Transmission electron micrograph of exocytosis in a mucus-secreting cell from a human colon. Exocytosis is a process by which a cell directs secretory vesicles out of the cell membrane. The accompanying micrograph shows mucus being discharged into the lumen from one of the secretory granules in the apical cytoplasm of a mucus-secreting cell from the colon. The limiting membrane of the granule is subsequently incorporated into the plasmalemma. The excess surface membrane accumulated during exocytosis is believed to be recycled to the Golgi complex in small vesicles.