Histological observations of the defect at 12 weeks (A-C: Hematoxylin-Eosin staining, D-F: Safranin-O staining, G-I: immuno-histological staining for type II collagen, original magnification ×2). Group I was filled with a proteoglycan-rich tissue, in which type 2 collagen was abundantly expressed (A, D, and G). In high magnification histology of Group I, no cleft was observed between the regenerated tissue and the normal cartilage tissue (j: A black bar shows 200 micrometer). In the regenerated tissue, fairly large round cells rich in cytoplasm were scattered singly or as an isogenous group in a proteoglycan-rich matrix (k, l, p). In Group II, the defect was filled with thin heterogeneous tissues, including cartilage, fibrous, and bone tissues at 12 weeks (B and E). In this tissue, type 2 collagen was not uniformly expressed (H, q). In this tissue, cells were rather small and sparse (m, n). In Group III, the defect was filled with the fibrous and bone tissues (Figure C and F), while the type 2 collagen expression was not found in this tissue (I, r).