REGULATION OF LUNG EXPANSION AND LUNG GROWTH BEFORE BIRTH

Fetal lung growth depends on the degree to which lungs are distended w ith luminal liquid. Fetal lungs are highly distended such that mean lu minal volume exceeds the static relaxation volume. This high level of expansion is maintained by fetal breathing movements and by resistive effects of the upper airway during apnea; both factors oppose lung rec oil. Mechanical stress in lung and other tissues stimulates cell divis ion and tissue remodeling. Potential transduction mechanisms involve d irect effects of cellular tension and mediation of locally released mi togenic factors. Further studies are required to further define links between lung tissue stress, increased growth, structural remodeling, a nd the endocrine environment. A common cause of fetal lung hypoplasia is a sustained reduction in mean lung expansion. Studies of mechanisms controlling fetal lung expansion have led to insights into the etiolo gy of fetal lung hypoplasia and how it may be remedied in utero. Fetal lung hypoplasia can have long-lasting effects on postnatal lung funct ion, as airway and alveolar formation may be compromised. Preterm birt h may also result in incomplete structural development of the lungs as it shortens the period of increased intrauterine lung expansion.