Cystidia may occur on the gill edge (cheilocystidia), on the face of the gill (pleurocystidia), on the surface of the cap (dermatocystidia or pileocystidia) or on the stipe (caulocystidia). Especially the pleurocystidia and cheilocystidia are important for identification within many genera. Sometimes the cheilocystidia give the gill edge a distinct colour which is visible to the naked eye or with a hand lens. Boletes also have cystidia; here, of course, the definitions should refer to pores, not gills.
Chrysocystidia are cystidia whose contents contain a distinct refractive yellow body, that becomes more deeply yellow when exposed to ammonia or other alkaline compounds. Chrysocystidia are characteristic of many (though not all) members of the agaric family Strophariaceae.
Due to their high refractive index, Gloeocystidia have an oily or granular appearance under the microscope. Like gloeohyphae, they may be yellowish or clear (hyaline) and can sometimes selectively be coloured by sulphovanillin or other reagents.
In general the purpose of the cystidia from the point of view of the fungus is a mystery. Cheilocystidia, which occur at the edge of the gills, have an excretory function. The margin of gills with abundant cheilocystidia are often beaded with tiny droplets of liquid (see photo, which can be expanded by clicking), so one function of cystidia may be to exude substances which in this case are visible as droplets.