TNF and intervertebral disk degenerative disorder: Furthermore, upstream inflammatory cytokines identified as key players in disc degeneration (TNF‐α, IL‐1β) were undetectable in all conditions, while mid‐ and downstream cytokines (IL‐6, IL‐8, PGE2, NO) were produced by the NP cells, implying that the extracellular environment directly affected NP‐cells or that other upstream mechanisms are involved.