Hormones are indispensable for bone development, growth, and maintenance. While many of the genes associated with osteogenesis are well established, it is the recent findings in endocrinology that are advancing the fields of bone biology and toxicology. Endocrine-disrupting chemicals (EDCs) are defined as chemicals that interfere with the function of the endocrine system. Here, we report recent discoveries describing key hormone pathways involved in osteogenesis and the EDCs that alter these pathways. EDCs can lead to bone morphological changes via altering hormone receptors, signaling pathways, and gene expression. The objective of this review is to highlight the recent discoveries of the harmful effects of environmental toxicants on bone formation and the pathways impacted. Understanding the mechanisms of how EDCs interfere with bone formation contributes to providing a comprehensive toxicological profile of a chemical.

To prevent congenital defects arising from maternal exposure, safety regulations require pre-market developmental toxicity screens for industrial chemicals and pharmaceuticals. Traditional embryotoxicity approaches depend heavily on the use of low-throughput animal models which may not adequately predict human risk. The validated embryonic stem cell test (EST) developed in murine embryonic stem cells addressed the former problem over 15 years ago. Here, we present a proof-of-concept study to address the latter challenge by updating all three endpoints of the classic mouse EST with endpoints derived from human induced pluripotent stem cells (hiPSCs) and human fibroblasts. Exposure of hiPSCs to selected test chemicals inhibited differentiation at lower concentrations than observed in the mouse EST. The hiPSC-EST also discerned adverse developmental outcomes driven by novel environmental toxicants. Evaluation of the early cardiac gene TBX5 yielded similar toxicity patterns as the full-length hiPSC-EST. Together, these findings support the further development of hiPSCs and early molecular endpoints as a biologically relevant embryotoxicity screening approach for individual chemicals and mixtures.

Introduction: Tobacco smoking has been implicated in an array of adverse health outcomes, including those that affect adult bone. However, little is known about the impact of tobacco products on developing bone tissue as it develops in the embryo. Aims and Methods: Here, human embryonic stem cells were differentiated into osteoblasts in vitro and concomitantly exposed to various concentrations of smoke solutions from two conventional, one additive-free and two harm-reduction brands of cigarettes. Differentiation inhibition was determined by calcium assays that quantified matrix mineralization and compared to the cytotoxicity of the tobacco product. Results: Exposure to mainstream smoke from conventional and additive-free cigarettes caused no inhibition of cell viability or mineralization, while sidestream smoke (SS) concentration-dependently produced cell death. In contrast, mineralization was inhibited only by the highest mainstream concentration of harm-reduction smoke solution. Additionally, sidestream smoke solution from the harm-reduction cigarettes impeded calcification at concentrations lower than those determined to be cytotoxic for conventional products. Conclusions: Sidestream smoke impaired in vitro osteogenesis at subtoxic concentrations. In addition, though often perceived as safer, smoke from harm-reduction cigarettes was more potent in inhibiting in vitro osteogenesis than smoke from conventional cigarettes. Implications: This study adds to a growing list of adverse outcomes associated with pre-natal tobacco exposure. Specifically, in vitro exposure to tobacco products interfered with osteogenic differentiation of human embryonic stem cells, a well-established surrogate model for human embryonic bone development. Contrasting a diverse array of tobacco products unveiled that sidestream smoke was generally more developmentally osteotoxic than mainstream smoke and that harm-reduction products may not be less harmful than conventional products, adverse effects that were seemingly independent of nicotine.

Epidemiological studies suggest cigarette smoking as a probable environmental factor for a variety of congenital anomalies, including low bone mass, increased fracture risk and poor skeletal health. Human and animal in vitro models have confirmed hypomineralization of differentiating cell lines with sidestream smoke being more harmful to developing cells than mainstream smoke. Furthermore, first reports are emerging to suggest a differential impact of conventional versus harm-reduction tobacco products on bone tissue as it develops in the embryo or in vitro. To gather first insight into the molecular mechanism of such differences, we assessed the effect of sidestream smoke solutions from Camel (conventional) and Camel Blue (harm-reduction) cigarettes using a human embryonic stem cell osteogenic differentiation model. Sidestream smoke from the conventional Camel cigarettes concentration-dependently inhibited in vitro calcification triggered by high levels of mitochondrially generated oxidative stress, loss of mitochondrial membrane potential, and reduced ATP production. Camel sidestream smoke also induced DNA damage and caspase 9-dependent apoptosis. Camel Blue-exposed cells, in contrast, invoked only intermediate levels of reactive oxygen species insufficient to activate caspase 3/7. Despite the absence of apoptotic gene activation, damage to the mitochondrial phenotype was still noted concomitant with activation of an anti-inflammatory gene signature and inhibited mineralization. Collectively, the presented findings in differentiating pluripotent stem cells imply that embryos may exhibit low bone mineral density if exposed to environmental smoke during development.

Air pollution, including particulate matter (PM10, PM2.5) and airborne metals (Cd, Hg, As, Pb) are a systemic health hazard, but osteotoxic effects remain underrecognized. We systematically synthesized human, animal, and in vitro evidence on airborne contaminants and outcomes to skeletal health. Following Office of Health Assessment and Translation (OHAT) guidance, we searched PubMed and Google Scholar (2014–2024; final search November 24, 2024) for English-language studies relating PM or airborne metals to skeletal outcomes, excluding studies limited to adult occupational exposures, age-related bone disease, or non-peer-reviewed reports. Studies were screened and data extracted in Covidence. Risk of bias was assessed using tools from the Quality Assessment and Risk of Bias Tool Repository, and certainty of evidence was graded with GRADE. We used a random-effects meta-analyses model to estimate relative risks (RRs) interpreted as directional consistency hazards for bone loss, impaired osteoblast differentiation, enhanced osteoclast activity, bone marrow adiposity, epigenetic changes, and altered bone microarchitecture. Fifty-three studies met the inclusion criteria. Across outcomes, meta-analyses showed higher hazard risk in exposed versus control groups, with RRs ranging from 7 to 10; for example, bone loss (RR 7.12, 95% CI 4.20–12.06) and altered bone microarchitecture (RR 10.28, 95% CI 4.67–22.61). Epidemiologic and experimental evidence may implicate oxidative stress, inflammation, and epigenetic dysregulation as key mechanisms. Overall certainty was low to moderate due to residual confounding, exposure misclassification, and heterogeneity. Growing evidence indicates that airborne PM and metals adversely affect bone health, particularly during critical developmental windows, underscoring the need to integrate skeletal endpoints into health assessment and policy.

Developmental bone toxicity encompasses impairments in skeletal growth and remodeling during embryonic and fetal development that can result in long-term s