OBJECTIVE:Marrow-stimulation techniques are used by surgeons to repair cartilage lesions although consistent regeneration of hyaline cartilage is rare. We have shown previously that autologous blood can be mixed with a polymer solution containing chitosan in a glycerol phosphate (GP) buffer (chitosan-GP), and that implantation of this polymer/blood composite onto marrow-stimulated chondral defects in rabbit and sheep leads to the synthesis of more chondral repair tissue with greater hyaline character compared to marrow-stimulation alone. In the current study, we examined the modulation of cell recruitment and repair tissue characteristics at early post-surgical time points (from day 1 to 56) in a rabbit model to elucidate potential mechanisms behind this improved repair outcome. DESIGN:Thirty-three skeletally mature New Zealand White rabbits underwent bilateral arthrotomies, with each trochlea receiving a cartilage defect (3.5 mm x 4.5mm) bearing four microdrill holes (0.9 mm diameter, approximately 4 mm deep) into the subchondral bone. One defect per rabbit was treated with a chitosan-GP/blood implant, while the other defect was left as a microdrilled control. Repair tissues were stained by histochemistry, for collagen types I, II, and X by immunohistochemistry and analyzed using quantitative stereological tools. RESULTS:Histological analyses demonstrated that control defects followed a typical healing sequence observed previously in marrow-stimulation animal models while chitosan-GP/blood implants led to three significant modifications in the healing sequence at early stages: (1) increased inflammatory and marrow-derived stromal cell recruitment to the microdrill holes, (2) increased vascularization of the provisional repair tissue in the microdrill holes, and (3) increased intramembranous bone formation and subchondral bone remodeling (BR). CONCLUSIONS:These results suggest that the greater levels of provisional tissue vascularization and BR activity are main factors supporting improved cartilage repair when chitosan-GP/blood implants are applied to marrow-stimulated cartilage lesions.