Immunoglobulin protein and gene transcripts in ovarian follicles throughout oogenesis in the teleost Dicentrarchus labrax.

Transfer of immunoglobulins (IgM-like) from the female to the teleost embryo has been demonstrated but mechanisms of uptake into and storage within the eggs remain to be clarified. The monoclonal antibody DLIg3 against Dicentrarchus labrax Ig light chain revealed an active role of both follicle cells and oocytes in the Ig uptake. The primordial follicular cells showed DLIg3 immunoreactivity even at a pre-vitellogenetic stage. Early vitellogenetic oocytes (lipid vesicle stages) had DLIg3 staining of pore canals, plasmalemma and outer cortex and of their follicular cells. In protein yolk granule oocytes, DLIg3 staining was also detected within vesicles of the outer-mid cortex and juxtanuclear yolk granules; therefore, a centripetal transport of Ig throughout oocyte development is apparently carried out. Immunoelectron microscopy confirmed the presence of Ig within thecal and granulosa cells (and in the interposed basement membrane) of pre-vitellogenic and vitellogenic follicles. Thus, the transport of Ig to the egg apparently occurs also by transcytosis across the follicle cells. Igs were localised in the pore canals surrounding the microvilli and in vesicles of outer-mid cortex of vitellogenic oocytes. Reverse transcription/polymerase chain reaction with primers designed for the constant region of sea bass Ig light chain detected Ig mRNA in hydrated oocytes, a smaller content in released eggs and no signal in larvae at day two post-hatching. These findings show that a significant level of Ig gene transcription in the oocyte and/or a transfer of transcripts may also occur.