Coordinating tissue development to build body and brain: Lessons from the gastrula organiser and its derivatives.

The Spemann-Mangold organiser has been a central paradigm in developmental biology for over a century, primarily studied through its ability to induce secondary axes in transplantation experiments. However, how the organiser and its derivatives act together to generate a well-proportioned body axis is still unknown. Here, we review evidence across four major vertebrate models (Xenopus, zebrafish, chick, and mouse) to evaluate the organiser's stage-specific requirements in neural development and body axis formation. We present a consensus view that initial neural specification and anterior-posterior patterning occur before morphological organiser formation, driven by planar signals from pre-organiser cells and species-specific contributions from extraembryonic tissues. We also discuss how the organiser-derived anterior axial mesendoderm (prechordal plate and head process) primarily acts to maintain anterior identity and ensure proper tissue proportions through continued signalling. The posterior organiser derivative, the notochord, remains essential for dorsal-ventral neural patterning, but also coordinates posterior body elongation through mechanical coupling with adjacent tissues, and the pace of axial mesoderm extension serves as a key coordinator of proportional body axis development. Future studies will improve our understanding of how organiser derivatives orchestrate the mechanical and molecular coordination of multiple tissues during axis formation.