Endogenous estrogen has beneficial effects on mature bone and negatively affects the developing skeleton, whereas the effect of environmental estrogens is not known. Methoxychlor (MXC) is a synthetic estrogen known as a persistent organochlorine and used as a pesticide. Methoxychlor and its metabolites display estrogenic, anti-estrogenic and anti-androgenic activity and may therefore influence bone. Fifty-eight male fetal and neonatal rats were exposed to either: a negative control (DMSO), 0.020, 100 mg/kg MXC, or 1 mg/kg β-estradiol-3-benzoate (EB; positive control). Rats were treated daily for 11 days, from embryonic day 19 to postnatal day (PND) 7 or for 4 days during the postnatal period (PND 0-7). All rats were analyzed at PND-84. Total body, femur, spine, and tibia areal bone mineral density (BMD) and content (BMC), lean body mass (LBM) and fat were measured by dual energy X-ray absorptiometry. Bone geometry and volumetric (v) BMD were measured using micro-computed tomography and biomechanical properties using three-point bending were assessed. Rats exposed to EB or MXC (at either the high and/or low dose), independent of exposure interval showed lower body weight, LBM, tibia and femur BMD and length, and total body BMD and BMC than DMSO control group (p ≤ 0.05). Methoxychlor and EB exposure increased cortical porosity compared to DMSO controls. Trabecular vBMD, number and separation, and cortical polar moment of inertia and cross-sectional area were lower due to EB exposure compared to control (p < 0.05). Early MXC exposure compromises cortical porosity and bone size at maturity, and could ultimately increase the risk of fracture with aging.

Estrogens have a well known physiological effect on bone. Chemicals in the environment with estrogenic activity may thus influence bone during development. Methoxychlor (MXC) is a chlorinated hydrocarbon pesticide with estrogenic properties and is commonly used in the United States. This study examines early MXC exposure in 25 male Fischer 344 rats (6‐7/group) randomized to control [DMSO (CTL)], MXC [0.02 mg/kg/day, low environmentally relevant dose (M‐low)], MXC [100 mg/kg/day, high dose (M‐hi)], or E2B [positive control; 1 mg/kg/day of β‐estradiol‐3‐benzoate]. Treatments were administered in utero (days 19‐22) and postnatally (days 0‐7). Body composition, and the femur and tibia bone mineral density (BMD) and content were examined at postnatal day 84 using dual energy x‐ray absorptiometry and biomechanical properties are currently being tested. Body weight was 92‐94% (p ≤ 0.05) and 80% (p < 0.001) of CTL in the MXC and E2B groups, respectively. Only the M‐low group showed a reduced % body fat (p < 0.01) compared to CTL, whereas lean body mass was lower (p ≤ 0.01) only in the M‐hi and E2B groups compared to CTL. Whole body BMD was lower (p < 0.05) in M‐hi and E2B versus CTL, but not after correcting for body weight. Low and high dose MXC reduced femur BMD (p < 0.01). These data show that early exposure to MXC will reduce growth and bone mineral density, and may be associated with an increase in fracture risk.