This experimental system was inspired by the work of Yusuke Marikawa using embryoid bodies derived from P19 embryonal carcinoma cells to investigate the principles of early mouse embryogenesis . Our studies of embryonic stem (ES) cell differentiation in vitro revealed tight schedules in gene expression that mimic the embryonic programs of gene expression [2, 3]. Using protocols developed in this work, we extended Marikawa’s model to mouse ES cell cultures under conditions that promote a primitive streak-like state and demonstrated that ES cells can undergo spontaneous symmetry-breaking, differentiation to all three germ layers and axial elongation morphogenesis . We named these structures gastruloids since they develop as free-floating, three dimensional aggregates that progress through a gastrulation-like process.
In collaboration with groups at the Hubrecht Institute (the Netherlands) and EPFL (Switzerland) we have used transcriptomic approaches to characterise the genome-wide changes in gene expression associated with gastruloid development. As gastruloids extend along the antero-posterior axis, we have described a spatial and temporal pattern of Hox gene activation that is consistent with this process in the developing embryo . By applying spatial transcriptomics to the extending posterior tissues, we have described the dynamic gene expression profile associated with somitogenesis in gastruloids . These studies have also revealed that the anterior domain contains progenitors of cranial, cardiac and hematopoietic mesoderm as well as elements from placodal and surface ectoderm. Other groups have built on this work to advance the system towards specific structures and tissues, namely the heart , somites [6,8], posterior neural tube [8,9] and brain  and to study cell movements during elongation  (see Prelights for a curated list).
We consider the reproducibility of the gastruloid system and the accessibility of the technique to be two major strengths of this approach (see Protocols). We have found them to be a useful tool in exploring mammalian embryogenesis, in part through attempting to replicate developmental processes in culture and to understand what is required to do so. We also see their potential as a source of progenitor cell populations and stem cell niches, offering the seeds of adult tissues, inviting us to learn how to engineer tissues and organs. When working with any organoid system, it is crucial to achieve reproducibility and to insist upon it; we hope that the Protocols and Updates will ensure that gastruloids can be robustly applied to an exciting new range of experimental questions.
Peter Baillie-Benson & Alfonso Martinez Arias, June 2020
 Marikawa, Y., Tamashiro, D. A. a, Fujita, T. C. & Alarcón, V. B. Aggregated P19 mouse embryonal carcinoma cells as a simple in vitro model to study the molecular regulations of mesoderm formation and axial elongation morphogenesis. Genesis 47, 93–106 (2009).
 Turner, D. A., Rué, P., Mackenzie, J. P., Davies, E. & Martinez Arias, A. Brachyury cooperates with Wnt/Beta-catenin signalling to elicit primitive-streak-like behaviour in differentiating mouse embryonic stem cells. BMC Biol. 12, 63 (2014).
 Turner, D. A., Trott, J., Hayward, P., Rué, P. & Martinez Arias, A. An interplay between extracellular signalling and the dynamics of the exit from pluripotency drives cell fate decisions in mouse ES cells. Biol. Open 3, 614–26 (2014).
 van den Brink, S. C. et al. Symmetry breaking, germ layer specification and axial organisation in aggregates of mouse embryonic stem cells. Development 141, 4231–4242 (2014).
 Beccari, L. et al. Multi-axial self-organization properties of mouse embryonic stem cells into gastruloids. Nature 1 (2018). doi:10.1038/s41586-018-0578-0
 van den Brink, S. C. et al. Single-cell and spatial transcriptomics reveal somitogenesis in gastruloids. Nature (2020). doi:10.1038/s41586-020-2024-3
 Rossi, G. et al. Embryonic organoids recapitulate early heart organogenesis. bioRxiv (preprint) 802181 (2019). doi:10.1101/802181
 Veenvliet, J. V, Bolondi, A., Kretzmer, H., Haut, L. & Scholze-wittler, M. Mouse embryonic stem cells self-organize into trunk-like structures with neural tube and somites. bioRxiv 1–72 (2020).
 Libby, A.R.G., Joy, D.A., Elder, N.H., Bulger, E.A., Krakora, M.Z., Gaylord, E.A., Mendoza-Camacho, F., McDevitt, T. C. Axial Elongation of Caudalized Human Pluripotent Stem Cell Organoids Mimics Neural Tube Development. bioRxiv (preprint) 4–6 (2020).
 Bérenger-Currias, N. M. L. P. et al. Early neurulation recapitulated in assemblies of embryonic and extraembryonic cells. bioRxiv 2020.02.13.947655 (2020). doi:10.1101/2020.02.13.947655
 Hashmi, A., Tlili, S., Perrin, P., Martinez-arias, A. & Lenne, P. Cell-state transitions and collective cell movement generate an endoderm-like region in gastruloids. bioRxiv (preprint) (2020).