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//NASAL AND OLFACTORY MUCOSAE

NASAL AND OLFACTORY MUCOSAE

Nasal mucosa

The lining of the anterior part of the nasal cavity and vestibule is continuous with the skin, and consists of keratinized stratified squamous epithelium overlying a connective tissue lamina propria. Inferiorly the skin bears coarse hairs (vibrissae) which curve towards the naris and help to arrest the passage of particles in inspired air. In males, after middle age, these hairs increase considerably in size. Further posteriorly, at the limen nasi, this grades into a mucosa which is lined initially by non-keratinizing stratified squamous epithelium, and then by pseudostratified ciliated (respiratory) epithelium rich in goblet cells. Respiratory epithelium forms most of the surface of the nasal cavity, i.e. it covers the conchae, meati, septum, floor and roof, except superiorly in the olfactory cleft, where the olfactory epithelium is present. It is adherent to the periosteum or perichondrium of the neighbouring skeletal structures. In some areas, cells of the respiratory epithelium may be low columnar or cuboidal, and the proportion of ciliated to non-ciliated cells is variable.

There are numerous seromucous glands within the lamina propria of the nasal mucosa. Their secretions make the surface sticky so that it traps particles in the inspired air. The mucous film is continually moved by ciliary action (the mucociliary escalator or rejection current) posteriorly into the nasopharynx at a rate of 6 mm per minute. Palatal movements transfer the mucus and its entrapped particles to the oropharynx for swallowing, but some also enters the nasal vestibule anteriorly. The secretions of the nasal mucosa contain the bacteriocides lysozyme, β-defensin and lactoferrin, and also secretory immunoglobulins (IgA). The mucosa is continuous with the nasopharyngeal mucosa through the posterior nasal apertures, the conjunctiva through the nasolacrimal duct and lacrimal canaliculi, and the mucosa of the sphenoidal, ethmoidal, frontal and maxillary sinuses through their openings into the meati.

The mucosa is thickest and most vascular over the conchae, especially at their extremities, and also on the anterior and posterior parts of the nasal septum and between the conchae. The mucosa is very thin in the meati, on the nasal floor and in the paranasal sinuses. Its thickness reduces the volume of the nasal cavity and its apertures significantly. The lamina propria contains cavernous vascular tissue with large venous sinusoids.

Olfactory mucosa

Olfactory mucosa (Fig. 32.6) covers approximately 5 cm2 of the posterior upper parts of the lateral nasal wall, including the upper part of the vertical portion of the middle concha (where it is interspersed with respiratory epithelium in a checkerboard fashion) and the opposite part of the nasal septum, the superior concha, the sphenoethmoidal recess, the upper part of the perpendicular plate of the ethmoid and the portion of the roof of the nose that arches between the septum and lateral wall, including the underside of the cribriform plate (constituting the olfactory cleft or groove). It consists of a yellowish brown pigmented pseudostratified epithelium, containing olfactory receptor neurones, sustentacular cells and two classes of basal cell, lying on a subepithelial lamina propria containing subepithelial olfactory glands (of Bowman) and bundles of axons derived from the olfactory receptor neurones which course through the mucosa on their way to the cribriform plate. The glands secrete a predominantly serous fluid through ducts which open onto the epithelial surface. These secretions form a thin fluid layer in which sensory cilia and the microvilli of the sustentacular cells are embedded.

  

Fig. 32.6  A, The chief cytological features of the olfactory epithelium. Receptor cells (neurones) (R) are situated among columnar sustentacular cells. The axons of the receptor cells emerge from the epithelium in bundles enclosed by ensheathing glial cells (G). Rounded globose basal cells (B) and flattened horizontal basal cells (not shown) lie on the basal lamina and the subepithelial glands (of Bowman) (S) open on to the surface via their intraepithelial ducts (I). At the surface are cilia of the receptor cells and microvilli of the supporting cells. B, C and D are longitudinal sections through human olfactory epithelium: B, Ciliated olfactory receptor neurones with characteristic expanded ends (N) project into the nasal lumen. M, microvillar cell; S, supporting or sustentacular cells containing electron dense material; B, basal cells resting upon the basal lamina. The edge of a Bowman’s gland (BG) lies deeper in the lamina propria. C, Higher power view of the expanded end of an olfactory receptor neurone. The electron dense, osmiophilic material within the adjacent supporting cell (S) is thought to contribute to the pigmentation of the olfactory epithelium. C, olfactory cilium; B, basal body with projecting ‘feet’. D, Section of human olfactory epithelium immunostained with anti-OMP (olfactory marker protein): an immunopositive olfactory receptor neurone lies between two unstained supporting cells (S). V, microvilli.
(Parts B, C and D by courtesy of Professor Bruce Jafek, Department of Otolaryngology, University of Colorado, Denver, USA.)

