Ultrastructural evaluation of lung maturation in a sheep model of diaphragmatic hernia and tracheal occlusion

In fetuses with diaphragmatic hernia (DH) lung development is impaired, and pulmonary hypoplasia is one of the main factors responsible for the poor o utcome of the disease. A possible treatment consists of occluding trachea d uring lung development to retain pulmonary fluid and to force the lung to e xpand. Although it appeared promising at first, this technique has recently been reported to decrease type II cell number and to induce surfactant def iciency. The aim of this study was to investigate lung maturation further t hrough ultrastructural examination in a fetal lamb model of DH created at 8 5 d, followed or not by endoscopic balloon tracheal occlusion (TO) at 120 d of gestation. The proportion of alveolar epithelial type I and type II cel ls was altered by both treatments: the type I/type II cell ratio, which was about 2 in control lungs, was decreased 4.5-fold in DH lungs but was incre ased 4.5-fold in DH+TO lungs. The proportion of undifferentiated cells was increased in DH lungs. Indeterminate cells sharing features of type II and type I cells that were not observed in controls were seldom seen in DH lung s and were numerous in DH+TO lungs. The number of lamellar bodies per type IT cell was decreased in both DH and DH+TO groups. In DH lungs, wall struct ure presented an immature appearance, with cellular connective tissue and p oor secondary septation of saccules. In DH+TO lungs, primary septa appeared more mature, with reduced connective tissue, but secondary septa were stil l buds, although elastin was present at their tips. A single capillary laye r was found in all three groups (control, DH, and DH+TO) with no sign of se ptal capillary pairing. This first investigation in DH and DH+TO lungs thro ugh transmission electron microscopy thus enabled us to show that compressi on and forced expansion of the lung are both responsible for alterations in type II cell differentiation and septal development.