PARATHYROID HORMONE-RELATED PEPTIDE-DEPLETED MICE SHOW ABNORMAL EPIPHYSEAL CARTILAGE DEVELOPMENT ALTERED ENDOCHONDRAL BONE-FORMATION

To elucidate the role of PTHrP in skeletal development, we examined th e proximal tibial epiphysis and metaphysis of wild-type (PTHrP-normal) 18-19-d-old fetal mice and of chondrodystrophic litter mates homozygo us for a disrupted PTHrP allele generated via homologous recombination in embryonic stem cells (PTHrP-depleted). In the PTHrP-normal epiphys is, immunocytochemistry showed PTHrP to be localized in chondrocytes w ithin the resting zone and at the junction between proliferative and h ypertrophic zones. In PTHrP-depleted epiphyses, a diminished [H-3]thym idine-labeling index was observed in the resting and proliferative zon es accounting for reduced numbers of epiphyseal chondrocytes and for a thinner epiphyseal plate. In the mutant hypertrophic zone, enlarged c hondrocytes were interspersed with clusters of cells that did not hype rtrophy, but resembled resting or proliferative chondrocytes. Although the overall content of type II collagen in the epiphyseal plate was d iminished, the lacunae of these non-hypertrophic chondrocytes did reac t for type II collagen. Moreover, cell membrane-associated chondroitin sulfate immunoreactivity was evident on these cells. Despite the pres ence of alkaline phosphatase activity on these non-hypertrophic chondr ocytes, the adjacent cartilage matrix did not calcify and their persis tence accounted for distorted chondrocyte columns and sporadic distrib ution of calcified cartilage. Consequently, in the metaphysis, bone de posited on the irregular and sparse scaffold of calcified cartilage an d resulted in mixed spicules that did not parallel the longitudinal ax is of the tibia and were, therefore, inappropriate for bone elongation . Thus, PTHrP appears to modulate both the proliferation and different iation of chondrocytes and its absence alters the temporal and spatial sequence of epiphyseal cartilage development and of subsequent endoch ondral bone formation necessary for normal elongation of long bones.