Respiratory bronchioles continue to divide to form alveolar ducts, passages lined solely with rows of alveoli that have rings of smooth muscle in knob-like interalveolar septa. The alveolar ducts terminate in alveolar sacs, enlarged spaces surrounded by clusters of alveoli that open into the spaces. The alveoli are lined with type I alveolar cells, extremely thin squamous cells that cover about 95% of the alveolar surface, and with type II alveolar cells, cuboidal cells that secrete surfactant, a surface-active agent that reduces surface tension at the air-epithelium surface. The tissue between adjacent alveoli is called the alveolar septum. This consists of the alveolar epithelial cells and their basal lamina, the basal lamina of the underlying capillary endothelium and the endothelial cells, themselves, and any other connective tissue elements that may lie between the two basal laminae. The alveolar septum is the site of the air-blood barrier.
Figure 1, terminal bronchiole, H&E x550.
The histologic features of the terminal bronchiolar wall are shown here. Ciliated epithelium extends from the top of the figure to the diamond. This is ciliated pseudostratified columnar epithelium (PsEp). Some basal cells are still present and, therefore, the designation pseudostratified columnar. Elsewhere, the epithelium might be ciliated simple columnar, and just before it becomes a respiratory bronchi-
Figure 2, respiratory bronchiole, H&E x550.
The wall of a respiratory bronchiole is shown here and in Figure 3. The alveoli (A) are terminal air spaces on the left in each of the two figures. The lumen of the respiratory bronchiole is on the right. Characteristically, the wall of the respiratory bronchiole consists of alternating thick and thin regions. The thick regions are similar to the wall of the
Figure 3, respiratory bronchiole, H&E x550.
The respiratory bronchiole shown in Figure 3 is slightly more distal than the area seen in Figure 2. Structurally, it
Figure 4, alveolus, H&E x800.
The alveolar wall is shown in Figure 4. The central component of the wall is the capillary (C) and, in certain locations, associated connective tissue. On each side, where it faces the alveolus (A), a flat squamous cell is interposed between the capillary and the air spaces. This is a pneumocyte type I cell. In some places, the type I cell is separated from the capillary endothelial cell by a single basal lamina shared by the two cells. This is the thin portion of the alveolar-capillary complex, readily seen in the upper part of the figure (arrows). Gas exchange occurs through the thin por-
ole, the epithelium may include cuboidal or low columnar nonciliated cells. These nonciliated cells are Clara cells (CC, beyond the diamond). Clara cells produce a surface-active agent that is instrumental in expansion of the lungs. The smooth muscle (SM) in the bronchiolar wall is organized in bundles; other cells under the epithelium and around the smooth muscle belong to the connective tissue.
bronchiole except that cuboidal Clara cells instead of columnar epithelium form the surface. Thus, as seen here, Clara cells (CC) are the surface-lining cells of the thick regions, and smooth muscle bundles (SM) are under the Clara cells, with a small amount of intervening connective tissue. The thin regions have a wall similar to the alveolar wall; this is considered below.
shows essentially the same features as those seen in Figure 2 except that there are fewer Clara cells and the smooth muscle is somewhat thinner.
tion of the alveolar-capillary complex. Elsewhere, connective tissue is interposed between the pneumocyte type I cell and the endothelial cell of the capillary; each of these epithelial cells retains its own basal lamina.
A second cell type, the pneumocyte type II cell or septal cell (SC), also lines the alveolar air space. This cell typically displays a rounded (rather than flattened) shape, and the nucleus is surrounded by a noticeable amount of cytoplasm, some of which may appear clear. The septal cell produces a surface-active agent different from that of the Clara cell, which also acts in permitting the lung to expand.
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