TGFB1 and intervertebral disk degenerative disorder: Various animal model studies suggest that early IDD involves a transient increase in collagen II, changing gelatinous NP into a cartilaginous entity before further remodeling into fibrous tissue [25,49], characterized by an increase in collagen I. Accumulation of collagen I has been evidenced in human IDD [6,21,50], animal models of IDD induced by aging [6,21,25,49,50,51], overloading [52] or injury [6,26,36,49,53,54,55,56,57,58,59], as well as NP cells under stress [36,50] or treatment of TGF-β [59,60].