Generating gastruloids with somite-like structures from mouse embryonic stem cells

Abstract Gastruloids are aggregates of mouse embryonic stem cells that can be used to study key aspects of mammalian post-implantation development in vitro1–4. Gastruloids generated with previously published protocols do not generate somite-like structures4–6. Here, we describe a modified version of the gastruloids culture protocol5,6 that results in gastruloids that do generate somite-like structures in vitro (van den Brink et al., Nature, 2020)7. Under these conditions, about 50% of the gastruloids generated form structures with features that are characteristic of somites7.This protocol takes 6 days, with relatively little hands-on time. The protocol starts with the aggregation of the cultured cells. Then, the Wnt-agonist Chiron is added 2 days (48h) later. The medium of the aggregates is replaced 3 days (72h) after aggregation. To induce somite-formation, gastruloids are embedded in Matrigel 4 days (96h) after aggregation. After 5 days (120h) of culture, gastruloids resemble E8.5 mouse embryos. At this timepoint they can be fixed (fixative is added on day 5 and washed away on day 6 after overnight incubation in PFA) to prepare them for staining or microscopy experiments.