Olfactory receptor neurones

Olfactory receptor neurones are bipolar. Their cell bodies and nuclei are located in the middle zone of the olfactory epithelium. Each neurone has a single unbranched apical dendrite, 2 μm diameter, which extends to the epithelial surface, and a basally directed unmyelinated axon, 0.2 μm diameter, which passes in the opposite direction, penetrates the basal lamina and enters the lamina propria. The tips of the dendrites project into the overlying secretory fluid and are expanded into characteristic endings (knobs) (Fig. 32.6B). Groups of up to 20 cilia radiate from the circumference of each ending and extend for long distances parallel to the epithelial surface. Internally, the short proximal part of each cilium has the ‘9 + 2′ pattern of microtubules typical of motile cilia, while the longer distal trailing end contains only the central pair of microtubules. The olfactory cilia lack dynein arms and are thought to be non-motile; their primary purpose is to increase the surface area of sensory receptor membrane available for the efficient detection of odorant molecules transferred across the mucous layer by odorant binding proteins. Mature olfactory neurones express olfactory marker protein (OMP), an abundant cytoplasmic protein involved in olfactory signal transduction (Fig. 32.6D). Each olfactory receptor neurone expresses receptors for a single (or very few) odorant molecules. In humans, over 1000 genes code for functional odorant receptors; the number of functional genes is much higher in macrosmotic animals (Buck & Axel 1991). Although neurones with the same receptor specificity are randomly distributed within anatomical zones of the epithelium, their axons all converge on the same glomerulus in the olfactory bulb. Specific odours activate a unique spectrum of receptor neurones which in turn activate restricted groups of glomeruli and their second order neurones.

The axons form small intraepithelial fascicles among the processes of sustentacular and basal cells. The fascicles penetrate the basal lamina, and are immediately surrounded by olfactory ensheathing cells. Groups of up to 50 such fascicles join to form larger olfactory nerve rootlets which pass through the cribriform plate of the ethmoid bone, wrapped in meningeal sheaths. They immediately enter the overlying olfactory bulbs, where they synapse in glomeruli with mitral cells and, to a lesser extent, with smaller tufted cells.

Microvillar cells

Microvillar cells occupy a superficial position in the olfactory epithelium. They are flask-shaped and electron-lucent, and the apical end of each cell gives rise to a tuft of microvilli that project into the mucus layer lining the nasal cavity (Fig. 32.6B). Cell counts in longitudinal sections reveal that microvillar cells occur with a density that is approximately one tenth of the density of ciliated olfactory neurones: their function and origin has yet to be determined.

Sustentacular cells

Sustentacular, or supporting, cells are columnar cells that separate and partially ensheathe the olfactory receptor neurones. Their large nuclei form a layer superficial to the neuronal nuclei within the epithelium. The cells are capped by numerous long, irregular, microvilli which lie in the secretory fluid layer that covers the surface of the epithelium, intermingled with the trailing ends of the cilia on the olfactory receptor endings. Their expanded bases contain numerous lamellated dense bodies, which are the remnants of secondary lysosomes, and which contribute significantly to the pigmentation of the olfactory area (Fig. 32.6B,C). The granules gradually accumulate with age, and because these cells are long-lived, the intensity of pigmentation also increases with age. Neighbouring sustentacular cells are linked by desmosomes close to the epithelial surface, an arrangement that helps to stabilize the epithelium mechanically. Sustentacular cells and olfactory receptor neurones are linked by tight junctions at the level of the epithelial surface.

Basal cells

There are horizontal and globose basal cells. Horizontal basal cells are flattened against the basal lamina. Their nuclei are condensed and their darkly staining cytoplasm contains numerous intermediate filaments of the cytokeratin family, inserted into desmosomes between the basal cells and surrounding sustentacular cells. Globose cells are rounded or elliptical in shape, and have pale, euchromatic nuclei, and pale cytoplasm. They form a distinct zone that is slightly internal to the basal surface of the epithelium and characterized by mitotic figures: globose basal cells are the immediate source of new olfactory receptor neurones.

Olfactory ensheathing cells

Olfactory ensheathing cells share properties with astrocytes and non-myelinating Schwann cells, but also possess distinctive features that indicate they are a separate class of glia. Developmentally they are derived from the olfactory placode rather than the neural crest. They ensheath olfactory axons in a unique manner throughout their entire course, and accompany them into the olfactory bulb, where they contribute to the glia limitans. In recent years, olfactory ensheathing cells have been the focus of intense experimental scrutiny in the search for a source of transplantable glia capable of supporting neuronal regeneration within the CNS, possibly in the treatment of paraplegia.

Olfactory glands

Olfactory (Bowman’s) glands are branched tubuloalveolar structures that lie beneath the olfactory epithelium and secrete their products onto the epithelial surface through narrow, vertical ducts. Their secretions, which include defensive substances, lysozyme, lactoferrin, IgA and sulphated proteoglycans, together with odorant-binding proteins which increase the efficiency of odour detection, bathe the dendritic endings and cilia of the olfactory receptors. The fluid acts as a solvent for odorant molecules, allowing their diffusion to the sensory receptors.

Turnover of olfactory receptor neurones

Olfactory receptor neurones are lost and replaced throughout life. Individual receptor cells have a variable lifespan, thought to average 1–3 months. Stem cells situated near the base of the epithelium undergo periodic mitotic division throughout life, giving rise to new olfactory receptor neurones which then grow a dendrite to the olfactory surface and an axon to the olfactory bulb. The cell bodies of these new receptor neurones gradually move apically until they reach the region just below the supporting cell nuclei. When they degenerate, dead neurones are either shed from the epithelium or are phagocytosed by sustentacular cells. The rate of receptor cell loss and replacement increases after exposure to damaging stimuli, but declines slowly with age, a phenomenon that presumably contributes to diminishing olfactory sensory function in old age. Biopsy specimens from normosmic adults have revealed that patchy replacement of olfactory with respiratory epithelium occurs even in young healthy adults (Paik et al 1992, Holbrook et al 2005).

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