Zinc physiology and biochemistry in oocytes and embryos.

The essential role of zinc in embryogenesis was identified through studies of its presence in eggs and embryos, the effects of its deficiency and its role in metallo proteins required for organ development and formation. The Xenopus laevis oocyte zinc content varies during oogenesis. It increases from 3 to 70 ng zinc/oocyte as it progresses from stage I to VI. The oocyte zinc is derived from the maternal liver as part of a metallo-complex with vitellogenin. The latter transports the metal in plasma and into the oocyte. Once internalized, most of the zinc is stored within yolk platelets bound to lipovitellin, one of the processed products of vitellogenin. About 90% of the total zinc is associated with the yolk platelet lipovitellin while the remaining 10% is in a compartment associated with hitherto unknown molecule(s) or organelle(s) of the cytoplasm. The bi-compartmental distribution remains constant throughout embryogenesis since the embryo behaves as a closed system for zinc after fertilization. The yolk platelet zinc is used after the tadpole is hatched while we proposed that the 10% of the zinc in the non-yolk platelet pool is the one used for embryogenesis. It provides zinc to newly synthesized molecules responsible for the development of zinc-dependent organ genesis. Interference with the availability of this zinc by the chelating agent 1,10-phenanthroline results in the development of embryos that lack dorsal organs, including brain, eyes and spinal cord. The extensive teratology is proposed to be due to altered or absent zinc distribution between the cytosolic pool and zinc-transcription factors. The data identify the components of a zinc transport, storage and distribution system in a vertebrate organism.