Figure 3 Cell-surface markers for NP cell differentiation Adapted from Sakai, D. et al. Exhaustion of nucleus pulposus progenitor cells with ageing and degeneration of the intervertebral disc. Nat. Commun. 3, 1264 (2012), which is published under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License (http://creativecommons.org/licenses/by-nc-sa/3.0/) Sakai, D. & Andersson, G. B. J. (2015) Stem cell therapy for intervertebral disc regeneration: obstacles and solutions Nat. Rev. Rheumatol. doi:10.1038/nrrheum.2015.13