Spherical, heterogeneous aggregates of proliferating, quiescent, and necrotic cells in culture that retain three-dimensional architecture and tissue-specific functions. The ability to form spheroids is a characteristic trait of CULTURED TUMOR cells derived from solid TUMORS. cells from normal tissues can also form spheroids. They represent an in-vitro model for studies of the biology of both normal and malignant cells. (From Bjerkvig, Spheroid culture in Cancer research, 1992, p4)