Cell surface characterization of nucleus pulposus clones. (A) Cluster of differentiation 24 (CD24) histograms (top panels; green) of representative responsive and nonresponsive nucleosus pulposus (NP-R and NP-nR, respectively) subtypes (NP-R115 and NP-nR105) and an immunoglobulin G (IgG)–negative control (middle panels; grey). Phase-contrast images are shown in the lower panels. FITC, Fluorescein isothiocyanate. Black bars = 20 μm. (B) Percentage of CD24+ cells in three independent NP-R and NP-nR clones. (C) CD marker scores for the indicated cell lines: NP marker CD24, the hyaluronic acid receptor CD44 and the mesenchymal stem cell markers CD73, CD90 and CD105. (D) Gangliosidase 2 (GD2) histograms of subconfluent cultures of representative NP subtypes in maintenance medium (Mmed): NP-R115 and NP-nR105 (top panels) and 7-day differentiated cultures in differentiation medium (Dmed) (bottom panels). The U-CH1 chordoma cell line was used as a reference.