Smell receptor. Coloured high resolution scanning electron micrograph of the surface of the olfactory (smell) epithelium of the rat. At centre (blue) is the ending of an olfactory receptor cell process. This small swelling gives rise to a number of long, modified non-motile cilia (projections). The cilia are thought to be the sites of interaction between odiferous substances and the receptor cells. There are about 12 cilia in human cells, but over 20 cilia in the rat cell here, an indication of the rat's greater sense of smell. The surrounding tissue consists of the microvilli of the supporting cells.