Trace element metabolism in the developing avian embryo: a review.

Trace element metabolism in the developing avian embryo involves a variety of interrelated processes beginning with metal deposition in the nascent egg. Vitellogenin, the yolk precursor protein, plays a fundamental role in the transport of trace elements from hepatic stores in the hen to the developing oocyte, where it facilitates the uptake and storage of metals in the egg yolk. Egg white proteins such as ovalbumin and conalbumin (ovotransferrin) may function in an analogous manner by effecting the transfer of trace elements from the oviduct to the egg. The uptake and metabolism of trace elements by the developing embryo are mediated by the extraembryonic membranes, predominantly the yolk sac, which are in direct contact with the egg contents. Specific metal-binding proteins play important roles in the interorgan transport, cellular uptake, and intracellular partitioning of trace elements in the developing embryo. Models are proposed to explain the deposition of trace elements in the newly formed egg and the uptake and transfer of trace elements from the egg yolk and white to the tissues of the developing embryo and its associated membranes.