After the irst breath, the terminal sacs must remain expanded despite the pressure imparted by the tissue-to-air interface, and surfactant keeps them from collapsing. Surfactant is formed in type II pneumonocytes that line the alveoli. These cells are characterized by multivesicular bodies that produce the lamellar bodies in which surfactant is assembled. During late fetal life, at a time when the alveolus is characterized by a waterto-tissue interface, the intact lamellar bodies are secreted from the lung and swept into the amnionic fluid during respiratorylike movements that are termed fetal breathing. At birth, with the first breath, an air-to-tissue interface is established in the lung alveolus. Surfactant uncoils from the lamellar bodies and spreads to line the alveolus to prevent alveolar collapse during expiration. hus, the fetal lungs' capacity to produce surfactant establishes lung maturity.
Surfactant Composition. Gluck (1972) and Hallman (1976) and their coworkers approximated that 90 percent of surfactant's dry weight is lipid, speciically glycerophospholipids. Proteins account for the other 10 percent. Nearly 80 percent of the glycerophospholipids are phosphatidylcholines (lecithins). The principal active component that constitutes half of surfactant is a speciic lecithin, which is dipalmitoylphosphatidylcholine (DPPC or PC). Phosphatidylglycerol (PG) accounts for another 8 to 15 percent. Its precise role is unclear because newborns without PG usually do well. he other major constituent is phosphatidylinositol (PI). The relative contributions of each component are shown in Figure 7-12.
Surfactant Synthesis. Biosynthesis takes place in the type II pneumocytes. he apoproteins are produced in the endoplasmic reticulum, and the glycerophospholipids are synthesized by cooperative interactions of several cellular organelles. Phospholipid is the primary surface tension-lowering component of surfactant, whereas the apoproteins aid the forming and reforming of a surface ilm.
The major apoprotein is surfactant A (SP-A), which is a glycoprotein with a molecular weight of 28,000 to 35,000 Da (Whitsett, 1992). It is synthesized in the type II cells, and its content in amnionic fluid increases with gestational age and fetal lung maturity. SP-A gene expression is demonstrable by 29 weeks (Mendelson, 2005). Specifically, SP-Ai and SP-A2 0: : o .
15 -.a are two separate genes on chromosome 10, but their regulation is distinctive and diferent (McCormick, 1994).
Several smaller apoproteins such as SP-B and SP-C are likely important in optimizing the action of surfactant. For example, deletions.in SP-B gene are incompatible with survival despite production of large amounts of surfactant (Hallman, 2013).
Corticosteroids and Fetal Lung Maturation. Since Liggins (1969) observed accelerated lung maturation in lamb fetuses given glucocorticosteroids prior to preterm delivery, many suggested that fetal cortisol stimulates lung maturation and surfactant synthesis. It is unlikely that corticosteroids are the only stimulus for augmented surfactant formation. However, when these are administered at certain critical times, they may improve preterm fetal lung maturation. As fetal lung therapy, antenatal betamethasone and dexamethasone use and neonatal replacement surfactant therapy are discussed in Chapter 34 (p. 637).