ENDOCYTIC ROUTES TO THE GOLGI-APPARATUS

The endocytic routes of labelled lectins as well as cationic ferritin were studied in cells with a regulated secretion, i.e. pancreatic beta cells, and in constitutively secreting cells, i.e. fibroblasts and He pG2 hepatoma cells, paying particular attention to routes into the Gol gi apparatus. Considerable amounts of internalised molecules were take n up into the trans Golgi network (TGN) and into Golgi subcompartments in all three cell types as well as in secretory granules of the pancr eatic beta cells. The internalised materials did not pass rapidly the TGN and Golgi stacks, but were still present hours after internalisati on, being then particularly concentrated in TGN-elements and in the tr ansmost Golgi cisterna. Endocytosed materials reached forming secretor y granules present in the TGN. Further, direct fusion between endocyto tic vesicles and mature secretory granules was observed. Golgi subcomp artments as well as endocytic TGN containing endocytosed materials wer e in close apposition to specialised regions of the endoplasmic reticu lum. The Golgi apparatus including its parts containing endocytosed ma terials were transformed into a tubular reticulum upon treatment with the fungal metabolite Brefeldin A. Rarely, internalised material was o bserved in the lumen of the endoplasmic reticulum, thus providing evid ence for an endocytic plasma membrane to endoplasmic reticulum route.