Safranin-O and anti-type-2 collagen immunohistochemical staining at 4 weeks (2x original magnification). The defect in G-I, II, and III was filled with a sufficient volume of the proteoglycan-rich tissue with regenerated subchondral bone tissue when the final vacant space was set at 2.0 mm. The DN gel implantation in G-II and III also induced a sufficient volume of the proteoglycan-rich tissue in the defect having 3.0-mm depth. Immunohistochemical staining for type-2 collagen were consistent with Safranin-O staining. G-I; a 0.5-mm gel sheet was implanted at the bottom of the defect. G-II; a 1.0-mm gel sheet was implanted at the bottom of the defect. G-III; a 5.0-mm gel plug was implanted at the bottom of the defect. G-IV; the control group has a defect left untreated. Black scale bars show a length of 1 mm.