Primitive node

The primitive node, also referred to as Hensen's node in avian embryos, is a transient embryonic structure that emerges at the cranial end of the primitive streak during the third week of human development, serving as the primary organizer for gastrulation and the establishment of the three germ layers (ectoderm, mesoderm, and endoderm).^[1] It forms through the ingression of epiblast cells at a higher rate around the primitive pit, a small depression within the node, initiating epithelial-to-mesenchymal transition and driving the migration of cells to form definitive endoderm and mesoderm while preserving ectoderm on the surface.^[2] This structure is maintained by key transcription factors such as hepatocyte nuclear factor 3β (HNF-3β), encoded by the FOXA2 gene, which regulate its signaling pathways including TGF-β, Wnt, Nodal, and BMP to pattern the embryo.^[2] Functionally, the primitive node plays a critical role in axial development by giving rise to the notochordal process, a midline structure that extends cranially from the node toward the prechordal plate and later induces neural tube formation through secretion of morphogens like Sonic hedgehog (Shh), fibroblast growth factor (FGF), and retinoic acid (RA).^[3] It establishes the embryo's craniocaudal, dorsoventral, and left-right axes, ensuring proper orientation and polarity during early organogenesis, and its caudal migration during gastrulation contributes to the regression of the primitive streak by the end of the fourth week.^[1] Disruptions in primitive node formation or function, such as failure to regress, can lead to congenital anomalies including sacrococcygeal teratomas, highlighting its essential role in normal embryonic patterning.^[3]

Overview and Definition

Definition and Structure

The primitive node is a transient organizer structure located at the anterior end of the primitive streak in amniote embryos during gastrulation.^[4] It serves as a critical site for embryonic patterning in mammals, analogous to Hensen's node in avian embryos and the Spemann-Mangold organizer in amphibians.^[5] Structurally, the primitive node arises as a localized thickening of epiblast cells, forming a distinct primitive knot or node at the cranial terminus of the primitive streak.^[3] This thickening features a central depression known as the primitive pit, which communicates with the underlying notochordal canal and facilitates cellular movements during early development.^[6] The overall morphology presents as a compact, mound-like elevation on the embryonic disc.^[7] The cellular composition of the primitive node consists primarily of epiblast-derived cells that have undergone partial epithelial-to-mesenchymal transition.^[7] These include presumptive notochord progenitors destined to contribute to the axial midline and organizer cells that maintain the structural integrity of the node.^[4] In early mammalian embryos, such as those of the mouse, the node comprises a small cluster of approximately 200-300 tightly packed cells, exhibiting a diameter on the order of 100-200 μm.^[8]

Embryonic Location and Timing

The primitive node is located at the cranial (anterior) end of the primitive streak, situated on the epiblast surface of the bilaminar embryonic disc.^[1] It lies adjacent to the primitive pit, a slight depression within the node itself, and is positioned between the amniotic cavity superiorly and the yolk sac inferiorly.^[9] This placement positions the node as a key landmark on the dorsal aspect of the early embryo. In human development, the primitive node emerges around days 15-16 post-fertilization, during the third week (Carnegie stages 7-8), coinciding with the initial formation and elongation of the primitive streak.^[1] It remains active throughout much of week 3, facilitating processes such as cell ingression during gastrulation, before beginning to regress as the notochordal process extends cranially in early week 4.^[9] The node is oriented on the dorsal side of the embryonic disc, directly opposite the hypoblast (or primitive endoderm) layer on the ventral side.^[1] In preserved human embryos, it appears as a distinct nodal thickening, observable through techniques such as scanning electron microscopy, which reveals its surface morphology and cellular organization.^[10]

