Fig. 4

a Adult splenic monocytes from hCD68-GFP⁺ donors were transferred by intracardiac injection at time of MI surgery (50 ± 10 × 10³ cells). P1 neonates were then harvested at day 3 (for imaging) or day 21 (for histology). b GFP Fluorescence imaging demonstrating adoptively transferred GFP⁺ monocytes were present in the anterior wall of the recipient P1 at day 3 post-MI (scale bar: 1 mm). c Masson’s trichrome staining of heart sections showing that at day 21 post-MI, P1 neonates which received PBS vehicle showed histological evidence of regeneration. d P1 neonates which received adult monocytes show residual scar formation. Scale bar: 1 mm. e Adoptive transfer of adult monocytes led to a significant increase in scar size at 21 days, measured as percentage of the left ventricular area (scar percentage 3.99 ± 0.74% vs 0.81 ± 0.25%, n = 5 in PBS group, n = 6 in monocyte group, **p < 0.01, data are mean ± SEM, 2-tailed, unpaired Student’s t-test). f Splenic monocytes transferred from adult GFPtpz-collagen mice to P1 recipients undergoing MI. g Confocal imaging of immunofluorescence-stained heart cryosections at day 3 post-MI revealed emergent GFP⁺ fibres within the scar region and a subset of α-CD68⁺ macrophages co-labelled for GFP indicating that they had arisen from donor monocytes and had activated the GFPtpz-collagen 1. Scale bars, 500 μm (low power panel), 100 μm (high power panels). h, i By day 21 post-MI, recipient P1 mice revealed GFP⁺ scars within the non-regenerated myocardium (schematised hearts; black inset boxes relate to immuno-stained regions in the vicinity of the ligating suture) (h) or within the injury area (top two panels in i; black dashed inset box relates to injury region depicted in bottom two panels in i). Both GFP fluorescence and α-GFP staining revealed evidence of donor-monocyte-derived GFP⁺ collagen fibrils which were Col1⁺ within the scar at day 21 post-MI. Scale bars, 500 μm (low power panel), 100 μm (high power panels). ep, epicardium; my, myocardium; P1, post-natal day 1; sc, scar. Representative images of n = 3 per group. Source data are provided as a Source Data file.