In vivo ectopic chondrogenesis of BMSCs directed by mature chondrocytes.

In vivo niche plays an important role in determining the fate of exogenously implanted stem cells. Due to the lack of a proper chondrogenic niche, stable ectopic chondrogenesis of mesenchymal stem cells (MSCs) in subcutaneous environments remains a great challenge. The clinical application of MSC-regenerated cartilage in repairing defects in subcutaneous cartilage such as nasal or auricular cartilage is thus severely limited. The creation of a chondrogenic niche in subcutaneous environments is the key to solving this problem. The current study demonstrates that bone marrow stromal cells (BMSCs) could form cartilage-like tissue in a subcutaneous environment when co-transplanted with articular chondrocytes, indicating that chondrocytes could create a chondrogenic niche to direct chondrogenesis of BMSCs. Then, a series of in vitro co-culture models revealed that it was the secretion of soluble factors by chondrocytes but not cell-cell contact that provided the chondrogenic signals. The subsequent studies further demonstrated that multiple factors currently used for chondroinduction (including TGF-β1, IGF-1 and BMP-2) were present in the supernatant of chondrocyte-engineered constructs. Furthermore, all of these factors were required for initiating chondrogenic differentiation and fulfilled their roles in a coordinated way. These results suggest that paracrine signaling of soluble chondrogenic factors provided by chondrocytes was an important mechanism in directing the in vivo ectopic chondrogenesis of BMSCs. The multiple co-culture systems established in this study provide new methods for directing committed differentiation of stem cells as well as new in vitro models for studying differentiation mechanism of stem cells determined by a tissue-specific niche.