B.E. Pippenger1, M. Ventura2, K. Pelttari1, S. Feliciano1, C. Scotti3, C. Jaquiery1, A. Scherberich1, F. Walboomers2, A. Barbero1, I. Martin1
1: University Hospital Basel, Institute for Surgical Research, Basel, Switzerland
2: UMC St.Radboud, Department of Biomaterials, Nijmegen, the Netherlands
3: I.R.C.C.S. Istituto Ortopedico Galeazzi, Milano, Italy
Differentiated adult human nasal chondrocytes (hNC) exhibit a regenerative capacity for cartilage repair and, accordingly to their neuro-ectodermal origin, a phenotypical plasticity. While literature suggests that hNC are also capable of osteoblastic differentiation, their bone forming capacity has never been investigated. The aim of this study is to assess the potential of hNC to follow one or both of the archetypal routes of bone formation (endochondral and intramembranous). HNC-based cartilage grafts developed features typical of hypertrophy in vitro (type X collagen: xx fold upregulation), but did not proceed throughout endochondral ossification upon subcutaneous implantation into nude mice, yielding stable hyaline-like cartilage. However, when subjected to direct osteoblastic differentiation culture in a ceramic-based scaffold, hNC deposited a highly mineralized matrix and unregulated the expression of the key osteogenic genes cbfa-1 (15-fold) and BMP-2 (80-fold). While subsequent ectopic implantation into nude mice indeed resulted in hNC-containing bone, the process was inefficient (<5% total bone). To explain this inefficiency, we performed a clonal study that demonstrated the relatively low (<30%) percentage of osteogenic hNC clones present in nasal cartilage. Hypothesizing orthotopic environmental influences could boost the osteogenic subpopulation towards efficient bone formation, we then tested the hNC-containing ceramic constructs in a calvarian defect model (intramembranous bone repair). Remarkably, orthotopic implantation demonstrated that hNC were not only able to survive in a homotopic boney environment, but also able to actively contribute to new bone formation, both contrary to the gold standard heterotopic cell source (bone marrow-derived mesenchymal stem cells).
Our findings show that orthotopic, and perhaps homotopic, environmental cues are essential for hNC to survive and actively produce intramembranous bone. From a broader biological perspective, we demonstrate that environmental cues can suffice for the direct conversion of human chondrocytes into functional osteoblasts.