Imaging analysis of osteogenic transformation of Meckelʹs chondrocytes from green fluorescent protein-transgenic mice during intrasplenic transplantation

Our previous studies demonstrated that Meckelʹs chondrocytes, which are derived from ectomesenchyme, have the potential to transform into osteogenic phenotypes. The present study aimed to clarify the role of cell origin in the phenotypic transformation of chondrocytes. Cell pellets from ectomesenchyme-derived Meckelʹs cartilage and mesoderm-derived costal cartilage from green fluorescent protein (GFP)-transgenic mice were transplanted into the spleen for up to 4 weeks. Chondrocyte pellets from both cartilages adapted well to the splenic tissues and formed an alizarin red-positive calcified matrix, with increasing duration of transplantation. Following the production of cartilage-specific type II and type X collagens, newly-formed type I collagen appeared in the chondrocyte pellets from Meckelʹs cartilage during the late stage of transplantation. Although the bone-marker proteins: osteocalcin, osteopontin, osteonectin and bone morphogenetic protein-2, were detected in pellets from both Meckelʹs and costal cartilage, only type I collagen in Meckelʹs cartilage was a significant marker protein for detecting transformation. These bone-type protein-producing cells represented osteogenic cells transformed from GFP-expressing cells, rather than from recipient cells. These results indicate that neural crest-derived Meckelʹs cartilage displays a higher potential for phenotypic switching than mesoderm-derived costal chondrocytes under in vivo conditions.