The effects of four organophosphate ester flame retardants and BDE-47 on endochondral ossification in a murine ex vivo limb bud culture model

Introduction: Flame retardants (FRs) are added to a plethora of consumer products, from couches to computers, to slow fire propagation. In past decades, the most popular FRs in North America have been the brominated flame retardants (BFRs), particularly, polybrominated diphenyl ethers (PBDEs). By the mid-2000s, restrictions on the use of PBDEs were put in place due to their persistence, bioaccumulative properties and toxicity. Following this global phase-out, the use of alternative flame retardants, such as the organophosphate esters (OPEs), has increased [1]. Indeed, the levels of OPEs detected in the environment now often surpass those of PBDEs [2]. However, we know relatively little about the safety of these replacement chemicals. Endochondral ossification is the process by which most bones in the vertebrate skeleton are formed. A complex signalling network drives its normal progression from the initial cartilage anlagen formation (chondrogenesis) through to bone matrix deposition and mineralization (osteogenesis). An external insult, such as a chemical exposure, may cause dysregulation, resulting in adverse effects on bone quality during skeletal development. PBDE exposures have been associated with the suppression of osteogenesis. Previously, our lab showed that in utero exposure to an environmentally relevant PBDE mixture increased the incidence of ossification anomalies in gestational day 20 Sprague-Dawley rat fetuses and postnatal day 4 pups [3-4]. Little is known about whether OPE exposures affect bone formation. The goal of this study was to test the hypothesis that various OPEs have an impact on endochondral ossification in the developing limb and to compare their potential effects to those of a predominant PBDE congener, brominated diphenyl ether-47 (BDE-47).