Frontiers in tissue engineering.  In vitro modulation of chondrogenesis. 
Cartilage tissue engineering can provide functional cartilaginous constructs that can be used for controlled in vitro studies of chondrogenesis and potentially for in vivo articular cartilage repair.  Ideally, engineered cartilage should be indistinguishable from native articular cartilage with respect to zonal organization, biochemical composition, and mechanical properties.  In the model system presented here, chondrogenic cells are expanded in vitro as required, seeded onto three-dimensional polymeric scaffolds, and cultured in bioreactor vessels.  During the course of in vitro cultivation, construct cellularity plateaus at a physiologic level, fractions of glycosaminoglycan and Type II collagen increase progressively, and the scaffold biodegrades.  Construct structure (composition, morphology) and function (biosynthetic activity, mechanical properties) depend on cultivation conditions.  This paper reviews recent studies of in vitro modulation of chondrogenesis by: (1) cell seeding density and source; (2) the tissue regeneration template; (3) biochemical regulatory signals; (4) mixing, mass transport and hydrodynamic forces; and (5) cultivation time.  Key requirements and some of the critical research needs for successful cartilage tissue engineering are discussed.