Reproduction in Domestic Animals

Dietary Pollutants as Risk Factors in Reproduction 17Level ofexposureClass of compound – volatility, hydrophobicity,degradability, solubility, binding properties, etcTrophic level/biomagnificationEnvironmental concentrationUptakeDietary exposure(food/water)• Food type• Degradation in intestinal tract• Efficiency of absorption from tract• Dietary chelatorsNon-dietary exposure(skin/integument/lungs/gills, etc)• Structure/permability of barrier• Active transport processes (?)• Octanol/water or octanol/air coefficient• SequestrationPost-uptakeWithin animal• Metabolism• Excretion• Lipid contentFig. 1. Determinants of level ofexposure to environmentalpollutants, of uptake by animals,rates of exposure of their organsand effects on themIndividual organ• Permeability of organ membrane(e.g. brain; testis)• Sensitivity of target tissues/targetgenesOffspring• Susceptibility of gametes• Efficiency of trans-placentaltransfer• Fetal metabolism/excretionassociated gene expression. Furthermore, disruption ofthe germ cell line in one generation can be expressed insubsequent generations (Bøgh et al. 2001; Anway andSkinner 2006; Edwards and Myers 2007), probablythrough altered gene methylation, and therefore geneexpression, during the period of sex differentiation of thedeveloping gonad (Anway and Skinner 2006). Althoughdemonstrated in vertebrates, these fundamental mechanismsare likely to pertain to most species.How Important is Exposure Through Ingestion?The complex relationship between rates of pollutantingestion and effects exerted may be further complicatedby related factors such as additional uptake throughdrinking water, inhalation or, particularly in smaller,invertebrate species, through dermal absorption ofpollutants.Exposure through foodFood is generally considered to be the most importantroute of exposure in vertebrates, except, perhaps, at thelowest trophic levels (Fries 1995; Kavlock et al. 2002;Norstrom 2002). Since pollutants accumulate in thetissue of animals at each trophic level, the position of theanimal in the food chain determines the exposure rate;owing to the effects of biomagnification (increasedconcentrations as a result of food chain energetics),the highest tissue concentrations are frequently recordedin carnivorous species at or near to the top of the foodchain (Johnson et al. 1996; Darnerud et al. 2002; Davidand Gans 2003; Law et al. 2003; Naert et al. 2007). Forexample, in such species, concentrations of polybrominateddiphenyl ethers (PBDEs) often can be>100 lg ⁄ kg lipid (Darnerud et al. 2001) and PCBs>100 lg ⁄ kg dry matter based on the whole body(Johnson et al. 1996).Herbivores, being much closer to the primary sourceof food production, are not subject to multiple levels ofbiomagnification. Accordingly, tissue accumulation (theincrease in concentration tissues attributable to uptake)is frequently much lower with PBDEs being