Nasal epithelium mucus protection

In vertebrates, most of the olfactory neurons of the nasal epithelium protrude from the apical dendritic knob a variety of cilia into the protective mucus layer, thus enlarging the sensory surface area of the cells. A subpopulation of sensory neurons in the main olfactory epithelium and all of the neurons in the vomeronasal organ (VNO) are structurally different their apical region of the dendrite extends in an array of microvilli. 

One method by which mucus protects the nasal epithelium is by acting as a physical barrier and respiratory mucus has been reported to retard the diffusion of water and a range of //-lactam antibiotics used to treat respiratory infections. The use of mucolytics, which alter the viscoelasticity of mucus, has been shown to increase the absorption of intranasally administered human growth hormone (hGH, M. Wt.=22 kDa). However, other studies have shown that antibodies (150-970 kDa) are able to diffuse through cervical mucus relatively unimpeded these latter studies tend to suggest that the diffusion barrier presented by mucus in the nasal cavity would be insignificant. 

Local host defenses of both the upper and lower respiratory tract, along with the anatomy of the airways, are important in preventing infection. Upper respiratory defenses include the mucodliary apparatus of the nasopharynx, nasal hair, normal bacterial flora, IgA antibodies, and complement. Local host defenses of the lower respiratory tract include cough, mucodliary apparatus of the trachea and bronchi, antibodies (IgA, IgM, and IgG), complement, and alveolar macrophages. Mucus lines the cells of the respiratory tract, forming a protective barrier for the cells. This minimizes the ability of organisms to attach to the cells and initiate the infectious process. The squamous epithelial cells of the upper respiratory tract are not ciliated, but those of the columnar epithelium of the lower tract are. The cilia beat in a uniform fashion upward, moving particles up and out of the lower respiratory tract. 

The entire nasal cavity is lined by pseudostratilied squamous epithelium with a cover of a thin layer of mucus providing immune and mechanical protection. Cilia beat in the mucus about 1000 times per minute and surface material is moved along at a rate of 3-25 mm/min. This transport is unidirectional and moves mucus and any trapped inhaled particles back toward the nasopharynx, where it is periodically swallowed. Drug delivery to the lung is therefore likely to be markedly reduced in the presence of excess mucous seCTetion. Further details of the anatomy and physiology of the nasal cavity can be found in published reviews (24,29,30). 

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