Evolutionary and Comparative Aspects

Diversity Across Species

In mammals such as mice and humans, the primitive node is a compact structure forming at the anterior end of a relatively short primitive streak, measuring approximately 0.3–0.5 mm in length. This node consists of a small cluster of cells with a central pit from which notochord precursors directly ingress, and it undergoes rapid regression shortly after formation, typically within hours during early gastrulation stages around embryonic day 6.5–7.5 in mice.^[11][12] In birds, exemplified by chickens, the equivalent structure known as Hensen's node is larger and more prominent, forming a distinct mound up to 300 μm in diameter at the anterior terminus of a longer primitive streak spanning about 1.2–2 mm. This node migrates posteriorly along the streak over a distance of roughly 2 mm as gastrulation progresses, allowing for extended cell ingression and contributing to a more prolonged organizational role compared to mammals.^[13][11] Reptiles and monotremes exhibit intermediate morphologies, often featuring variable primitive streak lengths or homologous structures like blastoporal plates rather than a fully elongated streak, reflecting their oviparous nature and larger yolk reserves. In reptiles such as turtles (Trachemys scripta) and chameleons (Chamaeleo calyptratus), gastrulation involves a bi-modal process with a slit- or circular-shaped opening (approximately 50 μm in diameter in chameleons) that facilitates ingression without a pronounced node, though conserved cell fates indicate homology to the primitive node in other amniotes. Monotremes like the platypus develop a primitive streak within the embryonal area of the yolk sac, bridging reptilian and therian mammalian forms, but these remain less studied due to limited accessible embryos.^[14][15][16] These variations highlight species-specific adaptations, particularly longer persistence of organizer structures in oviparous amniotes like birds and reptiles to accommodate extended gastrulation on yolky eggs, in contrast to the swift, compact process in viviparous mammals. Shared gene expression patterns, such as those involving Brachyury, further support the homology of these organizers across amniotes.^[15][11]

Homologies with Non-Amniote Organizers

The primitive node in amniotes exhibits functional homologies with the Spemann-Mangold organizer in amphibians, particularly in their roles during gastrulation. In amphibian embryos, the Spemann-Mangold organizer is located at the dorsal lip of the blastopore, where cells ingress to form mesodermal structures and induce the overlying ectoderm to differentiate into neural tissue.^[17] This organizer's capacity for axis induction mirrors that of the primitive node, which similarly orchestrates the formation of the anterior-posterior axis through ingression of cells at the anterior end of the primitive streak.^[18] In teleost fish such as zebrafish, the embryonic shield serves as a homologous organizing center, positioned at the dorsal margin of the gastrula where it drives the convergence of cells to establish axial mesendoderm without forming a distinct primitive streak.^[19] The shield induces secondary axis formation and contributes to midline structures, akin to the primitive node's role in amniotes, with both relying on nodal-related signaling for their inductive properties.^[20] These homologies reflect evolutionary conservation across vertebrates, where organizers like the primitive node arise from signaling centers analogous to the amphibian Nieuwkoop center, which patterns the dorsal mesoderm via underlying endodermal interactions.^[21] Fate-mapping studies further support this, showing that cells from these organizers contribute to shared derivatives, including the notochord and somites, underscoring a common developmental logic.^[20] Key evidence for these homologies comes from classical transplantation experiments, beginning with Spemann and Mangold's 1924 work demonstrating that grafting the dorsal blastopore lip induces a complete secondary axis in host amphibian embryos.^[17] This principle extends to amniotes, where transplanting Hensen's node (the avian equivalent of the primitive node) to ectopic sites induces secondary axes with neural and mesodermal components, as shown in chick embryos.^[22] Similar heterospecific grafts, such as node tissue into fish hosts, highlight conserved inductive potential across taxa, though with varying efficiency due to species-specific contexts.^[23] Gene expression patterns, such as overlaps in goosecoid and chordin, provide molecular corroboration for these functional parallels.^[24]

Formation and Dynamics

Induction and Initial Formation

The formation of the primitive node in amniote embryos is induced by signaling from the posterior marginal zone (PMZ), a region analogous to the Nieuwkoop center in non-amniotes, which establishes the initial bilateral symmetry breaking and axis formation.^[25] In chick embryos, the PMZ emits diffusible signals that promote epiblast competence for gastrulation, preventing streak formation elsewhere in the epiblast.^[13] This induction process relies on cooperative interactions between canonical Wnt signaling, which activates downstream targets like brachyury in the posterior epiblast, and Vg1 (a TGF-β family member), which synergizes with Wnt to initiate streak gene expression.^[26] FGF signaling further supports this by regulating cell motility and proliferation in the nascent streak, ensuring proper convergence without directly initiating the axis.^[11] The hypoblast contributes indirectly by creating inhibitory gradients of Nodal, Wnt, and BMP antagonists (such as Cerberus), which position the streak posteriorly while restricting ectopic induction.^[27] Initial cellular events begin with the convergence of epiblast cells toward the posterior midline, driven by polarized cell rearrangements and chemotactic responses to PMZ signals.^[28] These cells, originating from the posterior epiblast adjacent to Koller's sickle, accumulate at the prospective anterior end of the emerging primitive streak, forming a localized thickening through oriented proliferation and early ingression via epithelial-to-mesenchymal transition.^[29] The first node cells express brachyury (T), a T-box transcription factor marking nascent mesendoderm, which is activated in this converging population prior to full node maturation.^[30] In chick embryos, primitive node formation coincides with primitive streak initiation at Hamburger-Hamilton stage 4, approximately 18-19 hours post-fertilization, when the streak reaches approximately 1.8 mm in length and Hensen's node becomes distinct at its anterior tip.^[31] In humans, this process occurs around day 15 post-fertilization, aligning with the onset of the definitive streak in the bilaminar disc.^[11] These events establish the node's architecture as a transient organizer, preceding its posterior regression along the streak.

