Chick and duck embryos in the evaluation of pesticide toxicity

Avian Models for Toxicity Testing

The use of birds as test models in experimental and environmental toxicology as related to health effects is reviewed, and an overview of descriptive tests routinely used in wildlife toxicology is provided. Toxicologic research on birds may be applicable to human health both directly by their use as models for mechanistic and descriptive studies and indirectly as monitors of environmental quality. Topics include the use of birds as models for study of teratogenesis and embryotoxicity, neurotoxicity, behavior, trends of environmental pollution, and for use in predictive wildlife toxicology. Uses of domestic and wild-captured birds are discussed.

[Current status and future progress of reproductive/developmental toxicity test]

Currently, the European Centre for the Validation of Alternative Methods in the EU appears to be at the forefront of the development of alternative methods for developmental toxicity test (reproductive/developmental toxicity test). Why is it difficult to develop alternative methods for developmental toxicity test in comparison with other toxicity tests? In developmental toxicity test, chemical substances first enter the bloodstream and then reach the placenta via metabolism in the liver and other organs. After further metabolism in the placenta, chemical substances finally reach the fetus, where they affect fetal development. The difference in the in vivo route of chemical substances is an important reason for the difficulty in the establishment of new methods for developmental toxicologic test in comparison with general toxicity tests. According to the EU, the use of "in silico" techniques for developmental toxicity test may be difficult, and I agree with this. The in silico technique is basically a method for prediction of toxicologic effects from existing data, and cannot predict new effects, because data obtained by developmental toxicologic test are too complex. Three techniques are now being examined to overcome the difficulty in changing the method of developmental toxicologic test: the technique utilizing embryonic stem cells; micromass culture technique; and the whole embryonic culture technique. In this symposium, the current status of developmental toxicity tests and the three techniques being examined in the EU are introduced, and opinions on future progress are presented.

Development of esterase activities in the chicken before and after hatching

The embryonic chick has long been a model for developmental biology and has often been recommended as a model system in developmental toxicology. More recently, several investigators have shown that the chick embryo also provides a good model for identifying the neurotoxic effects of environmental pollutants, especially cholinesterase-inhibiting pesticides. Although numerous studies detail the structural development of chick embryos, few describe embryonic levels of enzyme synthesis and their changes during development. In this study, the development of esterase activity in chick embryos was measured from day 9 of incubation until 46 days after hatching. Brain acetylcholinesterase (AChE) activity was detected on day 9 of incubation at a concentration of 0.364 mumoles/min/g tissue. An increase between AChE activity and age of the embryos was observed. In the liver, the nonspecific cholinesterases (ChE) and carboxylesterase activities during incubation were not different from activities after the chicks had hatched. Plasma ChE and carboxylesterase activities did not change with age after hatching. Brain neuropathy target esterase (NTE) activity was not detected on day 9 of incubation and was extremely low (6.12 nmoles/15 min/mg protein) the next day, but increased rapidly with increasing age. This study demonstrates that chick embryos have developed esterase activities in the brain and liver by day 10 of incubation and again confirms that the insensitivity of chick embryos and young chicks to organophosphorus ester-induced delayed neurotoxicity is not due to absence of NTE. In addition, the results provide baseline data for evaluating the response of embryonic and immature chicks to neurotoxicants and teratogens.

Effects of fenthion, fenitrothion and desbromoleptophos on gait, acetylcholinesterase, and neurotoxic esterase in young chicks after in ovo exposure

Previous studies have demonstrated that gait is affected in chicks exposed to organophosphorus esters (OPs) that induce delayed neurotoxicity (OPIDN) in adult hens. To investigate the developmental relationship between such functional deficits and OPIDN, chicks were exposed to 3 OPs with different OPIDN potential. Desbromoleptophos (DBL) induces OPIDN in adult hens; fenthion (FEN) has uncertain OPIDN potential; fenitrothion (FTR) does not induce OPIDN. Chicks were treated by injection into the egg on day 15 of incubation, after the presumed period of OP-induced structural teratogenesis. AChE and neurotoxic esterase (NTE) were assayed during incubation and in parallel with post-hatching evaluations of gait. DBL, 125 mg/kg in ovo, caused paralysis in 70% of chicks after hatching. The gait of surviving chicks was affected for at least 6 weeks and marked by toes curling under. NTE was inhibited until 10 days post-hatching and AChE until hatching. FEN did not inhibit NTE significantly, but AChE was significantly inhibited until hatching. Chicks exposed as embryos to FEN were hyperactive and aggressive. Gait was still affected 6 weeks after treatment with 3 mg/kg FEN. FTR at 125 mg/kg inhibited AChE until day 10 post-hatching, but neither inhibited NTE nor affected gait. The growth of OP-exposed chicks was not significantly decreased, so the decreased length and increased width of the stride could not be ascribed to stunted growth. We conclude that OPs cause irreversible effects on gait that are not related to their defined neurotoxic effects, since altered gait (1) occurs below the age of sensitivity to OPIDN, (2) is seen in the absence of NTE inhibition and (3) does not invariably accompany AChE inhibition.

Comparison of cytotoxicity and thin-layer chromatography methods for detection of mycotoxins

Thirty-three standard mycotoxins were assayed by thin-layer chromatography and by cytotoxicity in HEp-2 and Chang cells. Various levels of detection were found. The cytotoxicity test was significantly more sensitive than thin-layer chromatography for the trichothecenes and should be useful for screening extracts from animal feedstuffs for the presence of unknown mycotoxins.

Use of chick embryo in screening for embryotoxicity

In vitro systems afford a unique opportunity for investigating the direct interaction of substances with developing morphogenetic systems (MGSs), since maternal influences are excluded. As a carrier of a complete set of MGSs, the chick embryo in ovo manifests an advantage over those in vitro systems that employ isolated embryos or embryonic tissues that have only limited survival. Under controlled experimental conditions including standardization of subjects, administration technique and mode of evaluation according to the basic principles of teratology, the chick embryo test was demonstrated to be reliable and to afford quantifiable endpoints for evaluation (e.g., specific embryotoxicity effect level, positive dose response, and a stage response and effect). Individual compounds, mixtures of compounds, and even poorly identified extracts can easily be tested. The chick embryo possesses its own basic enzyme-catalyzed drug-transformation capacity and, moreover, it can be used for screening specific human metabolites. Comparative studies have indicated a high predictive value of the chick embryo system relative to other in vitro systems as well as to whole-animal testing.