Abstract 1969

Growth retardation is a major consequence of chronic renal failure in children. Recent studies have demonstrated the roles of parathyroid hormone-related peptide (PTHrP) and the PTH/PTHrP receptor in the regulation of endochondral bone formation. Little is known, however, about the molecular regulation of endochondral bone formation in renal failure. Thus, we utilized a neonatal bone transplant model to characterize the effects of renal failure on chondrocyte proliferation and differentiation. Tibiae harvested from newborn mice were implanted subcutaneously into adult FVB mice that have previously undergone two-stage nephrectomy (Nx) or sham operation (C). Four neonatal bones were transplanted into each adult animal. Transplant recipients were sacrificed two weeks after bone implantation and blood was obtained for blood urea nitrogen (BUN) determination. Tibial length was measured and the results were expressed as the percentage increase from values obtained at the time of transplant. Tibiae were fixed in 4%PFA/PBS, decalcified in 20%EDTA-4%PFA/PBS and embedded in paraffin. Five µm bone sections were obtained and the growth plate width at the proximal tibia was measured by quantitative histomorphometry. In situ hybridization was performed using 35S-labeled riboprobes for PTH/PTHrP receptor and type II and type X collagen. BUN was greater in Nx, 52±14 mg/dl, than in C, 23±8 mg/dl, p<0.01. Transplant bones exhibited linear growth and maintained normal shape. The increase in tibial length after implantation and growth plate width were less in bones transplanted into Nx (n=8) compared to C (n=8); values were 96±20% vs 111±22%, p<0.05, and 250±47 µm vs 301±18 µm, p<0.001, respectively. PTH/PTHrP receptor mRNA was localized to the zone of transition between proliferating and hypertrophic chondrocytes. Type II collagen mRNA was seen both in proliferating and in hypertrophic chondrocytes whereas type X collagen expression was seen both within hypertrophic chondrocytes and in chondrocytes adjacent to the secondary ossification center. The results indicate that bone growth and the expression of various key markers of chondrocyte proliferation and differentiation can be readily evaluated in murine neonatal bone transplants. This experimental model may be useful for examining the effects of renal failure on endochondral bone formation and development.