Higher-order teleosts (Acanthomorpha) express three distinct forms of vitellogenin (VtgAa, VtgAb, and VtgC) and these have been characterized in a number of fish species. The VtgAa and VtgAb are considered “complete” and contain five yolk protein domains (lipovitellin heavy chain, lipovitellin light chain, phosvitin, beta’-component, and c-terminal component), however the VtgC is “incomplete” and only contains the lipovitellin heavy and light chains. These egg yolk precursors are produced by the liver in response to estrogen, with gene expression mediated by estrogen receptor alpha. Vitellogenins are released into the circulatory system where they are taken up specifically by growing oocytes via receptor-mediated endocytosis within clathrin- coated pits. Two vitellogenin receptors (LR8 and Lrp13 ) have been characterized in fishes: The Lrp13, which localizes throughout the zona radiata and granulosa cells and specifically binds VtgAa, and the LR8, which localizes to the oolemma and zona radiata interna and binds VtgAb. To date, no known lipoprotein receptor has been shown to bind VtgC in Acanthomorphs and it might enter oocytes through the endocytosis fluid phase or escorted by Y-box binding protein 2a, to which it binds. Additionally, VtgC localizes exclusively to lipid inclusions within growing oocytes, whereas VtgAb localizes to the ooplasm and yolk globules; VtgAa has not yet been evaluated in this manner. The VtgAb primarily enters growing oocytes during early- to mid-vitellogenesis, whereas VtgAa enters oocytes from mid- to post-vitellogenesis. The VtgAb is typically the predominant form in blood plasma and egg yolk. The VtgC is accumulated by oocytes beginning at pre-vitellogenesis and continues until post-vitellogenesis and its composition in the yolk can vary widely between species. The overall proportional accumulation of different Vtgs within the yolk influences egg buoyancy. In higher-order teleosts, yolk proteins derived from VtgAa are degraded into peptides and free amino acids that drive oocyte hydration during ovarian maturation. A link between egg diameter and buoyancy were observed in striped bass, indicating that more buoyant eggs have a larger outer diameter due to hydration and also a greater proportion of VtgAa yolk content. The proportional deposition of different Vtgs in this species was influenced by water salinity, such that eggs of the correct specific gravity were ovulated and tailored to the estuarine environment.
Lower-order teleosts also possess different complete Vtg forms , although, with the exception the Ostariophysian fishes, they all appear to be functionally similar. These species also express VtgC. In salmonids , the c omplete VtgAs form binds both LR8 and Lrp13 receptors. In salmonids, t he VtgAs and its derived lipovitellin (LvAs) are always pre dominant (95%) in the circulation and in the yolk of vitellogenic females. Understanding the functions of these multiple vitellogenins is relevant to egg quality, since yolk components not only provide nutrition to embryos and larvae at specific developmental stages, but contribute to egg buoyancy as well.