The Ontogeny and Induction by Zinc of Hepatic Chick Embryo Metallothionein 1

Abstract

Abstract

To avoid the maternal influences inherent in mammalian models, the chick embryo was examined for the effects of development on the ontogeny of hepatic metallothionein (MT) induction. Livers from embryos were examined for total and cytosolic zinc (Zn) and copper (Cu), and cytosolic MT on 12, 14, 16, and 18-21 days of incubation (DI) as well as 1, 7, and 14 days posthatch (ph). Cytosolic Zn levels fell, from a high on 12 DI (45.04 ± 1.05 ng/μg DNA), to 25.79 ± 1.05 ng/μg DNA on 19 DI and then increased to 44.73 ± 3.47 ng/μg DNA by 1 day ph. Cytosolic Cu followed a reverse pattern, with a high of 24.10 ± 0.74 ng/μg DNA on 16 DI. Both ¹⁰⁹Cd radioassay analysis and gel filtration chromatography of hepatic cytosols showed a developmental pattern for MT similar to that of cytosolic Zn. These results are in contrast to the developmental pattern of the rat. To establish if yolk Zn levels limit hepatic MT in the 1-day neonate, the yolks of unincubated fertile eggs were supplemented with 26, 52, or 78% of the endogenous Zn (768 μg/yolk). As a result, hepatic MT of 1-day neonates increased by 3.9-, 4.7-, and 7.1-fold, respectively. Thus the initial level of Zn in the yolk has a significant impact on the final concentration of MT in neonatal liver. A study of MT induction in the 18-DI embryo revealed that yolk sac administration of Zn (550, 2750, 4950 μg/yolk) increased hepatic MT by 5.8-, 23-, and 39-fold, respectively. This demonstrates the inducibility of MT even at the point of minimum endogenous MT (18 DI). In summary, our results show that (1) a marked difference exists between the developmental patterns of MT in avian and mammalian species, and (2) chick embryo hepatic MT is highly responsive to exogenous Zn introduced into the yolk.