3. Notochord formation and body axis establishment

Specification of Axial Mesoderm and Chordamesoderm Progenitors

In humans, direct mechanistic evidence for notochord formation remains comparatively limited; consequently, the cellular and molecular framework is derived principally from experimentally tractable vertebrate models, particularly mouse, chick, Xenopus, and zebrafish. During gastrulation, axial mesoderm progenitors arise from organizer-associated territories, including the primitive streak and node in amniotes and the dorsal organizer region in amphibians, and subsequently generate chordamesodermal and notochordal lineages. Nodal signaling plays central, context-dependent roles in mesendoderm induction, organizer formation, and specification of axial progenitors, whereas canonical Wnt/β-catenin signaling promotes primitive streak formation and posterior mesodermal development. FGF signaling contributes to mesodermal specification, progenitor maintenance, and morphogenetic cell behaviors. These pathways operate through temporally and spatially restricted regulatory networks rather than as independent linear cascades.

The transcription factors Brachyury (T) and Foxa2 are important components of the regulatory networks controlling primitive streak, axial mesoderm, and midline development. In mouse, T is required for normal posterior mesodermal and axial development, whereas Foxa2 is required for organizer/node function and the development of axial midline structures.

Cellular Mechanisms of Notochord Morphogenesis

Notochord morphogenesis depends on coordinated cell rearrangements, including convergence and extension and mediolateral intercalation, which lengthen and narrow the developing axial tissue. In Xenopus, planar cell polarity-associated mechanisms and Rho-family GTPase signaling regulate polarized cellular behaviors underlying convergent extension. Cadherin-dependent adhesion, actomyosin contractility, and cytoskeletal remodeling further contribute to tissue organization and axial morphogenesis. Additional processes, including localized cell-shape changes and apical constriction, contribute to specific organizer-, node-, or axial-region morphogenetic events in a model- and developmental-stage-dependent manner.

In teleosts, notochord cells subsequently undergo pronounced vacuolization, generating large intracellular vacuoles whose formation involves lysosome-related organelles and which contribute mechanically to body-axis elongation. This specialized process is particularly well characterized in zebrafish and should not be assumed to occur identically in all vertebrate species.

Molecular Pathways in Anteroposterior and Dorsoventral Axis Formation

BMP antagonism within organizer-associated territories establishes locally low-BMP signaling environments that contribute to dorsoventral patterning. The developing notochord expresses Sonic hedgehog (Shh) and functions as a major early axial signaling source. Notochord-derived Shh contributes to the induction and patterning of ventral neural tube identities, after which the floor plate also becomes an important Shh-expressing signaling center. Shh additionally influences adjacent paraxial mesoderm and contributes to somite patterning.

Anteroposterior patterning emerges through interactions among Wnt, FGF, retinoic acid, and other spatially and temporally regulated developmental signals. Left–right asymmetry is initiated through node-associated mechanisms that establish asymmetric activation of the Nodal signaling pathway. The notochord also functions as a structural midline and signaling tissue that contributes to the organization and maintenance of bilateral embryonic architecture.

Experimental Approaches and Reagents for Studying Notochord and Axis Establishment

Mechanistic studies combine fate mapping and genetic lineage tracing, tissue transplantation, explant and recombination assays, genetic loss- and gain-of-function approaches, and live imaging. Mouse Cre/loxP-based lineage tracing, chick grafting experiments, Xenopus morpholino-mediated knockdown and mRNA overexpression, and zebrafish mutant, transgenic, and CRISPR-based perturbations have enabled direct analysis of axial progenitors and notochord development. Whole-mount in situ hybridization and immunofluorescence are routinely used to resolve the expression of markers such as T, Foxa2, Shh, and species-specific axial or notochordal markers. Pharmacological perturbation of FGF, Wnt, BMP, or Hedgehog signaling complements genetic approaches but requires careful interpretation because of dose dependence, developmental timing, pathway cross-regulation, and potential indirect effects on tissue morphogenesis.