Neurobiology and Cultivation of Olfactory Receptor Neurons on a Chip - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 95

olfactory cilia
dendritic knob
sustentacular cell

ciliary layer

mature ORN layer

mature ORN

immature ORN layer

immature ORN
precursor cell
globose basal cell

Lamina propria

a b

Fig. 6.1 Olfactory Epithelium. a OE consists of three principal cell types; globose basal cell,
ORN and sustentacular cell. Apical dendritic extension of ORN forms dendritic knob. Dendritic
knob contains non-motile cilia where ORs are highly expressed and detect odorants. b OE section
labeled by two cell type markers, GAP43 and OMP. GAP43 is expressed in immature neurons, and
OMP is expressed exclusively in terminally differentiated ORNs. ORNs migrate apically to the top
of the epithelium. Thus people can recognize age of neurons by position

6.2 Cellular Structures of Olfactory Cells

The peripheral olfactory system is structurally well adapted to detect environmen-
tal chemicals. For example, the localization of ORNs in the olfactory epithelium
(OE) facilitates their direct contact with inhaled odorous chemicals (Fig. 6.1 ). When
considering the OE, it is important to be mindful of the three OE cell types: ORNs,
sustentacular (or supporting) cells (SCs), and basal cells (BCs) [ 3 , 4 ].

6.2.1 ORNs

ORNs are bipolar neurons, extending apical dendrites to the surface of the epithe-
lium and sending unmyelinated axons projecting to glomeruli on mitral and tufted
neurons in the olfactory bulb OB of the brain. The apical dendrites of ORNs form
dendritic knobs upon which non-motile cilia are located and it is here that the initial
events of olfactory signal transduction occur [ 5 - 7 ]. Results of electrophysiological
studies have indicated that odorant sensitivity and the odorant-induced current are
uniformly distributed along the cilia, suggesting that all the components of the im-
mediate responses to odorants are localized to the cilia [ 7 ]. Furthermore, results of
immunoelectron microscopic studies have confirmed the ciliary localization of many
of these olfactory signaling molecules [ 8 , 9 ]. ORNs account for 75-80 % of the cells
in the OE (Fig. 6.2 ) [ 10 ]. They are functionally homogeneous in that they all detect
odorants. As they mature, ORNs migrate apically to the surface of the OE, which
allows for determination of neuronal age by position [ 11 ]. Replacement ORNs are
supplied by the differentiation of globose basal cells (GBCs) [ 12 - 14 ]. These unique
characteristics of neurogenesis allow for cultivation of ORNs in vitro [ 15 - 17 ].