1. A tissue construct comprising: a self-assembled, scaffold-free, high-density cell aggregate in the form of a cell sheet, the cell aggregate comprising: a plurality of chondrogenic progenitor cells; a plurality of biocompatible and degradable nanoparticles and/or microparticles that are incorporated within the cell aggregate, wherein the nanoparticles and/or microparticles comprise a gelatin hydrogel cross-linked with genipin and TGF-beta, wherein said TGF-beta is controllably released by the nanoparticles and/or microparticles; and a self-secreted extracellular matrix that promotes adherence of the cells to the nanoparticles and/or microparticles.