The maturity of tissue-engineered cartilage in vitro affects the repairability for osteochondral defect

Cartilage tissue engineering using cells and biocompatible scaffolds has emerged as a promising approach to repair of cartilage damage. To date, however, no engineered cartilage has proven to be equivalent to native cartilage in terms of biochemical and compression properties, as well as histological features. An alternative strategy for cartilage engineering is to focus on the in vivo regeneration potential of immature engineered cartilage. Here, we used a rabbit model to evaluate the extent to which the maturity of engineered cartilage influenced the remodeling and integration of implanted extracellular matrix scaffolds containing allogenous chondrocytes. Full-thickness osteochondral defects were created in the trochlear groove of New Zealand white rabbits. Left knee defects were left untreated as a control (group 1), and right knee defects were implanted with tissue-engineered cartilage cultured in vitro for 2 days (group 2), 2 weeks (group 3), or 4 weeks (group 4). Histological, chemical, and compression assays of engineered cartilage in vitro showed that biochemical composition became more cartilagenous, and biomechanical property for compression gradually increased with culture time. In an in vivo study, gross imaging and histological observation at 1 and 3 months after implanting in vitro-cultured engineered cartilage showed that defects in groups 3 and 4 were repaired with hyaline cartilage-like tissue, whereas defects were only partially filled with fibrocartilage after 1 month in groups 1 and 2. At 3 months, group 4 showed striking features of hyaline cartilage tissue, with a mature matrix and a columnar arrangement of chondrocytes. Zonal distribution of type II collagen was most prominent, and the International Cartilage Repair Society score was also highest at this time. In addition, the subchondral bone was well ossified. In conclusion, in vivo engineered cartilage was remodeled when implanted; however, its extent to maturity varied with cultivation period. Our results showed that the more matured the engineered cartilage was, the better repaired the osteochondral defect was, highlighting the importance of the in vitro cultivation period.