Immunoelectron Microscopical Study of Prolactin in Pituitary of Tilapia (Orochromis mossambicus).

Abstract

Protein A-gold method was used for cytochemical localization of prolactin (PRL) in the secretory cells in the tilapia (Oreochromis mossambicus) pituitary. Antiserum to chum salmon prolactin (sPRL) was used for this study. Effects of fixation on the antigencity of PRL were examined, and the antigenicities of PRL in pituitaries of seawater (SW) -and freshwater (FW) -adapted tilapia were compared.The antiserum to sPRL reacts selectively only with the secretory granules in the specific cells, i, e., the presumptive PRL cells, in the rostral pars distalis, but never reacts with the other cellular compartment of the PRL cells nor with other cell types in the tilapia pituitary. The findings have been confirmed by the control experiments in which preincubating the antiserum with purified sPRL abolished all the immunoreactivity. PRL cells which have been fixed with 4% paraformaldehyde for 2 to 12 hr, do not show any significant difference in antigenicity preservation, but show a better ultrastructure in longer fixation periods. Addition of 2.5% glutaraldehyde to paraformaldehyde could greatly enhance the ultrastructural preservation but concomitantly might cause a loss of 30% of the antigenicity. Post fixation with 1% osmium tetroxide improves the membranous ultrastructure of organelles, however reduced the PRL antigenicity by 50%. The PRL antigenicity (number of protein A-gold binding per PRL granule) in FWadapted tilapia is 1.5 times that in SW-adapted one. Thus, protein A-gold immunocytochemical method is suggested to be suitable for evaluation of the activity of PRL cells in tilapia reared in different environments.