Effect of gestational age on the increase in fetal lung growth following tracheal obstruction

The growth response of the fetal lung to increased expansion was compared at two gestational ages. In fetal sheep, lung expansion was increased by occluding the trachea for 48 h at either 112-114 days (younger fetuses) or 125-127 days (older fetuses) of gestation (term is ~ 145 days). After 24 h of tracheal occlusion, the volumes of liquid that could be drained from the lungs were increased by 64.7 and 158% above control in younger and older fetuses respectively; the volumes were not increased further after 48 h. In younger fetuses, 48 h of tracheal occlusion increased (p < .05) fetal lung wet weights (21% above control) and protein contents (43% above control) but not DNA contents. In older fetuses, 48 h of tracheal occlusion increased (p < .05) fetal lung wet weights (61% above control). However, 48 h of tracheal occlusion did not alter total lung hydroxyproline content at either age, resulting in a reduction in the hydroxyproline protein ratio of the fetal lungs. The results suggest what the lung growth response to tracheal occlusion is greater accumulation of lung liquid and hence a greater increase in lung expansion, in older fetuses.