Membrane recycling in secretory cells: pathway to the Golgi complex

The pathway taken by membrane that is recovered by endocytosis from the surface of secretory cells was investigated with electron-dense tracers 9dextrans and cationized ferritin). The cell types examined included exocrine cells of the parotid and lacrimal glands, endocrine cells of the anterior pituitary gland, and immunoglobulin-secreting cells from lymph nodes or myeloma cell lines. In all cases, when the cells were incubated at 37 degrees C the tracers were initially taken up by endocytosis and they later appeared in the stacked Golgi cisternae, in immature secretion granules or vacuoles and in lysosomes. Similar results were obtained after covalent labelling of surface membrane constituents when myeloma cells were radioiodinated and the fate of the labelled components was followed by autoradiography. Initially only the cell surface was labelled, and the autoradiographic grains were concentrated over the plasmalemma. After incubation at 37 degrees C some of the labelled components were internalized (by endocytosis), and the majority of the internal autoradiographic grains were found over Golgi cisternae and over associated secretory vacuoles, which were the only organelles significantly labelled. The findings indicate the existence of considerable membrane traffic from the plasmalemma to the stacked Golgi cisternae and forming secretion granules or vacuoles in all these cell types. Membrane is thus continually recovered from the cell surface of secretory cells and funnelled through the Golgi complex; moreover, the plasmalemma-to-Golgi traffic appears to represent a major route of membrane traffic in secretory cells. A large portion of this traffic appears to be associated with the recycling of the membrane containers used in the packaging of secretory products.