Figure 1814

Photomicrograph showing an alveolar sac with adjacent alveoli.

This photomicrograph shows the terminal components of the respiratory system, namely, the alveolar sac (AS) and the surrounding alveoli (A). The alveoli are surrounded and separated from one another by a thin connective tissue layer, the interalveolar septa, containing blood capillaries. On the right is the lung surface, which is covered by visceral pleura containing simple squamous epithelium and an underlying layer of connective tissue. x360.

of smooth muscle are present in the knob-like interalveolar septa (see below).

• Alveolar sacs are spaces surrounded by clusters of alveoli. The surrounding alveoli open into these spaces.

Alveolar sacs usually occur at the termination of an alveolar duct but may occur anywhere along its length. Alveoli are surrounded and separated from one another by an exceedingly thin connective tissue layer that contains blood capillaries. The tissue between adjacent alveolar air spaces is called the alveolar septum or septal wall (Fig. 18.15).

Alveolar epithelium is composed of type I and II alveolar cells and occasional brush cells

The alveolar surface forms a vulnerable biologic interface that is subject to many destabilizing surface forces and to continuous exposure to inhaled particles, pathogens, and toxins. The alveolar epithelium is composed of several specialized cells and their products, some of which play defensive and protective roles:

• Type I alveolar cells, also known as type I pneumocytes, are extremely thin squamous cells that line most (95%) of the surface of the alveoli (see Fig. 18.15). These cells are joined to one another and to the other cells of the alveolar epithelium by occluding junctions (Fig. 18.16). The junctions form an effective barrier between the air space and the components of the septal wall. Type I alveolar cells are not capable of cell division.

• Type II alveolar cells, also called type II pneumocytes or septal cells, are secretory cells. These cuboidal cells are interspersed among the type I cells but tend to congregate at septal junctions. Type II cells are as numerous as type I cells, but because of their different shape they cover only about 5% of the alveolar air surface. Like Clara cells, type II cells tend to bulge into the air space (see Fig. 18.16). Their apical cytoplasm is filled with granules that are resolved with the TEM (Fig. 18.17) as stacks of parallel membrane lamellae, the lamellar bodies. They are rich in a mixture of phospholipids, neutral lipids, and proteins that is secreted by exocytosis to form an alveolar lining, surface-active agent called surfactant. In addition to secretion of surfactant, type II alveolar cells are progenitor cells for type I alveolar cells. Following lung injury, they proliferate and restore both types of alveolar cells within the alveolus. Hyperplasia of type II alveolar cells is an important marker of alveolar injury and repair of alveoli.

• Brush cells are also present in the alveolar wall, but they are few in number. They may serve as receptors that monitor air quality in the lung.

Surfactant decreases the alveolar surface tension and actively participates in the clearance of foreign materials

The surfactant layer produced by type II alveolar cells reduces the surface tension at the air-epithelium interface.

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