Figure 1815

Electron micrograph of lung alveoli. This electron micrograph shows two alveolar spaces separated by an alveolar septum containing capillaries, some of which contain red blood cells. Note the areas of thin and thick portions of the alveolar septum. These are shown at a higher magnification in Figure 18.19. x 5,800. inset. Photomicrograph of an alveolus for comparison with the alveolar wall as seen in an electron micrograph. Arrows indicate alveolar capillaries containing red blood cells. X480.

The most critical agent for air space stability is a specific phospholipid called dipalmitoylphosphatidylcholine (DPPC), which accounts for almost all surface tension-reducing properties of surfactant. Surfactant synthesis in the fetus occurs after the 35th week of gestation and is modulated by a variety of hormones, including Cortisol, insulin, prolactin, and thyroxine. Without adequate secretion of surfactant, the alveoli would collapse on each successive exhalation. Such collapse occurs in premature infants whose lungs have not developed sufficiently to produce surfactant, causing neonatal respiratory distress syndrome (RDS). Prophylactic administration of exogenous surfactant at birth to extremely premature infants and administration to symptomatic newborns reduces the risk of RDS. In addition, administration of Cortisol to mothers with threatened premature delivery decreases neonatal mortality.

Surfactant proteins help organize the surfactant layer and modulate alveolar immune responses

In addition to phospholipids, hydrophobic proteins are necessary for the structure and function of surfactant. These proteins are

• Surfactant protein A (SP-A), the most abundant surfactant protein. SP-A is responsible for surfactant homeostasis (regulating synthesis and secretion of surfactant by type II alveolar cells). It also modulates immune responses to viruses, bacteria, and fungi.

• Surfactant protein B (SP-B), an important protein for the transformation of the lamellar body into the thin surface film of surfactant. SP-B is a critical surfactant-organizing protein responsible for adsorption and spreading of surfactant onto the surface of the alveolar epithelium.

alveolar cells that are joined to the type II cell by occluding junctions. Both cell types rest on the basal lamina (BL). The secretory vesicles (G) in this specimen are largely dissolved, but their lamellar character is shown to advantage in Figure 18.17b. x24,000.

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