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, Oncorh ynchus mykiss. Northern blot confirmed the presence of a transcript (approx imately 3.9 kilobases [kb]) that was specific to the ovary. The expression of VTGR mRNA varied throughout ovarian development and was highest in previ tellogenic ovaries and in ovaries at the onset of vitellogenesis containing ovarian follicles (OF) from 35 to 600 pm in diameter. In situ hybridizatio n 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 ex clusively localized in the ooplasm. A dramatic increase in mRNA synthesis o ccurred during previtellogenic growth (OF from 50 to 200 mu m); this was fo llowed by a gradual decrease during the vitellogenic growth phase. VTGR mRN A was not detected in OF greater than 1000 mu m in diameter (oocytes active ly sequestering VTG). Immunocytolocalization of yolk proteins derived from VTG demonstrated that oocytes started to sequester VTG when they were aroun d 300 mu m in diameter, shortly after the time of maximal density of VTGR m RNA in the ooplasm. The timing of transcription of the VTGR gene, predomina ntly during previtellogenesis, suggests that the VTGR is recycled to the oo cyte surface during the vitellogenic growth phase.