> arrange into a polarized rosette, with apical domains directed toward the centre and basal surfaces toward the <basement membrane>. Apical membrane separation and lumen opening produce the <proamniotic cavity>, followed by expansion into the cup-shaped <epiblast> of the <egg cylinder>. This architecture places <epiblast> adjacent to <extraembryonic ectoderm> and enveloped by <visceral endoderm>, creating a geometry for reciprocal signalling. Live <embryo> observations and matrix-dependent pluripotent-<cell cultures> demonstrated that obligatory central <apoptosis> is an inadequate explanation of normal cavity formation.<Self-organizing properties of mouse pluripotent cells initiate morphogenesis upon implantation by Ivan Bedzhov and Magdalena Zernicka-Goetz>[2014, pp. 1032–1044, embryo imaging, integrin/matrix dependence and rosette formation]{ref} <apoptosis> can occur and affect abnormal cultures, but its presence does not show that the physiological lumen is carved by <cell death>.
The morphological change accompanies exit from <naive pluripotency>, through acquisition of formative competence and toward a primed <epiblast> state. Naive regulators are downregulated, signalling dependence and <chromatin accessibility> change, and the <cells> become responsive to germ-layer-inducing cues. These states are developmental transitions within a lineage, not three separate <cell> ancestries.<Pluripotent state transitions coordinate morphogenesis in mouse and human embryos by Marta N. Shahbazi and others>[2017, pp. 239–243, state manipulation and epithelial organization]{ref} Embryonic stem-<cell> and <epiblast>-stem-<cell cultures> model parts of the transition, but culture media and matrix can uncouple molecular state from embryonic timing. Competence should therefore be tested by response to defined differentiation signals, with marker expression used as a readout rather than the sole definition of <pluripotency>.
By approximately E5.5–E6.5, communication among <epiblast>, <visceral endoderm> and <extraembryonic ectoderm> establishes an anterior–posterior asymmetry. <Nodal signalling> participates both in posterior <epiblast> specification and in patterning the overlying <visceral endoderm>. Distal <visceral endoderm> <cells> become positioned anteriorly, establishing the <anterior visceral endoderm>. Antagonists including <CER1>, <LEFTY1> and <DKK1> limit posteriorizing Nodal/Wnt activity anteriorly; extraembryonic <BMP signalling>, Wnt and Nodal interactions favour the posterior primitive-streak region. Genetic analysis demonstrates that Nodal has different effects in the embryonic and neighbouring extraembryonic compartments, rather than acting as one uniform instruction everywhere.<Nodal signalling in the epiblast patterns the early mouse embryo by Jane Brennan and others>[2001, pp. 965–969, epiblast signalling and extraembryonic patterning]{ref} A simple isolated <epiblast> culture therefore lacks important spatial inputs of the <embryo>.
\b[At <gastrulation>, a <cell>'s position, competence, signal history and time of passage through the <primitive streak> jointly influence its fate.] Posterior <epiblast> <cells> activate <mesendoderm>-associated programmes, locally dismantle <basement membrane> and undergo an <epithelial-to-mesenchymal transition>. They lose or reorganize apical junctions, cross the streak and migrate between existing layers. <FGF signalling> through <FGFR1> helps coordinate this behaviour by regulating <Snail>, <E-cadherin> and <mesoderm>-associated <genes> including <Brachyury> and <Tbx6>. Fate and movement are experimentally separable, even though a regulator can influence both.<FGF signaling regulates mesoderm cell fate specification and morphogenetic movement at the primitive streak by Brian Ciruna and Janet Rossant>[2001, pp. 37–49, FGFR1, Snail/E-cadherin and mesodermal marker analysis]{ref}
The <cells> contributing to <definitive endoderm> activate factors including <SOX17> and <FOXA2> and insert into the outer endodermal <epithelium>; <mesoderm> spreads between <endoderm> and the remaining <epiblast>. Earlier or more anterior streak contributions include axial and anterior <mesendoderm>, while other positions and times supply paraxial, lateral, cardiac and extraembryonic mesodermal populations. These associations are a dynamic <fate map>, not proof that every pre-streak <cell> is irreversibly determined. Stronger or more prolonged Nodal/SMAD activity supports particular anterior <mesendoderm> and definitive-<endoderm> contributions in a context-dependent manner. Conditional <SMAD2> loss can preserve streak formation and <gastrulation> movements while disrupting anterior <definitive endoderm> and prechordal progenitor specification, directly distinguishing fate choice from a general inability to gastrulate.<Cell fate decisions within the mouse organizer are governed by graded Nodal signals by Stéphane D. Vincent and others>[2003, pp. 1646–1662, epiblast-conditional Smad2 perturbation and organizer-derived fates]{ref}
The <epiblast> remaining outside the ingressing populations forms <ectoderm>. Its neural versus surface-<ectoderm> choices depend on the local balance of BMP, FGF and antagonistic organizer-derived signals, alongside the timing of competence. Posterior progenitors can continue to supply neural and mesodermal tissues during axis elongation, so <gastrulation> is not one instantaneous partition of a homogeneous sheet. Nor does <primitive endoderm> simply become the entire embryonic gut: <definitive endoderm> formation and intercalation must be followed by <lineage tracing>. Conversely, <visceral endoderm> descendants are not all excluded from the gut <epithelium>: direct tracing demonstrates intercalation of embryonic and extraembryonic histories.<The endoderm of the mouse embryo arises by dynamic widespread intercalation of embryonic and extraembryonic lineages by Gloria S. Kwon and others>[2008, pp. 509–520, live imaging and lineage labels]{ref} To test the proposed sequence, timed <genetic perturbations> should be paired with live imaging, endogenous lineage-marker combinations and descendant tracing. Mosaic or tissue-restricted perturbation distinguishes an <epiblast>-autonomous requirement from loss of a supporting extraembryonic signal, and measurements of proliferation and <apoptosis> prevent altered <cell> numbers from being mistaken for fate conversion. \b[The central mechanism is reciprocal signalling acting on a changing, polarized and competent <epiblast>, with <cell> movements both implementing and modifying the resulting pattern.]
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