Olfactory epithelium is found to line the nasal cavity. It sub serves a special function i.e. olfaction. Olfactory area is confined to the following narrow zones of the nasal cavity:
1. Upper 1/3 of the lining mucosa of the nasal septum
2. Corresponding area of the medial aspect of superior turbinate
3. Corresponding area of the narrow roof of the nasal cavity
This portion of mucosa can be readily identified from the rest of the nasal mucosa by its unique yellowish color.
Embryologically, olfactory receptors derive from neuroblasts. These neuroblasts differentiates to form olfactory placodes. The central part of each placode invagiantes giving rise to olfactory sac. Olfactory sac opens anteriorly. Olfactory organ is the only part of the body in which the cell bodies of neurons lie at the surface, directly in contact with the external environment.
Histological organization of olfactory mucosa consists of
1. Epithelial layer
2. Basal lamina
3. Lamina propria - directly adheres to the underlying bony / cartilagenous tissue.
The olfactory area is covered by pseudostratified columnar epithelium.
Olfactory epithelium consists of 3 cell types:
1. Olfactory cells
2. Supporting cells
3. Basal cells
Olfactory cells: These cells are essentially bipolar neurons. These are flask like cells containing rounded nuclei. These cells are provided with proximal and distal processes. The distal process also known as dendrites pass towards the surface between the supporting cells. Dendrites on reaching the surface expands slightly to form olfactory rods. These olfactory rods contains a large number of small vesicular structures. Seen projecting from its surface are several cilia lying in a fluid layer. The fluid layer is formed from secretions of Bowman's glands located in the lamina propria of olfactory mucosa.
Figure showing olfactory epithelium
Supporting cells: These cells help in metabolic and physical support of olfactory cells. These cells are tall columnar cells having microvilli and a prominent terminal web. The nuclei of supporting cells are more terminally located than those of other olfactory epithelial cells.
Basal cells: Rests on the basal lamina of olfactory epithelium. These are stem cells capable of division and differentiation into either supporting or olfactory epithelial cells. This layer ensures that the olfactory epithelium is replaced every 2-4 weeks.
Bowman's glands: are tubular acinar glands (exocrine) lying deep to the basal lamina. The ducts of these glands traverse to reach the surface. Secretions from these glands bathe the surface of olfactory epithelium. It is in this fluid the odoriferous substances are thought to be dissolved before they can effectively stimulate the sensory cells.