PROTEIN AND LIPOPROTEIN UPTAKE BY DEVELOPING OOCYTES OF THE HAWKMOTH MANDUCA-SEXTA - AN ULTRASTRUCTURAL AND IMMUNOCYTOCHEMICAL STUDY

The ultrastructure of developing Manduca sexta oocytes is described wi th respect to the endocytic pathway for protein incorporation. Three m ajor (lipo)protein components of mature M. sexta eggs, lipophorin, vit ellogenin and microvitellogenin, were localized along this pathway by immuno-fluorescence and immuno-gold labeling techniques. Labeling of t he antigens was observed in the extracellular spaces of the follicle. In those cases where fixation and en bloc staining procedures did not destroy antigenicity, antigens were detected in coated pits and coated vesicles near the plasma membrane of the oocyte. All three antigens w ere demonstrated to be present in endosomes in the cortex of the oocyt e. Both the morphology and the labeling pattern of the endosomes indic ate that this organelle is a compartment of uncoupling of receptor and ligand. Tubular elements at the surface of the endosome, interpreted to be involved in the recycling of receptors and membrane to the oocyt e surface, were not labeled. Strong labeling of lipophorin, vitellogen in and microvitellogenin was observed in the developing yolk bodies, t he main protein storage compartment of the oocyte. The uptake and stor age of hemolymph proteins and lipoproteins by M. sexta oocytes is disc ussed in comparison with other insect and vertebrate endocytic systems .