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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