Induced pinocytosis and cytoskeletal organization in Amoeba proteus - a combined fluorescence and electron...

Induced pinocytosis was studied in Amoeba proteus by combining fluorescence microscopy, low-light-level-TV intensification, image processing and electron microscopy. After external incubation of living cells with fluorochromed bovine serum albumin (BSA-TRITC) or a complex of colloidal gold and fluorochromed bovine serum albumin (Au(12)-BSA-TRITC) the fate of internalized material was followed for more than 2 h. Endosomes containing the ingested marker/membrane receptor-complex gather rapidly (15-30 min) within the uroid region and fuse successively to one large secondary endosome which either segregates its content into the external medium by exocytosis or is pinched off with some surrounding cytoplasm from the cell body. In cases where endosome-fusion does not occur, the distinct vacuoles release their content exocytotically in a progressive fashion. The fluorescence and electron microscopic results are in agreement with the in vivo investigations and support the suggestion that the microfilament system is responsible for the generation of motive force necessary to promote endocytosis, endosome gathering, fusion, sequestration and exocytosis.