Cellular structure, function and organization in the lower respiratory tract.

First and foremost, the lung is considered an organ of gas exchange. The earliest electron microscopical studies concentrated on features related to this function, notably the nature, origin and organization of the alveolar lining epithelium, contentious issues that had not been answered with the limited resolution of the light microscope. More recently, the perspective of relationships between the various pulmonary cell populations has changed rapidly, with wider recognition of the lung as a hemodynamic, immunologic and metabolic organ, necessarily equipped with complex systems for the maintenance, protection and repair of normal structures in the face of constant exposure to pollutants or infectious agents derived from the external or interval environment. Cells of the Pulmonary Airways The tracheobronchial lining is a tall-columnar, pseudostratified epithelium in which at least 13 cell types, 11 epithelial and 2 mesenchymal, have been identified by electron microscopy (1). Not all of these cells are found in every species, and most of the epithelium consists of ciliated and mucus-secreting cells, those which move or elaborate the surface mucous layer. Scattered along the length of the conducting airways, from the trachea to the bronchioles, are intraepithelial corpuscles known as neuroepithelial bodies. An important function of the membrane lining larger airways is the production of mucus, which covers the luminal surface. The mucus-secreting apparatus consists of the epithelial serous and goblet cells and the submucosal mucous glands, which are largely lined by serous and mucous cells. Glands are sparse in small bronchi and absent from the bronchioles. Only six cell types, the ciliated, brush, basal, K (Kultschitsky-like), nonciliated bronchiolar epithelial (Clara) cells and the globule leukocyte have been recorded in the bronchioles, where the epithelium is a simple, single layer. Mucussecreting cells are sparse in the bronchioles of most species, except the cat, in health.