Yolk Proteins in Vertebrate: A Review

In all the oviparous vertebrates, also showing different reproductive modes, that is, oviparity, ovoviviparity, and viviparity, the oocytes always contain yolk proteins that represent the nourishment store for the developing embryo. Yolk proteins derive from the cleavage of a large maternal serum lipoglycophosphoprotein called vitellogenin (VTG) produced by the liver under estrogen stimulation (Wallace, 1985 and references therein; Rosanova et al., 2002). The mammalian (eutherian) liver lost the ability tomakeVTGbut the genetic bases of this acquired inability are still unknown (Rothchild, 2003). Once synthesized by the liver, VTG reaches the ovarian follicles through the bloodstream, crosses between the granulosa cells and is incorporated into the oocytes by micropinocytosis, a mechanism of receptor-mediated endocytosis (Opresko and Wiley, 1987; Schneider, 1996; Romano and Limatola, 2000). VTG internalization occurs in coated pits of the oocyte plasma membrane; they rapidly detach and give rise to coated vesicles in the cortical oocyte cytoplasm; the latter rapidly lose their clathrin coat and coalesce to form the primordial yolk globules subsequently transformed into yolk platelets (Ghiara et al., 1968, 1970; Neaves, 1972; Wallace, 1985; Limatola and Filosa, 1989). Similar ultrastructural features were described also in the mosquito oocytes (Roth and Porter, 1964). In the platelets, VTG are enzymatically cleaved in lipovitellins and phosvitins that represent the two principal classes of yolk proteins. Lipovitellins are high molecular-weight proteins with a strongly hydrophobic nature; phosvitins show a lower molecular weight and are highly phosphorylated. The cleavage of VTG in yolk proteins is generally indicated as primary degradation; furthermore, in almost all vertebrates studied, a secondary degradation of yolk proteins occurs at oocyte maturation or later during embryo development. This review represents an attempt to summarize the numerous data of the literature about the VTG derived yolk proteins during the oogenesis and their utilization during the embryogenesis