Migration, Regression, and Cellular Changes

Following its initial formation, the primitive node in amniote embryos undergoes posterior regression along the primitive streak, a process that deposits notochord precursors to establish the midline structure of the developing embryo. In the chick, this regression begins around Hamburger-Hamilton (HH) stage 7 and progresses caudally, covering approximately 1-2 mm over 12-18 hours as the node moves from the center of the area pellucida toward the posterior margin.^[32][33] This movement facilitates the sequential addition of axial mesoderm, contributing to body axis extension in a single sentence.^[34] During regression, significant cellular changes occur within the node. Cells ingress through the primitive pit, a central depression in the node, undergoing epithelial-to-mesenchymal transition to form prechordal mesoderm that migrates anteriorly ahead of the notochord.^[32] Concurrently, resident node cells differentiate into the notochordal plate, which later cavitates to form the definitive notochord, while the node's epithelial structure maintains its organizer function.^[35] These transformations involve shape changes, such as cells adopting a bottle-like morphology during ingression, increasing local cell density.^[32] The regression of the primitive node is primarily driven by differential proliferation and apoptosis at its margins, coupled with convergent cell movements in the underlying mesoderm. Higher proliferation rates in posterior regions of the primitive streak relative to the node promote caudal displacement, while targeted apoptosis refines the structure and prevents overextension.^[36] Intercalation of axial mesoderm cells further facilitates this posterior shift by reorganizing the tissue.^[37] By the endpoint of regression, remnants of the primitive node integrate into the tail bud, serving as a source for posterior neuromesodermal progenitors. In the chick, complete regression occurs by HH stage 10 (approximately 33 hours post-incubation), while in humans, this process concludes around day 18 post-fertilization at Carnegie stage 9.^[32][38]

Developmental Roles

Gastrulation and Germ Layer Specification

During gastrulation in the mouse embryo, the primitive node serves as a critical site for the ingression of epiblast cells, which undergo epithelial-to-mesenchymal transition (EMT) primarily at the primitive pit within the node and along the adjacent primitive streak. This process initiates around embryonic day 6.5 (E6.5), where epiblast progenitors destined for the definitive endoderm and mesoderm migrate toward and through the node, delaminating from the epithelial epiblast layer to become mesenchymal cells. The ingressing definitive endoderm cells displace the pre-existing visceral endoderm (hypoblast equivalent) layer, progressively replacing it to form the embryonic endoderm that will line the gut tube. This displacement occurs as the new endodermal cells intercalate and expand, pushing the visceral endoderm toward extraembryonic regions by E7.5-E8.0.^[39][40] Mesoderm formation similarly depends on the primitive node's role in orchestrating cell ingression, with progenitors from the node's flanks contributing to distinct mesodermal subtypes. Cells ingress through the lateral aspects of the node to generate axial mesoderm (including notochord precursors) and paraxial mesoderm, while more posterior streak regions supply lateral and extraembryonic mesoderm. This orderly allocation ensures that axial mesoderm arises from the earliest ingressions at the node during mid-gastrulation (around E7.0), followed by lateral mesoderm from later ingressions. The EMT at the primitive pit facilitates this by allowing cells to lose epithelial adhesions and gain migratory properties, enabling their dispersal beneath the epiblast.^[41][42] The remaining epiblast cells that do not ingress during gastrulation differentiate into the ectoderm layer, which subsequently specifies into neurectoderm and surface ectoderm. By the end of gastrulation (E7.5), these non-migratory epiblast cells form the dorsal layer of the trilaminar embryonic disc, positioned above the newly formed mesoderm and endoderm. This establishes the three definitive germ layers—ectoderm, mesoderm, and endoderm—setting the foundation for organogenesis, with the primitive node's ingression dynamics ensuring precise spatiotemporal allocation of progenitors.^[43]

