Endocytosis in secretory cells

Abstract

Membranes of secretion granules inserted during exocytosis into the luminal plasma membrane of glandular cells are retrieved by endocytosis as revealed by electron dense tracers applied selectively to the apical cell surfaces. Two major pathways that endocytic vesicles may take are described: (1) a direct route to the Golgi complex (e.g. in parotid and exocrine pancreas) with later appearance of the tracer in the periphery of mature secretion granules; (2) an indirect route with lysosomes as a first station and the subsequent appearance of tracer in stacked Golgi cisternae. It is presumed that some of the retrieved membrane follows the same pathways and is reutilized in the secretory cycle.