Figure 1. Approximate location of three chemosensitive areas in the human. The olfactory epithelium is located in the upper regions of the nasal airway. The olfactory receptor cells send axons through the cribriform plate and into the olfactory bulb. The vomeronasal organ (exaggerated in size for the purposes of this illustration) is a tubular structure (<1–2 mm) located in the nasal septum. The tongue contains the majority of taste buds in the oral cavity.

Figure 2. The cellular organization of the olfactory epithelium. The olfactory receptor neurons (the thin light cells of the figure) send cilia from a single dendritic projection into the mucus-covered air space of the nasal cavity. The single axon passing from the cell body centrally bundles with axons of other receptor neurons to form cranial nerve I and synapse with other neurons in the olfactory bulb. Olfactory stimuli are recognized by receptors on the cilia. Other cell types within the epithelium include sustentacular (supporting) cells, which have a secretory role, and the basal progenitor cells, from which new olfactory receptor neurons emerge. The Bowman's glands secrete the watery mucus of the nasal cavity. Other tissues include blood vessels and submucosal glands.

Figure 3. The cellular organization of the taste bud. The taste bud contains ~100–150 cells that are grouped together in an onion-shaped organelle. Several types of cells have been distinguished, including the so-called dark cells and light cells (named as such due to their electron density as revealed by electron micrographs) and basal cells. The basal cells are thought to be the progenitors for replacing the active taste receptor cells. Some taste cells receive innervation from sensory nerve fibres at synapses. Stimuli are recognized by receptors of cells in the taste pore (TP) region, and this recognition is transduced into an electrical signal causing the innervating nerve fibre to alter its firing rate. The taste bud is divided into an apical and a basolateral portion by the tight junctions (TJ) that restrict the penetration of most taste stimuli into the basolateral portions of the bud. EC, epithelial cell.

Figure 4. The cellular organization of the vomeronasal epithelium. The major structural aspects of the vomeronasal epithelium parallel those of the olfactory epithelium. However, the dendrites of the VNO receptor neurons project microvilli into the lumenal space. Axons of these neuronal cells innervate the accessory olfactory bulb (AOB) which in most vertebrates is a small distinct region of the main olfactory bulb. The VNO epithelium also contains supporting cells and basal cells.