Fish egg yolk is largely derived from vitellogenins, which are synthesized in the liver, taken up from the maternal circulation by growing oocytes via receptor-mediated endocytosis and enzymatically processed into yolk proteins that are stored in the ooplasm. Lipid droplets are another major component of fish egg yolk, and these are mainly composed of neutral lipids that may originate from maternal plasma lipoproteins. This review aims to briefly summarize our current understanding of the molecular mechanisms underlying yolk formation in fishes. A hypothetical model of oocyte growth is proposed based on recent advances in our knowledge of fish yolk formation.

Recent investigations have revealed multiplicity in maternal yolk precursors and their corresponding ovarian lipoprotein receptors (LRs) in diverse oviparous vertebrates, including fishes. This mini-review describes further evidence for the system of fish egg yolk formation mediated by multiple ovarian LRs, which have been obtained by studies utilizing a combination of conventional molecular and biochemical analyses, and modern proteome and transcriptome technologies. A hypothetical "multiple ovarian LR" model is proposed based on our current and previous knowledge of fish yolk formation.

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Elasmobranchs (sharks and rays) exhibit unique reproductive characteristics and, in contrast to the situation in teleosts, very little is known about the identity, structure and physical characteristics of their egg yolk proteins. The aims of this study were to (1) detect and purify the vitellogenin (Vtg; egg yolk precursor) and yolk proteins (YPs) of the cloudy catshark (Scyliorhinus torazame), (2) examine the relationships between Vtg and YPs and (3) characterize and classify the deduced primary structure of the Vtg transcript (vtg). The apparent molecular weights of purified Vtg and putative Vtg-related YPs (lipovitellin: Lv, phosvitin: Pv) were determined by gel filtration and were ~560, >669 and ~58 kDa, respectively. Following SDS-PAGE, these purified products (i.e., Vtg, Lv and Pv) appeared as bands of ~210, ~110 and ~22 kDa, respectively. On Western blots, antisera against purified Vtg, Lv and Pv recognized the ~210 kDa Vtg band. Catshark Pv, in contrast to teleost Pvs, had a very low serine content. The catshark Vtg cDNA sequence (vtg) appeared to contain an open-reading frame consisting of domains encoding Lv, Pv and β'-component (β'-c). A phylogenetic analysis, with a consideration of genome duplication events, placed catshark vtg into the 'vtgAB type.' It is concluded that at least a single major type of Vtg protein, which is transcribed and translated from catshark vtgAB gene, is the precursor of three egg yolk proteins (Lv, Pv and β'-c) in catshark.

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The gene, vitellogenin (vtg) was cloned and characterized in the dojo loach (Misgurnus anguillicaudatus), an indigenous freshwater species in East Asia, in order to develop tools for detecting the effects of estrogenic endocrine-disrupting chemicals (EEDCs). Full-length cDNAs encoding seven distinct vtg transcripts (vtg1-7) were obtained. The corresponding deduced amino acid sequences (Vtg1-7) were divided into two types; type I (Vtg1-6; 89-99% identical), which contained both lipovitellin (Lv) and phosvitin (Pv), and type II (Vtg7), which contained Lv alone. Phylogenetic analysis revealed that the type I and type II Vtgs in the loach could be classified as VtgAo1 and VtgC types, respectively. Immuno-biochemical analyses using type-specific Vtg antisera revealed that VtgAo1 proteins appeared to be the major Vtg type in this species. Males were administered (aqueous exposure) either 17β-estradiol (E2) or 17α-ethinylestradiol (EE2), the results from which were used to determine that hepatic vtgAo1 expression was estrogen-sensitive. The precise classification of the loach vtg/Vtg products, as well as their induction profiles following the estrogenic stimulation, provide a basis for their use as sensitive biomarkers when EEDC activities are evaluated in the freshwater environments in East Asia.