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