OOCYTE SIZE AND STRUCTURE IN RELATION TO BLOOD-PLASMA STEROID-HORMONES IN INDIVIDUALLY MONITORED, SPAWNING ATLANTIC COD

A series of seasonal blood and ovarian tissue samples were taken from five female Atlantic cod Gadus morhua at different times between vario us batches of eggs and subsequently examined for oocyte size, plasma s teroid hormones and ultrastructure of the cortex oocyte cytoplasm and follicle layer. This detailed protocol demonstrated that the spawning cod ovary is vitellogenically very active. Individual oocytes were abl e to develop at different rates, and at least two cohorts of oocytes w ith a different developmental status were present simultaneously. The concentration of estradiol-17 beta varied between batches and decrease d over the season. The levels of 17,20 beta-P and 11-deoxycortisol wer e generally low. Oocytes of an advanced developing cohort showed numer ous coated vesicles and closely apposed oocytic and follicular microvi lli located in the chorionic pores. The number of oocytic microvilli w as estimated at about 1.8 million cell(-1), which increased the oocyte surface area by a factor of 12. Furthermore, the oocytic microvilli w ere found to stretch during final maturation. This stage specific mobi lity of microvilli is discussed in relation to further incorporation o f yolk and the relevant actin-based cytoskeleton observed for other cl asses of animals. (C) 1996 The Fisheries Society of the British Isles