Dense granules (also known as dense bodies or delta granules) are specialized secretory organelles.
In unicellular organisms
They are found in animals and in unicellular organisms including Apicomplexa protozoans.
They are also found in Entamoeba.
In multicellular organisms
The dense granules of human platelets contain adenosine diphosphate (ADP), adenosine triphosphate (ATP), ionized calcium (which is necessary for several steps of the coagulation cascade), histamine and serotonin.
A deficiency of CD63 can be associated with Hermansky-Pudlak syndrome.