Zinc deficiency and the developing embryo

Abstract

The effect ofin utero zinc deficiency on fetal development in rats is reviewed. Attention is paid to the primary biochemical lesion associated with zinc-related teratogenesis and special consideration is given to the central nervous system. Evidence is presented that the thymidine kinase salvage pathway, used for the synthesis of thymidine monophosphate in DNA synthesis, is depressed more in fetal brain tissue than in the liver. In addition, greater reliance appears to be placed on this pathway than onde novo synthesis in the fetal brain than in other tissues. Some consideration is given to the use of in vitro embryo culture in studies relating to neurogenesis, but evidence is presented of a greater capacity of explanted rat embryos to obtain zinc from maternal serum than occurs in vivo.The rapid onset of a teratogenic zinc deficiency following dietary zinc restriction is again highlighted and further studies are described which demonstrate the critical impact of a single feeding cycle, of 4 d duration, on maternal plasma zinc levels and on the extent and nature of the observed fetal abnormalities. Evidence is presented that by shifting the timing of the high dietary intake/low plasma zinc peak to coincide with a particular 48 h period between days 6 and 10 of pregnancy, the pattern of malformations thus obtained reflected the coincidence of the high dietary intake of zinc-deficient diet and the critical time of morphogenesis of several organ systems.Whereas diminished plasma zinc levels at term in zinc-deficient animals are generally well correlated with reduced growth and dysmorphogenesis of the offspring, the same is not always found in human studies. In some cases, elevated plasma zinc levels at parturition are found in mothers with growth-retarded children, or vice versa. Experimental studies with rats are reported that suggest that maternal zinc status at term may be higher in dams bearing pups stunted by exposure to a transient zinc deficiency early in pregnancy, which in turn may have reduced the demand for maternal zinc in the later stages of gestation.The protective effect of zinc on cadmium-induced teratogenesis is discussed, particularly in relation to findings concerning an interaction of these metals in the embryonic yolk sac and thus on preplacental embryonic nutrition. Possible interactions between alcohol and zinc deficiency are also considered and data are presented pointing to increased fetotoxicity and teratogenesis in the presence of both treatments and to a more specific interaction with respect to reduced cell numbers in the developing rat hippocampus. Malondialdehyde levels, which reflect the extent of lipid peroxidation in tissue, are reported to be substantially higher in microsomes from fetal rat livers whenin utero deficiency and gestational alcoholism are combined. The suggestion is made that alcohol and zinc deficiency act independently in the body, but overlap to some extent at the common biochemical locus of membrane lipid peroxidation.