Histological sections stained with Safranin O. A) Empty defect, B) Scaffold alone, C) Scaffold + rhFGF-18, D) Scaffold + BMP-7. In A, B and C there is good tissue fill. Differences in subchondral bone formation can be observed. The empty defect A shows subchondral hypertrophy, whilst in C and D there appears to be a cartilage cleft, with significant proteoglycan staining extending down into the subchondral bone. In D there is a large subchondral cyst. The arrows denote the margins of the defect.