secretory granule

A secretory granule is a tiny membrane-wrapped storage bag inside an endocrine cell that holds peptide hormones ready for instant release. Picture a pre-loaded delivery package sitting by the door: when the signal comes, the cell does not have to manufacture anything — it simply ships what it already has.

Granules form at the Golgi apparatus, where the mature hormone is concentrated and packaged behind a membrane. They cluster near the cell surface, and when a trigger such as rising blood glucose arrives, calcium floods in and the granule fuses with the outer membrane, dumping its hormone into the blood. This calcium-driven fusion is called regulated exocytosis.

Because storing a ready supply allows release within seconds to minutes, secretory granules are the reason water-soluble hormones like insulin can respond so quickly. Steroid hormones, which are made on demand, are not stored this way — another consequence of the deep split between water-soluble and fat-soluble hormones.