Gassmann et al. C57/BL6 and nonresponsive DBA mouse strains (Ema et al. 1994). The irresponsive­ ness of hNPCs toward AhR modulation was hence likely due to very low AhR expression combined with a human low­ affinity recep­ tor for ligand binding. Because several poly­ morphisms have been identified in the human AhR gene, interindividual differences in ligand responsiveness between hNPCs from different donors cannot be excluded. However, even though single AhR polymorphisms (e.g., at codon 517 or 570) found in people of African descent were under suspicion to affect the expression of single genes such as CYP1A1, it is widely accepted that none of the AhR polymorphisms described so far is of any func­ tional consequence regarding the overall out­ come of AhR response (Connor and Aylward 2009; Harper et al. 2002). Species differences similar to those reported here were observed in a compara­ tive study of human and mouse palate organ cultures (Abbott et al. 1999). This earlier work was driven by the facts that TCDD induces cleft palate in mouse embryos and that the risk for humans to develop such malformations upon in utero exposure to TCDD or related compounds was not known. Abbott et al. (1999) concluded that it seems highly unlikely that human embryos are exposed to sufficient amounts of TCDD to cause interruption of palatal differentiation because human palate organ cultures expressed several hundred times less AhR mRNA than did the mouse cultures, and human palates required 200 times more TCDD to produce a cleft palate in vitro than did the respective mouse model. Our results support those findings in a different organ system, the developing brain, and suggest that studies on AhRd­ependent DNT in C57/BL6 mice in vivo overestimate the risk of distur­ bances of human brain development resulting from AhR activation. Our data also imply that the use of toxic equivalency factors, which cor­ respond to the relative potency of a chemical to generate AhR­mediated effects compared with TCDD, based on data derived from rodents for risk assessment of POPs may not necessarily be useful for humans. How can the epidemiological evidence for POPr­elated DNT in humans be explained if the AhR is not involved in chemically induced DNT? Kodavanti (2005) comprehensively reviewed three alternative mechanisms for how POPs might interfere with human brain development. First, POPi­nduced changes in neurotransmitters such as dopamine or sero­ tonin could affect learning, memory, and other functions. Because these end points extend beyond the basic processes of brain develop­ ment, the basic “neurosphere assay” (Breier et al. 2009) cannot detect such changes. Second, PCBs were found to alter intracellular phosphokinase C (PKC) signaling and Ca2+ 1576 homeostasis in rodents. Because PKC mod­ ulation by the inhibitor BisI (bisindolylma­ leimide I) or the stimulator PMA (phorbol 12m­yristate 13a­cetate) causes inhibition and stimulation of hNPC migration, respectively (Moors et al. 2007), it is unlikely that the POPs used in this study affected PKC signaling. Third, effects on thyroid hormone (TH) ba l­ ance might contribute to POPi­nduced DNT. Some POPs can elicit direct biological effects on target organs such as the brain by inter­ fering with cellular TH signaling in vivo and in vitro (Fritsche et al. 2005; Schreiber et al. 2010; Zoeller and Crofton 2000). Because the neurosphere assay is able to detect endo­ crine disruption of TH signaling by PCBs and polybrominated diphenyl ethers (Fritsche et al. 2005; Schreiber et al. 2010), this mecha­ nism is hence unlikely to be involved in AhR­ independent DNT of “classical” AhR ligands such as 3­MC or TCDD in humans. The most likely mechanism through which POPs interfere with human brain development is via systemic effects on TH homeostasis result­ ing from direct effects on the thyroid gland and decreased synthesis of TH, reduced blood TH levels secondary to enhanced TH metabo­ lism, or displacement of natural ligand (thy­ roxine) binding to the TH plasma transport protein transthyretin. Evidence that TCDD also affects systemic TH balance in humans was reported by Baccarelli et al. (2008), who found that maternal exposure in the highly TCDD­exposed Seveso cohort was associ ­ ated with effects on neonatal thyroid function (Baccarelli et al. 2008). However, we cannot exclude the possibility that AhR expression in fetal brains increases with gestational age, resulting in AhRm­ediated toxicity of POPs in NPCs at later gestational stages. 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Methods 25:402–408. ovlume 118 | number 11 | November 2010 • Environmental Health Perspectives