Histological and immunohistochemical analysis of mono-cultured and co-cultured cell pellets after 21 days of culture. The expression of matrix proteoglycans (glycosaminoglycanated proteins) in pellet cultures was detected by Safranin O (series A) and Alcian blue/neutral red stain (series B). Extracellular matrix deposition of collagen types I (series C) and II (series D) was detected by immuno-histochemistry (IHC) in mono-cultured (MCI, MCO, and MSC) and co-cultured (MCI/MSCs or MCO/MSCs) cell pellets after 3 weeks of culture in chondrogenic medium (transforming growth factor-beta 1, dexamethasone, and ascorbate). Representative primary antibody control pellets of a 1:3 MCO/MSCs ratio is shown in E. All photomicrographs were captured via a 10× magnification lens. Scale bar is 100 μm. Ab, antibody; MCI, meniscus cells from inner meniscus region; MCO, meniscus cells from the outer meniscus region; MSC, mesenchymal stromal cells.