Normal Lung IV

FIG. 5. Electron micrographs of secretory, basal, and intermediate cells from human bronchi. A: Secretory cells (S) containing electron-dense granules (arrows). B: Several desmosomes (d) and keratin filaments (f) of a basal cell (B). C: An intermediate cell (I) with the same features as a basal cell (i.e., desmosomes, keratin filaments, and a high nucleus-to-cytoplasm ratio) but no basement membrane contact. Whereas a prominent nucleolus is typically found in basal cell nuclei, two nucleoli, as illustrated in the intermediate cell in C, were noted only in intermediate cells. Bar at the bottom right represents 2 µm.

Mucous Lining Layers
It has long been known that the lung clears or removes inhaled particulate matter by means of a mucociliary escalator mechanism. The mucous lining of the lung airways is composed of at least two physically and morphologically distinct layers: an underlying serous layer, in which the cilia beat (sol layer), is blanketed by a viscous layer (gel layer). Whether a continuous gel layer exists throughout the airways is a matter of debate. In general, studies focusing on the nasal epithelium and upper airways have found a continuous blanket, whereas studies focusing on more distal airways and bronchioles have not. More recent studies have demonstrated that the mucous lining layer of the airways contains a surface-active film at the air-fluid interface in addition to the two layers originally described.
The bronchial epithelium plays a critical role in both producing and moving mucus out of the lung. The rate of movement of mucus is slowest in the small airways and fastest in the large bronchi and trachea. The normal adult produces substantial quantities of lung secretions daily, virtually all of which are transported by ciliary clearance to the posterior pharynx, where they are unconsciously swallowed. The outer layer of the mucous coat is a highly viscous gel containing glycoproteins with molecular weights of several million daltons. In addition to mucous glycoproteins, the airways secretions contain immunoglobulins (primarily IgA), proteinase inhibitors, and antibacterial proteins (lysozyme and lactoferrin). Sixty to eighty percent of the cells in the airway epithelium are ciliated cells; the remaining cells are either basal or secretory cells.
Methods for preserving the mucous lining layer have included direct visual observation on dissected airway specimens, fixation by immersion, vascular perfusion fixation, quick freezing, and osmium tetroxide vapor fixation. Of these different methods of preservation, vascular perfusion fixation is the most generally applicable, as mechanical disruption from immersion or airway instillation of fixative is eliminated. Because the extensive capillary bed of the lungs is used to place the fixative in the immediate vicinity of the fluid lining layers, this method has been shown to improve significantly the preservation of mucous lining layers in the airways and the surface-active film of the alveolar region. Figure 6 demonstrates the changes in the mucous lining layer along the respiratory tract of a lung fixed by a combination of osmium vapor and vascular perfusion fixation. The gel layer is present in the airways from the trachea to the bronchi. In the distal and terminal bronchioles, the gel layer is attenuated and not always present. Tubular myelin and other surfactant debris are commonly found in both the gel layer of the upper airways (trachea to bronchi) and near the surface of the sol layer of the more distal airways, where the gel layer may be absent.

FIG. 6. Changes in the mucous lining layer along one airway path from trachea to alveolar surface. The thickness decreases from 10 to 20 µm in the trachea to 0.1 µm on the alveolar surface.