MARTIN A. VIDAL, BVSc, MS, Diplomate ACVS , SANDRA O. ROBINSON, BS , MANDI J. LOPEZ, DVM, MS, PhD, Diplomate ACVS et al. Veterinary Surgery December 2008 37 8 713 - 724 Objective—To compare the chondrogenic potential of adult equine mesenchymal stem cells derived from bone marrow (MSCs) or adipose tissue (ASCs). Study Design—In vitro experimental study. Animals—Adult Thoroughbred horses (n=11). Methods—BM (5 horses; mean [±SD] age, 4±1.4 years) or adipose tissue (6 horses; mean age, 3.5±1.1 years) samples were obtained. Cryopreserved MSCs and ASCs were used for pellet cultures in stromal medium (C) or induced into chondrogenesis±transforming growth factor-3 (TGFβ3) and bone morphogenic factor-6 (BMP-6). Pellets harvested after 3, 7, 14, and 21 days were examined for cross-sectional size and tissue composition (hematoxylin and eosin), glycosaminoglycan (GAG) staining (Alcian blue), collagen type II immunohistochemistry, and by transmission electron microscopy. Pellet GAG and total DNA content were measured using dimethylmethylene blue and Hoechst DNA assays. Results—Collagen type II synthesis was predominantly observed in MSC pellets from Day 7 onward. Unlike ASC cultures, MSC pellets had hyaline-like matrix by Day 14. GAG deposition occurred earlier in MSC cultures compared with ASC cultures and growth factors enhanced both MSC GAG concentrations (P<.0001) and MSC pellet size (P<.004) after 2 weeks in culture. Conclusion—Equine MSCs have superior chondrogenic potential compared with ASCs and the equine ASC growth factor response suggests possible differences compared with other species. Clinical Relevance—Elucidation of equine ASC and MSC receptor profiles will enhance the use of these cells in regenerative cartilage repair.