Double immunofluorescence microscopic findings. Photomicrographs showing serial sections of articular cartilage 5 weeks after injection in the hyaluronic acid (HA) + mesenchymal stem cell (MSC) group stained with (A) H & E and (B to D) double immunofluorescence (red, type II collagen; green, MSCs; blue, 4',6-diamidino-2-phenylindole (DAPI)). (B) corresponds to (A). (C) and (D) are magnified views of the boxes marked in (B). Bar = 100 μm (A, B), 50 μm (C, D).