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.