1. Primitive streak and gastrulation

Human Gastrulation and Experimental Models

Direct analysis of human embryos has identified transcriptionally distinct populations associated with the primitive streak and early germ-layer formation, including cells expressing TBXT (Brachyury), EOMES, and other mesendodermal regulators. These data provide evidence for the spatial and transcriptional organisation of early human gastrulation, although marker expression and lineage relationships are stage-dependent. Because experimental access to human embryos limits causal genetic manipulation, many molecular and cellular mechanisms of primitive streak formation, epithelial-to-mesenchymal transition (EMT), ingression, germ-layer specification, and tissue morphogenesis have been defined principally in mouse and chick embryos.

Signalling Networks Establishing the Primitive Streak

In the mouse, posterior embryonic axis patterning and primitive streak formation depend on coordinated Nodal, Wnt, BMP, and FGF signalling, with distinct functions for each pathway. Wnt3 is essential for primitive streak formation, while canonical Wnt signalling through CTNNB1/β-catenin and TCF/LEF-dependent transcription regulates primitive-streak-associated gene expression. Nodal signalling is required for posterior epiblast patterning and mesendoderm specification. Nodal is synthesised as a precursor and undergoes proteolytic activation involving Furin and PACE4; its activity is further regulated by antagonists including Lefty1 and Lefty2.

Extraembryonic tissues contribute to spatial patterning. The anterior visceral endoderm expresses antagonists including Cer1 and Dkk1, restricting posteriorising activities from anterior embryonic regions. BMP4 from the extraembryonic ectoderm participates in the regulatory network controlling posterior epiblast signalling and streak formation. FGF signalling contributes to EMT, cell migration, and mesoderm morphogenesis.

EMT, Ingression, and Germ-Layer Formation

At the primitive streak, epiblast cells undergo epithelial remodelling and ingression, involving changes in cell adhesion and polarity, basement-membrane remodelling, delamination, and acquisition of migratory properties. Snai1 contributes to the regulation of epithelial adhesion, including repression of Cdh1/E-cadherin. Eomes functions in primitive-streak-associated progenitors during definitive endoderm specification and the generation of particular mesodermal populations.

Gastrulation establishes all three primary germ layers. Cells entering the primitive streak generate mesodermal and definitive endodermal progenitors, whereas epiblast cells remaining outside the streak do not undergo ingression and contribute predominantly to the ectoderm. Primitive-streak formation therefore spatially segregates populations undergoing mesendodermal specification from epithelial cells retaining ectodermal potential; subsequent signalling interactions further pattern neural and non-neural ectoderm.

Progenitor Fate and Tissue Morphogenesis

TBXT and EOMES have distinct developmental functions. TBXT is required for normal posterior mesoderm development and axial morphogenesis, whereas Eomes is essential for definitive endoderm specification and contributes to specific mesodermal progenitor populations. Fate mapping and temporal analyses show that progenitor fates emerge in characteristic spatial and temporal patterns during passage through the streak, consistent with differential signalling responsiveness, transcriptional states, and progenitor history.

In chick embryos, fate mapping, grafting, electroporation, and live imaging have resolved cell ingression and mediolateral rearrangements. Wnt-dependent planar cell polarity signalling contributes to oriented cell behaviours, intercalation, and convergent extension.

Experimental Approaches

These mechanisms have been investigated using gene disruption, chimeric embryos, lineage tracing, in situ hybridisation, immunostaining, reporter alleles, and quantitative live imaging. Single-cell RNA sequencing and spatial transcriptomics extend these analyses to human and other vertebrate embryos. Micropatterned human pluripotent stem-cell colonies and gastruloids enable controlled perturbation using CHIR-99021, XAV939, LDN-193189, WNT3A, BMP4, Activin A, and FGF ligands. These models reproduce defined aspects of primitive-streak-associated signalling and germ-layer specification but complement rather than replace direct analysis of embryonic primitive streak formation and gastrulation.