Yolk utilization in stick insects entails the release of vitellin polypeptides into the perivitelline fluid

This study investigates the developmental fate of vitellin (Vt) polypeptides generated by limited proteolysis in an insect embryo. To this end, a number of polyclonal (pAb) and monoclonal antibodies (mAb) were raised against the yolk sac and the perivitelline fluid of late embryos of the stick insect Carausius morosus. Two dimensional immuno gel electrophoresis and Western blotting demonstrate that polypeptides resulting from Vt processing are present both in the yolk sac and the perivitelline fluid. At the confocal microscope, different labelling patterns were detected in the ooplasm depending on the stage of development attained by the embryo. At early developmental stages, label is associated with large unsegmented portions of the fluid ooplasm. During embryonic development, the fluid ooplasm is gradually transformed into yolk granules by intervention of vitellophages. Prior to dorsal closure, the yolk sac is separated from the perivitelline fluid by interposition of serosa cells (the so called serosa membrane). Several mAbs raised against the perivitelline fluid react specifically with this membrane suggesting that the release of Vt polypeptides from the yolk sac occurs by intracellular transit through the serosa cells. By immunocytochemistry, gold label appears associated with the cell surface and a number of vacuoles of the serosa membrane. These data are interpreted as suggesting that Vt polypeptides resulting from limited proteolysis in stick insect embryos are not exhaustively degraded within the yolk sac, but are instead transferred transcytotically to the perivitelline fluid through the serosa membrane.