Body Axis Organization and Notochord Formation

The primitive node serves as a key organizer in establishing the anterior-posterior (A-P) axis during mammalian gastrulation. Positioned at the anterior end of the primitive streak, it coordinates the ingression of epiblast cells that give rise to midline structures, thereby defining embryonic polarity. As gastrulation progresses, the primitive node's posterior regression is essential for A-P axis elongation; this movement deposits axial mesoderm along the midline, with the anterior node region specifying head organizers and the posterior node contributing to trunk and tail progenitors.^[44][45] This regression-driven process ensures that the notochord, derived from node cells, extends caudally to pattern the A-P axis, providing a scaffold for somitogenesis and neural tube formation. Disruptions in node regression, such as in certain mutants, lead to truncated axes and defective midline development.^[45][46] In parallel, the primitive node contributes to dorsal-ventral (D-V) patterning by secreting BMP antagonists, such as noggin and chordin, which inhibit ventralizing BMP signals in the overlying ectoderm. This localized inhibition promotes dorsal neural fates, preventing epidermal differentiation in the midline region.^[47][48] The node's influence aligns with the default model of neural induction, where BMP inhibition defaults ectodermal cells to a neural fate rather than epidermal. This mechanism underlies the formation of the neural plate along the dorsal midline, with the node acting as the primary source of inhibitory signals during early patterning.^[48] Central to these organizing functions is the induction of the notochord from the primitive node. Cells ingressing through the primitive pit—a depression within the node—form the notochordal process, a transient rod-like structure that integrates with the endoderm. This process subsequently canalizes, detaching to form the definitive notochord, which extends along the embryonic midline.^[44][47] The notochord, once established, induces the floor plate in the ventral neural tube, reinforcing D-V patterning and supporting neural tube closure. This induction ensures proper ventral midline specification, with the notochord providing sustained signals for floor plate differentiation throughout neurulation.^[44][49] These roles of the primitive node in axis organization and notochord formation are conserved across vertebrate species, reflecting an ancient developmental strategy.^[45]

Molecular Mechanisms

Key Genes and Expression Patterns

The primitive node, as the mammalian equivalent of the Spemann-Mangold organizer, exhibits spatially restricted expression of core transcription factors that delineate its subregions and functional domains. Goosecoid (Gsc) is prominently expressed in the central and anterior portions of the node during mid-gastrulation (around embryonic day 6.5-7.0 in mice), marking cells destined for head organizer activity.^[50] In contrast, Brachyury (T) shows strong expression in the posterior node and adjacent primitive streak, identifying nascent paraxial and lateral mesoderm progenitors that ingress during gastrulation.^[51] Notochord precursors within the node's pit and emerging midline express characteristic markers that persist into axial elongation. Sonic hedgehog (Shh) initiates in the node pit at early somite stages, delineating the midline notochord domain, while Foxa2 (also known as HNF-3β) is co-expressed in the same region, overlapping with Shh to specify definitive axial mesendoderm.^[52] These patterns highlight the node's role in generating midline structures. Distinct expression zones further subdivide the node along its anterior-posterior axis. The anterior node compartment features BMP antagonists such as Chordin and Noggin, which are restricted to organizer cells fated for prechordal mesoderm and neural plate induction.^[53] Posteriorly, Fgf8 and Wnt3a are enriched, promoting primitive streak elongation and posterior mesoderm specification through localized signaling gradients.^[54][55] Expression dynamics in the node evolve with gastrulation progression, particularly during posterior regression. Genes like Shh and Foxa2 exhibit upregulation as node cells migrate caudally to form the notochord, ensuring sustained midline patterning. These spatiotemporal patterns are largely conserved across amniotes, with similar anterior-posterior gradients observed in chick Hensen's node.^[11]

