We have analyzed the biology of embryonic, epidermal growth factor–responsive murine neural precursor cells cultured in the high-aspect ratio vessel (HARV). Within 2–3 d of rotary-cell culture, such cells formed multiple, macroscopic, three-dimensional structures that were orders of magnitude larger than the cellular clusters (“neurospheres”) formed by these cells in conventional stationary-flask cultures. Each HARV structure was composed of a multilayered cellular shell surrounding one or more central cavities that were bordered by pyknotic cell nuclei. Although the cells in the HARV structures were more pleomorphic than those in neurospheres, the structures did not appear to represent primitive neural tumors: the formation of HARV structures by precursor cells was not an irreversible phenotypic change, and the structures did not originate from the clonal expansion of single-progenitor cells; the growth rate and invasiveness of the cells in HARVs were less than those in flasks; and HARV-cultured cells did not form tumors after subcutaneous inoculation into the flanks of NOD-scid/scid mice. Immunohistochemical analysis suggested that HARV structures might be novel “proto-tissues” characterized by a crude, but organized, architecture, with a surface layer of immature proliferating cells (nestin- and proliferating cell nuclear antigen-positive) that enclosed strata of more differentiated cells (β-tubulin III– and glial fibrillary acidic protein-positive) within. Rotary-cell culture may have significant implications for the eventual utility of neural precursors for clinical neurotransplantation.