Nucleus pulposus is the jelly-like substance in the middle of the intervertebral disc. It functions to distribute hydraulic pressure in all directions within each disc under compressive loads. The nucleus pulposus consists of collagen fibrils, proteoglycan aggrecans and nucleus pulposus cells. Nucleus pulposus cells reside in an environment that has a limited vascular supply and generate energy through anaerobic glycolysis. The cells synthesize collagen II and X; express hypoxia-inducible factor-1, and secrete interleukins-1, -6, and -10 and granulocyte-macrophage colony-stimulating factor. Mechanical stress on nucleus pulposus cells promotes cell proliferation. Nucleus pulposus cells provide an in vitro model for the study of cellular and molecular events involved in disc degeneration, tissue engineering and cell therapy for spine disc disorders.