Signaling Pathways and Interactions

The primitive node orchestrates embryonic patterning through interconnected signaling pathways, primarily involving TGF-β family members, BMP antagonists, Wnt/FGF ligands, and Hedgehog signals. These pathways exhibit cross-talk and form morphogen gradients that specify cell fates and guide tissue organization during gastrulation. Nodal signaling, a core TGF-β pathway, operates via an autocrine loop within the primitive node to sustain organizer activity and epiblast competence. In mouse embryos, Nodal expression in the node is positively autoregulated through enhancers like the asymmetric enhancer (ASE), ensuring persistent signaling that induces proprotein convertases such as Furin and Pace4, which in turn activate Nodal ligands to form feedback loops supporting primitive streak extension.^[56] Nodal gradients, extending up to 500 μm from the node, pattern the primitive streak along the anteroposterior axis by differentially activating mesendodermal genes, with high anterior levels promoting organizer maintenance and lower posterior levels driving streak elongation; these gradients are modulated by inhibitors like Lefty to prevent ectopic signaling.^[56] BMP inhibition is a critical mechanism emanating from the primitive node to promote neural induction by antagonizing ventralizing BMP4 signals in the overlying ectoderm. Noggin and Chordin, secreted from node cells, directly bind and sequester BMP4, reducing its activity and allowing default neural fate specification in the ectoderm; in mouse Noggin mutants, initial neural plate formation occurs, but subsequent neural tube growth and ventral patterning are impaired due to unchecked BMP signaling.^[57] This antagonism integrates with Nodal pathways, as node-derived signals prime ectodermal responsiveness to BMP blockade, forming a cooperative gradient for broad neural competence during early gastrulation stages.^[58] Wnt and FGF pathways intersect at the primitive node to regulate mesoderm specification and migration. Posterior Wnt3a expression in the primitive streak, influenced by node signals, promotes paraxial mesoderm formation by stabilizing β-catenin and activating T-box transcription factors, while node-derived FGF8 drives epithelial-to-mesenchymal transition and directed migration of mesodermal progenitors away from the streak; Fgf8 null mice exhibit arrested cell exodus from the primitive streak, preventing node maturation and axial structure development.^[54] This integration ensures posterior mesoderm identity, with FGF8 gradients overlapping Wnt domains to coordinate streak regression and somitogenesis onset. Sonic hedgehog (Shh) signaling from the primitive node establishes ventral midline identities and induces floor plate formation through interactions with Gli transcription factors. Shh secreted by node cells creates a ventral-high gradient that activates Gli2 in midline progenitors, specifying floor plate fate during early somitogenesis; transient high Shh levels (≥4 nM) are required for floor plate elaboration, after which signaling attenuates to prevent conversion to neuronal progenitors, with FoxA2 mediating this down-regulation.^[59] Gli3 acts repressively in adjacent domains, sharpening the ventral boundary and integrating Shh with BMP/Wnt signals for dorsoventral patterning. Recent studies highlight heterogeneity in primitive node subpopulations, influencing signaling potency and pathway integration. Single-cell analyses reveal dynamic shifts in node cellular composition, with anterior Gsc-positive cells dominating early inductive phases for cephalic fates via enhanced Nodal/BMP antagonism, while posterior Lmo1-positive cells prevail during regression, boosting Wnt/FGF outputs for trunk mesoderm; these changes modulate overall organizer potency, as grafting experiments show subpopulation-specific induction outcomes.^[60] Complementary work positions the node as a cooperative of signaling subsets, each expressing distinct ligands (e.g., Fgf8-enriched groups), whose heterogeneity ensures robust cross-talk across pathways for neural and axial development.^[61]

