Brain asymmetry as a potential biomarker for developmental TCDD intoxication: a dose-response study

Area of Science:

• Environmental toxicology
• Developmental neurobiology
• Comparative toxicology

Background:

• In ovo exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been linked to brain asymmetry in wildlife.
• This asymmetry, characterized by interhemispheric differences in the forebrain and tecta, was previously observed in species like herons and cormorants.
• Wildlife studies correlated brain asymmetry with dioxin toxic equivalency factors (TEFs) in eggs.

Purpose of the Study:

• To investigate the effects of in ovo TCDD exposure on brain development in a laboratory model (chicken).
• To determine if TCDD alone induces brain asymmetry in chickens.
• To assess the dose-response relationship and developmental timing of TCDD-induced brain asymmetry.

Main Methods:

• Chicken embryos (Gallus gallus) were exposed to varying doses of TCDD or a vehicle control in ovo.
• Embryos were sacrificed at multiple developmental stages (9, 11, 13, 15, 17, 20 days of incubation) and post-hatch (24 hr, 3 weeks).
• Brain asymmetry was measured in both chicken embryos and hatchlings.

Main Results:

• TCDD exposure alone induced significant brain asymmetry in developing chickens.
• The observed asymmetry in chickens mirrored that seen in wildlife exposed to dioxin-related contaminants.
• A dose-dependent increase in the frequency and severity of brain asymmetry was noted across all measured ages.
• Asymmetry was detectable as early as embryonic day 9, indicating a direct effect on brain development.

Conclusions:

• TCDD is a direct inducer of brain asymmetry in developing chickens.
• Chicken is a sensitive model for studying TCDD's neurodevelopmental effects.
• The findings suggest that TCDD's impact on brain development occurs early and is not secondary to effects on the braincase.