An injectable hydrogel composite consisting of oligo(poly(ethylene glycol)fumarate) (OPF) and gelatin microparticles has been developed as a novel carrier system for cells and growth factors. Rabbit marrow mesenchymal stem cells (MSCs) and gelatin microparticles (MPs) loaded with insulin-like growth factor-1 (IGF-1), transforming growth factor-beta1 (TGF-beta1), or a combination of both growth factors were mixed with OPF, a poly(ethylene glycol)-diacrylate crosslinker and the radical initiators ammonium persulfate and N,N,N',N'-tetramethylethylenediamine, and then crosslinked at 37 degrees C for 8 min to form hydrogel composites. Hydrogel composites encapsulating rabbit marrow MSCs and blank MPs served as controls. At day 14, confocal fluorescent images of OPF hydrogels showed a strong aggregation of rabbit marrow MSCs when encapsulated with IGF-1-loaded MPs with or without TGF-beta1-loaded MPs. Quantitative RT-PCR results showed that rabbit marrow MSCs encapsulated with MPs loaded with TGF-beta1 or both TGF-beta1 and IGF-1 had a significant increase in the expression of chondrocyte-specific genes such as collagen type II and aggrecan at day 14 as compared with the control group. Specifically, samples with both TGF-beta1-loaded MPs and IGF-1-loaded MPs exhibited a 121 +/- 20-fold increase of type II collagen gene expression and a 71 +/- 24-fold increase of aggrecan gene expression after 14 days of in vitro culture as compared with controls at day 0. These results suggest that hydrogel composites based on OPF and gelatin microparticles have great potential as carriers for MSCs and multiple growth factors for cartilage tissue engineering applications.