Expression and localization of messenger ribonucleic acid for the vitellogenin receptor in ovarian follicles throughout oogenesis in the rainbow trout, Oncorhynchus mykiss

The expression and localization of vitellogenin (VTG) receptor (VTGR) mRNA were identified throughout ovarian development in the rainbow trout, Oncorhynchus mykiss. Northern blot confirmed the presence of a transcript (approximately 3.9 kilobases [kb]) that was specific to the ovary. The expression of VTGR mRNA varied throughout ovarian development and was highest in previtellogenic ovaries and in ovaries at the onset of vitellogenesis containing ovarian follicles (OF) from 35 to 600 pm in diameter. In situ hybridization using S-35 riboprobes showed that the transcription of the VTGR gene was initiated in OF measuring 45-50 mu m in diameter, with transcripts being exclusively localized in the ooplasm. A dramatic increase in mRNA synthesis occurred during previtellogenic growth (OF from 50 to 200 mu m); this was followed by a gradual decrease during the vitellogenic growth phase. VTGR mRNA was not detected in OF greater than 1000 mu m in diameter (oocytes actively sequestering VTG). Immunocytolocalization of yolk proteins derived from VTG demonstrated that oocytes started to sequester VTG when they were around 300 mu m in diameter, shortly after the time of maximal density of VTGR mRNA in the ooplasm. The timing of transcription of the VTGR gene, predominantly during previtellogenesis, suggests that the VTGR is recycled to the oocyte surface during the vitellogenic growth phase.