A robust, reproducible, accessible and scalable protocol for generating three-dimensional human gastruloids

The rapid rise of stem cell-based human embryo models has reignited interest in studying early human development while offering a promising platform to de-risk drugs. Among these, three-dimensional human gastruloids provide a tractable system to model symmetry breaking, germ layer specification and axial organization. However, existing gastruloid protocols remain expensive, specialized, variable and evaluated in a limited number of human pluripotent stem cell (hPSC) lines, restricting broader adoption. Here, we present a simple, robust, standardized gastruloid protocol achieving greater than 90% elongation efficiency with low inter- and intra-experimental variability, developed primarily in BJNhem20, a well-characterized Indian-origin human embryonic stem cell line. Further, we show that the protocol is applicable in a diverse set of hPSC lines. Using a TBXT (Brachyury)-GFP reporter in BJNhem20, we optimized cell seeding density, induction medium and Wnt activation strength, guided by real-time, quantitative assessment of mesoderm induction and symmetry breaking, allowing precise titration of CHIR99021. Comparative testing identified an in-house "Essential 6" medium formulation as the most consistent condition for robust TBXT induction. Optimization of aggregation density produced reproducible gastruloids with polarized TBXT expression and consistent axial elongation, within 72 hours. Single-cell RNA sequencing of individual gastruloids confirmed high transcriptional reproducibility and conserved lineage clusters, aligned with developmental trajectories. Cell line-specific CHIR99021 titration was sufficient to successfully transfer the optimized protocol to two additional lines, BJNhem19 and RUES2-GLR. This simplified and robust protocol reduces costs and improves accessibility, enabling broader application of stem cell-based human embryo models.