Gastruloids employ an alternative morphogenetic route to generate a posterior body axis on adherent substrates

Morphogenesis requires cells to integrate genetic and environmental information to pattern and shape tissues. Recent studies in vivo and in vitro have shown how alterations in environmental cues can induce alternative morphogenetic programs. However, what determines which morphogenetic strategy cells deploy in different environments remains unknown. Using gastruloids, a model for mammalian posterior body-axis elongation where environmental cues can be fully controlled in vitro, we show that substrate availability determines which force-generation strategy cells employ to build the body axis. In free-floating conditions, gastruloids self-assemble a single posterior body axis through cell-cell interactions. When plated on laminin, gastruloids acquire a flat morphology, break symmetry multiple times, and produce several independently elongating body axes through collective cell migration. On laminin, formin activity and focal adhesion-mediated traction are required for tissue elongation, yet both are functionally dispensable in free-floating conditions. Transcriptomic analysis shows that formin inhibition blocks elongation on laminin without any detectable transcriptional changes, demonstrating that these cytoskeletal components play a purely mechanical role that is only required when cells engage a substrate. Furthermore, laminin biases cells toward posterior migratory fates while preserving the Hox expression pattern. Together, these results show that mammalian body-axis elongation can proceed through different morphogenetic modes that engage distinct mechanical effectors depending on substrate availability. These findings have implications for understanding how evolution explores morphological diversity and how tissue engineering might achieve desired forms through precise environmental control.