The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures

and lies on the roof of the nasal cavity about above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.

Structure

Olfactory epithelium consists of four distinct cell types:

  • Olfactory sensory neurons
  • Supporting cells
  • Basal cells
  • Brush cells

Olfactory sensory neurons

The olfactory receptor neurons are sensory neurons of the olfactory epithelium. They are bipolar neurons and their apical poles express odorant receptors on non-motile cilia at the ends of the dendritic knob, which extend out into the airspace to interact with odorants. Odorant receptors bind odorants in the airspace, which are made soluble by the serous secretions from olfactory glands located in the lamina propria of the mucosa. The axons of the olfactory sensory neurons congregate to form the olfactory nerve (CN I). Once the axons pass through the cribriform plate, they terminate and synapse with the dendrites of mitral cells in the glomeruli of the olfactory bulb.

Supporting cells

Analogous to neural glial cells, the supporting cells are non-neural cells in the olfactory epithelium that are located in the apical layer of the pseudostratified ciliated columnar epithelium. There are two types of supporting cells in the olfactory epithelium: sustentacular cells and microvillar cells. The sustentacular cells function as metabolic and physical support for the olfactory epithelium. Microvillar cells are another class of supporting cells that are morphologically and biochemically distinct from the sustentacular cells, and arise from a basal cell population that expresses the c-KIT cell surface protein.

Basal cells

Resting on or near the basal lamina of the olfactory epithelium, basal cells are stem cells capable of division and differentiation into either supporting or olfactory cells. While some of these basal cells divide rapidly, a significant proportion remain relatively quiescent and replenish olfactory epithelial cells as needed. This leads to the olfactory epithelium being replaced every 6–8 weeks.

Basal cells can be divided on the basis of their cellular and histological features into two populations: the horizontal basal cells, which are slowly dividing reserve cells that express p63; and globose basal cells, which are a heterogeneous population of cells consisting of reserve cells, amplifying progenitor cells, and immediate precursor cells.

Brush cells

A brush cell is a microvilli-bearing columnar cell with its basal surface in contact with afferent nerve endings of the trigeminal nerve (CN V) and is specialized for transduction of general sensation.

Olfactory (Bowman's) glands

Tubuloalveolar serous secreting glands lying in the lamina propria of the olfactory mucosa. These glands deliver a proteinaceous secretion via ducts onto the surface of the mucosa. The role of the secretions are to trap and dissolve odiferous substances for the bipolar neurons. Constant flow from the olfactory glands allows old odors to be constantly washed away.

The embryonic olfactory epithelium consists of fewer cell types than in the adult, including apical and basal progenitor cells, as well as immature olfactory sensory neurons. The axons of the immature olfactory sensory neurons, along with a mixed population of migratory cells, including immature olfactory ensheathing cells and gonadotropin-releasing hormone neurons form a "migratory mass" that travels towards the olfactory bulb. Early cranial sensory placodes are marked by expression of Six1, part of the Six family of transcription factors that regulate the preplacodal specification. In mice, the olfactory placode derives from an anterior portion of the neural tube, ~9-9.5 days into development and not long after the closure of the neural plate. The specification of the olfactory placode tissue involves signaling of multiple gene networks, beginning with signals from bone morphogenetic proteins (BMP), retinoic acid (RA), and fibroblast growth factor (FGF), specifically FGF8. The resulting regulated downstream expression of transcription factors, such as Pax6, Dlx3, Sox2, and others, within the presumptive olfactory placode are crucial for sub-regionalization within the future olfactory epithelium and is responsible for the diversity of cells that compose the future epithelium.

Similar to the other embryonic placodes, the olfactory placode gives rise to both neural and non-neural structures, ultimately resulting in the formation of the nasal epithelium. The specification of neural versus non-neural tissue involves signals both within the olfactory placode, and between the olfactory placode and the underlying mesenchymal compartment.

  • Neural: olfactory sensory neurons, LHRH-secreting neurons, and ganglion cells
  • Non-neuronal: basal cells, olfactory supporting cells, ciliated cells, Bowman's glands, Schwann cells, submucosal glands, and brush cells

However, there is significant evidence for an additional neural crest-origin for many of these cell types as well. The olfactory epithelium contains olfactory sensory neurons, whose axons innervate the olfactory bulb. In order for olfactory sensory neurons to function properly, they must express odorant receptors and the proper transduction proteins on non-motile cilia that extend from the dendritic knob in addition to projecting their axons to the olfactory bulb.

The cells of the olfactory epithelium, including olfactory sensory neurons, begin to differentiate soon after the induction of the olfactory placode. Once the olfactory sensory neurons differentiate, they express odorant receptors, which transduce odorant information from the environment to the central nervous system and aids in the development of the odorant map. The differentiated olfactory sensory neurons extend pioneering axons, which follow guidance cues released by the underlying mesenchyme, as well as other chemotrophic cues released from the telencephalon.

Clinical significance

The olfactory epithelium can be damaged by inhalation of toxic fumes, physical injury to the interior of the nose, and possibly by the use of some nasal sprays. Because of its regenerative capacity, damage to the olfactory epithelium can be temporary but in extreme cases, injury can be permanent, leading to anosmia.

Additional images

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Image:Riechschleimhaut.svg|Composition of the Olfactory receptor neuron (captions in German)

Image:Olfactorisch epitheel varken olfactory epithelium pig.jpg|olfactory epithelium pig

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See also

  • Phantosmia
  • List of distinct cell types in the adult human body

References

  • Embryonic origin of the olfactory sensory system: fate map, lineage analysis and specification of the avian olfactory placode and [https://books.google.com/books?id=HnhqQwAACAAJ]