CONICET | Buscador de Institutos y Recursos Humanos

Vitellogenin (Vtg), a large lipoglycophosphoprotein, is the most important precursor of the yolk proteins, and the major source of nutrients for the developing embryo in oviparous species. After its uptake by the oocytes, Vtg is converted into lipovitellins (LpvI and LpvII) and phosvitin, which are deposited into crystalline yolk platelets. We describe here the presence of two Lpv I isoforms in Bufo arenarum mature oocytes with molecular weights of 113 and 100 kDa. Amino acid sequence analysis showed high sequence homology between both peptides and the complete reported sequences of Xenopus vitellogenin. Using specific antibodies, we determined that Vtg uptake begins early during oogenesis, at the previtellogenic stage, and continues until oocytes have reached their mature status. In addition, we found that large endocytic vesicles mediate Vtg uptake in stage I oocytes, and that the size of the endocytic vesicles declines with oogenesis progression. In terms of Vtg protein trafficking, we detected Vtg precursor (190 kDa) in the liver of estradiol-stimulated females but not in their oocytes, suggesting that proteolytic cleavage occurs as soon as Vtg is internalized. Finally, we propose a subclassification of B. arenarum stage-II oocytes in three physiologically and morphologically distinct periods (early, mid and late).