Respiratory epithelium goblet cells

The highly vascularized respiratory epithelium is composed of five major cell types ciliated cells, nonciliated cells, columnar cells, goblet cells, and basal cells. Low numbers of neurosecretory cells are present in the basement membrane [17]. Approximately 20% of the total number of cells in the lower turbinate area is ciliated with fine projections ( 100 per cell) on the apical cell surface. Cilia are used to transport the mucus toward the nasopharynx. These long (4-6 fxm) and thin projections are mobile and beat with a frequency of 1,000 strokes per min. Ciliated and nonciliated columnar cells are populated with about 300 microvilli per cell, which help in enlarging the surface area. 

The pharynx, larynx, trachea and bronchi are lined with pseudostratified, ciliated columnar epithelium that contain at least eight cell types, including mucous secretory goblet and Clara cells, which produce a protective mucus layer of 5-10 jum thickness (see Table 9.2). Subepithelial secretory glands, present in the bronchial submucosa, also contribute to the mucus blanket [9]. Through coordinated ciliary movement a propulsive wave is created, which continuously moves the mucus layer up towards the larynx. Consequently, the mucosal surface of trachea and bronchi is constantly swept to remove inhaled materials. As the bronchi divide into bronchioli, the ciliated columnar respiratory epithelium is much thinner and changes to a simpler non-ciliated cuboidal epithelium. The epithelium in the terminal and respiratory bronchioles consists of ciliated, cuboidal cells and a small number of Clara cells. However, Clara cells become the most predominant type in the most distal part of the respiratory bronchioles [10]. 

Figure 9.2 Diagram of the four cell types in the nasal respiratory epithelium. A ciliated columnar cell covered by cilia and microvilli of uniform length B basal cell C goblet cell packed with mucus granules D nonciliated columnar cell, covered by microvilli of uniform length. (Redrawn from Mygind, N., 1979 Nasal Allergy, 2nd edition. Blackwell Scientific Publications Oxford).

However, the nasal epithelium has little ability to break down drugs. The extensive mucosal surface of the nose has a lining of pseudostratified epithelium as well as cilia and the goblet cells involved in the secretion of mucus. The lymphoid tissue primarily involved in the mucosal immune responses is the mucosal-associated lymphoid tissue (MALT). The different regions of the respiratory tract that play an influencing role in the immune system are as follows ... 

In chronic studies, male and female rats and mice tolerated up to 2.5 ppm chlorine gas for 6 h/day, 5 days/week for 2 years (CUT, 1993 Wolf et al, 1995). Concentration-dependent lesions confined to the nasal passages were observed in all animals. These lesions were most severe in the anterior nasal cavity and included respiratory and olfactory epithelial degeneration, septal fenestration, mucosal inflammation, respiratory epithelial hyperplasia, squamous metaplasia and goblet cell hypertrophy and hyperplasia, and secretory metaplasia of the transitional epithelium of the lateral meatus. Body weight was depressed compared to controls but no early deaths occurred. A similar lack of lower respiratory tract effects was seen in monkeys exposed to 2.3 ppm chlorine for 6 h/day, 5 days/week, for 1 year (Klonne et al, 1987). At these concentrations, chlorine is effectively scrubbed in the anterior nasal passages as indicated by the absence of lesions in the lung. 

Shimizu et al. (1996) produced hypertrophic and metaplastic changes of goblet cells in rat nasal respiratory epithelium by intranasal instillation of endotoxin. The number of goblet cells increased and that of nongranulated secretory cells decreased time-dependently after endotoxin instillations. Mitotic rates examined after a 6-h colchicine metaphase blockade were very low at any time point studied, and cell division did not play a major role in this process. 

From a cellular viewpoint, the respiratory tract is extraordirrarily complex, consisting of more than 40 different individual cell types. The airways of the nasopharyngeal and tracheobronchial compartments are lined by epithelial cells. The most anterior portion is lined by stratified squamous epithelium that in a short distance transitions to respiratory epithelium, with numerous ciliated cells and occasional goblet cells that serve as a source of mucus. 

Figure 1 Schematic representation of the respiratory epithelium showing ciliated type II cells (EC), dendritic cells (DC), goblet cells (GC), submucosal lymphocytes (LYM), mast cells (MC), alveolar macrophages (AM), and basal lamina (BL). Exposure to particles causes epithelial cells and macrophages to release a variety of immunoreactive mediators.

A layer of epithelium covers the interior of the airway tract. This epithehum contains various cell types such as ciliated, goblet, brush, and basal cells [34]. All cells are in contact with the basement membrane however, basal cells do not reach the airway lumen. Moving through the smaller bronchi and bronchioles toward the respiratory portion, the histological organization of both the epithelium and the underlying lamina propria gradually becomes more simple. 

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