FORMATION OF THE VITELLINE ENVELOPE PRECEDES THE ACTIVE UPTAKE OF VITELLOGENIN DURING OOCYTE DEVELOPMENT IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

In order to study the initial formation of the vitelline envelope and the appearance of vitellogenin in oocytes of rainbow trout, females we re sampled monthly from 19 to 5 mo before ovulation. Immunohistochemis try revealed that the formation of the vitelline envelope starts when the oocytes reach a diameter of about 450 mu m. Oocytes of this size w ere first found in females sampled a year before ovulation at the time when plasma levels of estradiol-17 beta increased from 0.2 to 0.6 ng/ ml. An antiserum directed against vitellogenin crossreacted with small vesicles (around 2 mu m) present just inside the oolemma, when the oo cytes reached a diameter of 600 mu m. This was interpreted as an activ e uptake of vitellogenin. Oocytes of this size were first found in fem ales sampled 9 mo before ovulation at the time when estradiol-17 beta levels increased from 0.6 to 1.0 ng/ml and the gonadal somatic index w as doubled. Oocytes with a diameter of 600 mu m had an immunoreactive vitelline envelope with a thickness of about 3 mu m. It is apparent th at the initial formation of the vitelline envelope starts before the a ctive uptake of vitellogenin and that the low previtellogenic plasma l evels of estradiol-17 beta observed in females are of physiological si gnificance. (C) 1994 Wiley-Liss, Inc.