(a) Lesion induction in the ventral spinal cord by lysolecithin injection. Parabiotic pairings between young (Y-Y) (white) or old (O-O) (dark gray) animals were compared to lesions in old animals following heterochronic pairing (Y-O) (light gray), with previous studies showing restored remyelination efficiency in old partners of Y-O pairings. (b) Lesions immunostained at 7 dpl against Arg-1 (red) and CD68 (white). Scale bar, 25 μm. (c) MR+ IB4+ M2 cells/0.1 mm2 ± s.e.m. at 7 dpl. 2-tailed Student’s t-test: Y/Y vs. O/O, P=0.048; Y/O vs. O/O, P=0.005 (n=4 (O/O), 6(Y/O), 4 (Y/Y), df=13). (d) CD16/32+ IB4+ cells/0.1 mm2 ± s.e.m. at 7 dpl. 2-tailed Student’s t-test: Y/Y vs. O/O, P=0.0027; Y/Y vs. Y/O, P=0.014 (n=4 (O/O), 6(Y/O), 4 (Y/Y), df=13). (e) Parabiosis between a GFP-expressing young mouse and wildtype (WT) old mouse with a spinal cord lesion. (f) GFP expressing iNOS+ CD68+ M1 macrophages and Arg-1+ CD68+ M2 macrophages. Scale bar, 5 μm.