In most oviparous animal species, oocyte growth occurs via the uptake of plasma egg yolk precursors, predominantly vitellogenins
(Vtg). These glycolipoproteins are members of the large lipid transfer protein superfamily and key players in reproduction.
While the vertebrate liver has been demonstrated to synthesize large amount of Vtg, mostly under 17beta-estradiol control,
the ability of other tissues to express significant amounts of Vtg had not been conclusively demonstrated. RT-PCR revealed
vtg1 transcripts in female zebrafish and rainbow trout white adipose tissue (WAT). It was also found to co-express mtp, known to perform the intracellular lipidation of Vtg prior to secretion. The liver and pancreas markers, apobb2 and ins or ela2, respectively, were not expressed in adipocytes. Whole-mount in situ hybridization and in situ RT-PCR on histological sections
revealed vtg1 signal in adipocytes, whereas no signal was detected in infiltrated pancreatic islets. Transcript expression of vtg1 was induced in WAT of 17beta-estradiol-treated males and the transcript and corresponding protein were detected in the thin
rim of cytoplasm surrounding the adipocyte. qPCR showed that rainbow trout perivisceral WAT vtg1 transcript levels were high during early compared to late vitellogenesis. Taking normalized mRNA levels and tissue somatic
index into account, vtg1 transcript levels at the beginning of oocyte yolk deposition were around 45-times lower in WAT than liver and these levels
were not correlated to plasma Vtg and 17beta-estradiol concentrations. These findings suggest that WAT Vtg is implicated in
providing components to the ovary during the early stages of vitellogenesis.