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Surfactant function in fetal lung maturation
[black square] Lung development
Fetal lung development can be divided into four broad stages: pseudoglandular, canalicular, saccular and alveolar. The pseudoglandular stage spans weeks 5-17 in the embryo with the formation of two budding lungs. These undergo several steps of branching morphogenesis that result in the production of three lung lobes on the right side and two lung lobes on the left [1,2]. The second canalicular stage takes place between gestational weeks 2 and 16 and is remarkable for the increased angiogenesis and differentiation of type I and II pneumocytes, which will be responsible for mediating gas exchange [3]. The future lung parenchyma is also canalized by capillaries and surfactant first appears during this period [1]. The saccular stage comprises of weeks 24-38 and involves the formation of transitory clusters of widened spaces in the peripheral airways [1]. The fourth alveolar stage begins around gestational week 36 and involves the formation of alveoli. Alveolarization begins with low ridges on both sides of the saccular walls; with increasing growth the saccules subdivide into smaller units named alveoli. During this stage, there is increased production of pulmonary surfactant by type II pneumocytes [1]. Lung development and alveoli formation continues until approximately 8 years of age [4].
[black square] Lung surfactant
Surfactant is synthesized in the rough endoplasmic reticulum of the alveolar type II pneumocytes. They are transported to the multivesicular bodies where they are packaged into laminated storage granules called lamellar bodies. The lamellar bodies are subsequently secreted by endocytosis. The secreted lamellar body unfolds to form tubular myelin and other large aggregates that are adsorbed into the expanded surface monolayer. Lamellar bodies are 1-5 µM or 1.7-7.3 fl [5,6] in diameter and appear around week 24 of gestation in the cytosol of type II pneumocytes, but surfactant is not measurable in the amniotic fluid until approximately week 32 [2].
The function of pulmonary surfactant is to coat the alveolar epithelium and decrease its surface tension. According to Laplaces' law, there is an inverse relationship between surface tension and alveolar radius. Therefore, assuming their surface tensions are equal, a small alveolus will have greater inward force than a large alveolus. Therefore, small alveoli are more likely to collapse. Surfactant reduces...