Oogenesis in Fundulus heteroclitus. III. Vitellogenesis

Abstract

Several physiological parameters associated with oocyte growth (vitellogenesis) in the killifish, Fundulus heteroclitus, are defined. Hepatocytes from females and estrogen-treated males are highly synthetic cells, whereas those from males are relatively quiescent. A 200,000-molecular weight phosphoprotein (vitellogenin) has been identified by sodium dodecyl sulfate (SDS)-gel electrophoresis in the plasma of females and estrogen-treated males and isolated chromatographically using a potassium phosphate gradient system on DEAE-cellulose. To inhibit proteolysis, the nontoxic inhibitor, aprotinin, was injected into fish prior to bleeding. The structure of vitellogenic follicles and the process of oocyte growth are described. Once follicles reach a diameter of 0.5 mm, their oocytes incorporate exogenous materials by micropinocytosis and sequestered material is immediately translocated to yolk spheres in the oocyte cortex. This process has been followed by using the tracer horseradish peroxidase. Exogenous proteins appear to leave the perifollicular capillaries via an intercellular route and pass through intercellular channels within the follicular epithelium and patent pore canals of the vitelline envelope before reaching the oocyte surface. [³²P]Vitellogenin isolated from F. heteroclitus, [³H]vitellogenin isolated from X. laevis, and the dye trypan blue have been used to determine which sized follicles are vitellogenic (i.e., incorporate exogenous materials into growing oocytes). Morphological and autora-diographic studies are presented to confirm that sequestered macromolecules are incorporated into yolk spheres in the growing oocyte. Follicles appear to grow from 0.5 to 1.3 mm in diameter due to vitellogenesis and subsequently undergo an additional increase in size, which is primarily due to hydration, during oocyte maturation (Wallace and Selman, '78).