Oocyte growth in the sheepshead minnow: uptake of exogenous proteins by vitellogenic oocytes.

The structure of the vitellogenic follicles of the sheepshead minnow, Cyprinodon variegatus, is described. Follicles enlarge primarily by protein yolk accumulation (vitellogenesis) and subsequently increase in size by hydration. This study uses the electron-dense tracer, horseradish peroxidase, and a larger heterologous protein, Xenopus laevis [3H]vitellogenin, to follow the fate of exogenous proteins from the maternal circulation to yolk spheres of the growing oocyte. Materials appear to leave the perifollicular capillaries via an interendothelial route, traverse the theca and the patent intercellular channels of the follicular epithelium and the pore canals of the vitelline envelope. At the oocyte surface they are incorporated via micropinocytosis and translocated to growing yolk spheres in the peripheral ooplasm. In contrast to other studies on oocyte growth in teleosts which suggest that yolk is an autosynthetic product, this study substantiates the importance of heterosynthetic processes during oocyte growth in C. variegatus.