FIGURE 19.3 • Olfactory mucosa of the nasal cavity. a. This diagram shows the three major cell types located within the olfactory epithelium: the olfactory cell, supporting (sustentacular) cell, and basal cell. The olfactory cell is the receptor cell; it has an apical expansion, the olfactory vesicle, from which long, nonmotile cilia extend. At its basal surface it extends an axon into the connective tissue that joins with axons of other olfactory cells to form an olfactory nerve. The basal cells are small and cuboidal. They are restricted to the basal part of the epithelium. The supporting cells, in contrast, are columnar and extend the full thickness of the epithelium; their nuclei are located in the upper portion of the cell. Note the olfactory (Bowman’s) gland and its duct that empties on the surface of the mucosa. b. Photomicrograph of the olfactory mucosa. The olfactory epithelium exhibits nuclei through much of its thickness, but the individual cell types to which they belong are not discernible. The underlying connective tissue is largely occupied by numerous olfactory (Bowman’s) glands, olfactory nerves, and blood vessels. Note that the ducts of the olfactory glands extend from the secretory portion of the gland to the epithelial surface. 240.
 Supporting or sustentacular cells are columnar cells that are similar to neuroglia cells and provide mechanical and metabolic support to the olfactory receptor cells. They synthesize and secrete odorant-binding proteins.  Basal cells are stem cells from which new olfactory receptor cells and supporting cells differentiate.  Brush cells are the same cell type that occurs in the respi ratory epithelium.
Olfactory receptor cells are bipolar neurons that possess an apical projection bearing cilia.
The apical domain of each olfactory receptor cell has a single dendritic process that projects above the epithelial surface as a knoblike structure called the olfactory vesicle. A number of long, thin cilia (10 to 23) with typical basal bodies arise from the olfactory vesicle and extend radially in a plane parallel to the epithelial surface (see Fig. 19.3). The cilia are usually up to 200 m long and may overlap with cilia of adjacent olfactory receptor cells. The cilia are regarded as nonmotile, although some research suggests that they may have limited motility. The basal domain of the cell gives rise to an unmyelinated  axonal process that leaves the epithelial compartment. Collections of axons from olfactory receptor cells do not come together as a single nerve, but instead they are grouped into bundles that pass through a thin cribriform plate of the ethmoid bone, course through the dura and arachnoid matters, and finally are surrounded by pia matter, entering the olfactory bulb of the brain. The collections of axons from  olfactory receptor cells form the olfactory nerve (cranial nerve I). The olfactory axons are very fragile, and can be harmed during traumatic head injury. They can be permanently severed, resulting in anosmia (loss of the sense of smell).
Autoradiographic studies show that olfactory receptor cells have a lifespan of about 1 month. If injured, they are quickly replaced. Olfactory receptor cells (and some neurons of the enteric division of the autonomic nervous system) appear to be the only neurons in the nervous system that are readily replaced during postnatal life.
Entire olfactory transduction pathways occurs within the cilia of the olfactory receptor cells.