The dynamics of oocyte growth during vitellogenesis in the rainbow trout (Oncorhynchus mykiss).

This report describes the dynamics of oocyte growth during vitellogenesis in a population of virgin female rainbow trout. Indices of ovarian development increased dramatically during the period of study: the gonadosomatic index (GSI) increased over 50-fold, reaching a peak of 20 just before ovulation; the mean oocyte diameter increased from less than 1 mm to 5.4 mm; and plasma levels of vitellogenin increased from less than 1.5 mg/ml to 25 mg/ml. There were no changes in the numbers of developing oocytes (measuring 0.5 mm or greater in diameter) from the time when the majority of oocytes undergoing secondary development had entered vitellogenesis in August to ovulation in February (averaging 4000 oocytes per fish). The increase in ovary weight during vitellogenesis was, therefore, due to an increase in the size of oocytes rather than to recruitment of more maturing oocytes. The numbers of vitellogenic oocytes in the ovary during the entire study also suggested that atresia of vitellogenic oocytes does not play a prominent role in determining fecundity. During early vitellogenesis, the volume of maturing oocytes within an ovary varied by as much as 250-fold. From September onwards, when all oocytes to be ovulated that season had entered vitellogenesis, a gradual uniformity in size began to develop, such that at ovulation, in February, all the eggs were very similar in size (there was less than a 2-fold variation in volume). The pattern of growth of oocytes in an ovary during vitellogenesis suggests that growth between oocytes is closely coordinated.