Nuclei size-regulated packing and phase transition on multicellular spheres*

Within curved epithelia of multicellular systems, cells arrange differently to form various structures and perform functions. These diversities can be due to topology, cell layer thickness, and various mechanical perturbations. However, the fundamental mechanisms of how cells regulate local packing within these curved multicellular epithelia which gives rise to global architecture is not clear. Furthermore, how their packing evolve during growth and development remains unknown. Here by performing experiments using human iPSC-derived lung alveolospheres and numerical simulation, we show that nuclei-to-cell size ratio regulate both epithelial local neighbor orders and phases. As alveolospheres grow bigger, the nuclei-to-cell ratio increases, resulting in more ordered cellular packing and a topological gas-to-liquid transition. This is because cell nuclei set the lower limit of cell-cell distance and thus prevent the cell collision. In simulation by setting smallest cell-cell distance ratio as cell nuclei ratios, we are able to reproduce the alveolospheres packing evolution during entire growth stages. By further artificially applying osmotic shock to increase the nuclei-to-cell size ratio in alveolospheres, we indeed observe that the cell packing become more ordered. Together, our finding highlights the importance of cell nuclei in regulating both nearest neighbor order and epithelia phases during the growth of alveolospheres. This finding also has the potential to identify the stages and physical phases of other developmental and structure formation processes, such as morphogenesis.