An in vitro culture system for studying vitellogenin uptake into ovarian follicles of the rainbow trout, Salmo gairdneri

The development of a short‐term culture system to study vitellogenin (VTG) sequestration in vitro into vitellogenic follicles of the rainbow trout, Salmo gairdneri , is described. The outer epithelial cell layer of the trout follicle restricted VTG uptake considerably and its removal was a prerequisite to study VTG uptake in vitro. Vitellogenic follicles divested of the surface epithelium sequestered VTG at rates of up to 170 ng·mm 2 follicle surface −1 ·h −1 (ng·mm −2 ·h −1 ). Uptake of VTG over an 8 h period of incubation was linear. Increasing the titre of VTG in the culture medium induced greater rates of sequestration in a dose‐dependent manner; follicles cultured in medium containing 45 mg VTG·ml −1 sequestered 170 ± 9ng·mm −2 ·h −1 whereas those follicles cultured in medium containing 2 mg·ml −1 only incorporated 6 ± 0.5 ng·mm −2 ·h −1 . Uptake of VTG was temperature dependent with higher temperatures inducing higher levels of incorporation. Sequestration of VTG persisted at temperatures below 5°C, showing that the uptake mechanisms are adapted to low temperatures in this species. Size of follicles also had a considerable bearing on the rate of VTG sequestration; follicles of less than 1 mm diameter (pre‐vitellogenic follicles) did not sequester VTG in amounts that were detectable. All follicles measuring 1 mm or greater in diameter sequestered VTG. As the follicles increased in size between approximately 1 and 3 mm in diameter there was an increase in both the total amount of VTG sequestered and the rate of uptake of VTG relative to surface area. Limited data suggested that in follicles closely approaching ovulation the rate of VTG sequestration was considerably reduced.