Accumulations of membrane-free clathrin-like lattices in the mosquito oocyte.

Mosquito oocytes assemble their protein yolk by internalizing extraovarian vitellogenin and therefore are highly specialized for receptor-mediated endocytosis. Large aggregates of filamentous material appear in the ooplasm during the previtellogenic development of endocytic organelles and consist of a meshwork of filamentous subunits and numerous membrane-free clathrin-like cages. The cages, 50 to 60 nm in diameter, are arranged in polyhedral lattices. The morphological characteristics of the aggregates strongly suggest that they represent accumulations of clathrin and possibly other proteins of the coat that are synthesized in large amounts prior to endocytic activity.