In vivo cartilage formation of osteoarthritic chondrocytes seeded on collagen scaffolds. Collagen scaffolds were seeded with human chondrocytes, implanted subcutaneously into SCID mice and harvested after eight weeks. (a,b) On empty scaffolds no cartilage formation occurred, as shown by the absence of dense Safranin O (a) or type II collagen (b) staining. (c,d) On scaffolds seeded with 1 × 106 osteoarthritic chondrocytes from osteoarthritis (OA) donor 1, moderate amounts of cartilage-like proteoglycan (c) and type II collagen (d) containing tissue could be detected. (g,i) However, hardly any type II collagen positive tissue was formed in scaffolds seeded with 1 × 106 chondrocytes from OA donors 2 (g) and 3 (i). (d,f,h,j) Seeding scaffolds at the higher density of 3 × 106 chondrocytes/cm2 resulted in the formation of type II collagen- and proteoglycan-rich cartilage by cells from all three donors in amounts comparable to those produced by (k) 1 × 106 healthy chondrocytes. (l,m) No type I collagen (l) or alkaline phosphatase activitiy (m) could be detected in these tissues. (c-f) OA donor 1, 78 years. (g,h,l) OA donor 2, 68 years. (i,j,m) OA donor 3, 50 years. (k) Healthy donor, 40 years. (a,c,e) Safranin O staining. (b,d,e-k) Type II collagen immunostaining. (m-o) Positive controls (OA cartilage, cartilage-bone interface) for type I (n), type II (o) and alkaline phosphatase activity (insert in (m)). Bar = 250 μm.