Masson’s trichrome staining at 16 weeks post-transplantation. Light red or pink indicates cytoplasm and dark brown to black shows cell nuclei. (A) Group A with hUCB-MSCs + 4% HA hydrogel composites and 4% HA only; (B) Group B with hUCB-MSCs + 3% alginate:30% pluronic (1:1, v/v) composites and 3% alginate:30% pluronic (1:1, v/v) only; (C) Group C with hUCB-MSCs + 4% hyaluronic acid:3% alginate:20% pluronic (2:1:1, v/v) composites and 4% hyaluronic acid:3% alginate:20% pluronic (2:1:1, v/v) only; (D) Group D with hUCB-MSCs + 4% hyaluronic acid:3% alginate:20% pluronic: chitosan (4:1:1:2, v/v) and 4% hyaluronic acid:3% alginate:20% pluronic: chitosan (4:1:1:2, v/v) only; (E) Defect only and normal cartilage. The transplantation of hUCB-MSCs and hydrogel composites resulted in improved articular cartilage defect repair compared with corresponding controls (hydrogels only). Of the four hydrogel compositions examined, the composite of hUCB-MSCs and HA hydrogel (group A) exhibited the most mature type of cartilaginous tissue repair with respect to cellular shape, number, lacunae formation, organization and articular contour. In group A cartilage with hUCB-MSCs and HA composites, hyaline cartilage with good column alignment of chondrocytes without cell clustering and lacunae formation were observed, which was similar to the morphology of native cartilage. In contrast, the size of chondrocyte with HA only was relatively smaller than a normal one in the mid zone. In groups B and C, moderate hypocellularity with small and flat cells in the mid zone and cell number decrease were observed. In group D with hUCB-MSCs and hydrogel, cellular alignment was reduced as well as the cell size. HA, hyaluronic acid; hUCB-MSCs, human umbilical cord blood derived mesenchymal stem cells.