Isotropic expansion of external environment induces tissue elongation and collective cell alignment

•Growing embryos are mathematically modeled as isotropically expanding fields.•Morphogenesis of tissues on the fields is mathematically and analytically studied.•Cell clusters are passively elongated on the isotropically expanding field.•Elongation of cell clusters is primarily caused by their intrinsic features.•Interactions between cell clusters and growing embryos enhance asymmetries.