An elongated or fiber-supported multicellular aggregation of multipotent cells, wherein multipotent cells are arranged in an oblong or longish arrangement with an aspect ratio of a prolate dimension to a perpendicular dimension of at least 2:1, or supported by a fibrous structure, and wherein the aggregate contains cells at different stages of differentiation, and the aggregate contains polar cells; methods of generating such aggregates; methods of developing the aggregates further into tissue organoids and kits for such methods.