Uptake of vitellogenin into oocytes during early vitellogenic development in the rainbow trout, Oncorhynchus mykiss (Walbaum)

Uptake of 3H‐vitellogenin (3H‐VTG) into oocytes of various sizes was investigated during early vitellogenic development in the rainbow trout, Oncorhynchus mykiss (Walbaum). Females were injected with 3H‐VTG and uptake into oocytes of different sizes (<0.4,0.4–0.59, 0.6–0.79, 0.8–0.99 and 1.0 1.2 mm in diameter) measured. Oocytes measuring less than 0.6 mm in diameter appeared unable to sequester VTG and were therefore considered pre‐vitellogenic. Oocytes measuring 0.6 mm or more all sequestered VTG. The larger the oocyte, the more 3H‐VTG it sequestered, even when uptake was expressed per unit surface area. The latter observation could be due to an increase in the number of VTG receptors per unit surface area, an increase in the rate of turnover of the VTG receptor, greater access of VTG to the receptors as oocytes grow, or a combination of any of these factors. The data suggest that the ability to sequester VTG is developmentally regulated.