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 fr om 3 to 70 ng zinc/oocyte as it progresses from stage I to VI. The oocyte z inc is derived from the maternal liver as part of a metallo-complex with vi tellogenin. The latter transports the metal in plasma and into the oocyte. Once internalized, most of the zinc is stored within yolk platelets bound t o 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 molecul e(s) or organelle(s) of the cytoplasm. The bi-compartmental distribution re mains constant throughout embryogenesis since the embryo behaves as a close d 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 n on-yolk platelet pool is the one used for embryogenesis. It provides zinc t o newly synthesized molecules responsible for the development of zinc-depen dent organ genesis. Interference with the availability of this zinc by the chelating agent 1,10-phenanthroline results in the development of embryos t hat lack dorsal organs, including brain, eyes and spinal cord. The extensiv e teratology is proposed to be due to altered or absent zinc distribution b etween the cytosolic pool and zinc-transcription factors. The data identify the components of a zinc transport, storage and distribution system in a v ertebrate organism.