Experimental Models and Insights

Studies in Model Organisms

Studies in model organisms have been instrumental in elucidating the function of the primitive node, drawing from classical transplantation experiments and modern genetic tools to demonstrate its role as an organizer in embryonic axis formation and patterning. Foundational work by Hans Spemann and Hilde Mangold in 1924 established the concept of the embryonic organizer through transplantation experiments in amphibian embryos, where grafting the dorsal lip of the blastopore (analogous to the primitive node) induced a secondary axis, revealing inductive capacities that pattern the body plan.^[62] This discovery laid the groundwork for identifying organizer homologs across vertebrates, influencing subsequent studies on node function. In chick embryos, Hensen's node serves as the avian equivalent of the primitive node and has been extensively studied via transplantation assays. In the 1930s, Conrad Hal Waddington demonstrated organizer activity by transplanting Hensen's node to ectopic sites, inducing secondary neural structures and confirming its inductive role in amniote embryos.^[63] More recent fate mapping experiments, such as those using vital dyes to label node cells, have traced their contributions to notochord, somites, and floor plate, highlighting the node's role in generating midline structures during gastrulation.^[64] Additionally, electroporation techniques enable targeted gene knockdown in Hensen's node; for instance, introducing morpholinos or RNAi constructs disrupts specific pathways, leading to axis elongation defects and providing insights into nodal regulation.^[65] Mouse models have advanced understanding through genetic manipulation and lineage tracing of the primitive node. Null mutants of the Nodal gene (Nodal^{-/-}) exhibit severe axis defects, including failure of primitive streak formation and absence of anterior structures, underscoring Nodal signaling's essential role in node specification and function. Node-specific Cre recombinase lines, such as Noto-Cre, allow precise lineage tracing of node-derived cells, revealing their contributions to the notochord and vertebral column while avoiding off-target labeling in other primitive streak lineages.^[66] These tools have facilitated conditional knockouts, confirming the node's organizer properties in mammals akin to those in other vertebrates. In zebrafish, the embryonic shield acts as the functional analog to the primitive node, amenable to grafting experiments that mirror classical organizer assays. Transplantation of the shield to ventral regions induces ectopic axes, demonstrating its inductive capacity for dorsal mesoderm and neural tissue formation, as shown in microsurgical studies at early-shield stages.^[19] These approaches, building on earlier work by Jane Oppenheimer in the 1930s, have clarified the shield's role in gastrulation movements and body axis establishment, providing a transparent model for live imaging of node-like dynamics.

Recent Advances and Clinical Relevance

Recent studies from 2020 to 2025 have elucidated the cellular heterogeneity within the primitive node, highlighting its dynamic composition during gastrulation. In avian embryos, Hensen's node, the equivalent organizer structure, undergoes temporal changes where early stages are enriched with anterior definitive endoderm cells that drive neural induction, while later stages incorporate posterior progenitors contributing to axial elongation. This segregation shapes the node's inductive capacity and has been characterized through single-cell transcriptomics of microdissected tissues.^[60] In vitro modeling using human pluripotent stem cells (hPSCs) has advanced the understanding of primitive streak and node formation, particularly for generating notochord progenitors. A 2024 protocol induces anterior primitive streak patterning in hPSCs via sequential activation of BMP, Wnt, and Nodal signaling, yielding FOXA2+ notochord-like cells that mimic node-derived lineages and enable high-throughput analysis of human gastrulation. These models reveal transcriptional trajectories from epiblast to organizer cells, bypassing ethical constraints of natural embryos.^[67] Live imaging techniques have provided insights into transcription factor dynamics during early fate segregation relevant to node specification. A 2025 study employed endogenously tagged reporters in mouse embryos to track Nanog, Sox2, and Gata6 fluctuations, demonstrating oscillatory patterns that bias epiblast toward primitive endoderm fates prior to primitive streak emergence. Such dynamics inform the regulatory networks active in nascent node cells.^[68] Dysfunctions in primitive node signaling contribute to human developmental disorders, notably neural tube defects (NTDs) like spina bifida through disrupted Sonic hedgehog (Shh) pathway activation. The node-derived notochord induces ventral neural tube patterning via Shh secretion; impairments in this axis, often linked to genetic variants in Shh regulators, lead to incomplete neural tube closure. Sonic hedgehog signaling plays a critical role in floor plate induction and NTD etiology.^[69] Primitive node errors also underlie situs inversus, where defective Nodal signaling disrupts left-right asymmetry. Cilia-driven Nodal flow in the node establishes asymmetric gene expression (e.g., left-sided Nodal and Pitx2); mutations in Nodal or its effectors cause randomized organ situs. Recent analyses link these to primary ciliary dyskinesia variants affecting node monocilia.^[70] Stem cell-based embryo models, including blastoids and gastruloids, recapitulate primitive node formation and hold promise for drug screening in node-related pathologies. Blastoids, derived from naive hPSCs, form primitive streak-like structures with organizer gene expression (e.g., Brachyury, Goosecoid), enabling toxicity assays for NTD therapeutics. Gastruloids further model post-node axial patterning, supporting high-content screens for Shh modulators. A 2025 overview of peri-gastrulation models emphasizes their scalability for identifying teratogens.^[71]