part of part of in taxon endomembrane system secretory granule secretory vesicle GO:0030141 Note that the term 'secretory vesicle' is sometimes used in this sense, but can also mean 'transport vesicle ; GO:0030133'. A small subcellular vesicle, surrounded by a membrane, that is formed from the Golgi apparatus and contains a highly concentrated protein destined for secretion. Secretory granules move towards the periphery of the cell and upon stimulation, their membranes fuse with the cell membrane, and their protein load is exteriorized. Processing of the contained protein may take place in secretory granules. secretory vesicle