The role of apoptosis in normal and accelerated lung development in fetal rabbits

During fetal development, the mammalian lung undergoes progressive parenchymal involution. Intrauterine tracheal occlusion induces accelerated architectural maturation of the fetal lungs associated with depletion of the surfactant-producing type II cells. This study investigates the spatiotemporal pattern of apoptosis during normal fetal lung development and its modulation in tracheal occlusion-induced accelerated fetal lung growth.

Fetal rabbit lungs were studied at 25 to 31 days' gestational age (DGA; term, 31 DGA), corresponding to late pseudoglandular through terminal air sac stages of fetal lung development. Intrauterine tracheal ligation (TL) was performed at 24 DGA. TL fetuses were monitored until 29 DGA, a time-point previously shown to coincide with significant type II cell depletion. Apoptotic cells were identified by light and electron microscopy, as well as terminal deoxynucleotidyl transferase-mediated dUTP-FITC nick-end labeling (TUNEL). Epithelial (type I and II) cell apoptosis was studied by TUNEL labeling in conjunction with antisurfactant protein and anticytokeratin immunohistochemistry. DNA fragmentation was analyzed by gel electrophoresis. Sham-operated littermates served as controls.

The number of apoptotic cells progressively increased with advancing lung growth and architectural maturation (apoptotic index [Al] 1.2 +/- 0.7 x 10(-3) at 25 DGA v 4.2 +/- 1.4 x 10(-3) at 31 DGA; P< .05, analysis of variance). In TL fetuses, the apoptotic rate was significantly higher than in non-TL fetuses from the third postligation day on, coinciding with the onset of significantly increased airspace distension (Al 4.9 +/- 1.3 x 10(-3) in TL v2.6 +/- 0.4 x 10(-3) in controls at 29 DGA; P< .05, Student's ttest). Apoptosis occurred in parenchymal cells and in isolated cells within the airspaces. The apoptotic activity of type II cells was significantly higher in TL fetuses than C fetuses at 29 DGA (type II Al 25.5 +/- 6.3 x 10(-3) in TL v2.3 +/- 0.8 x 10(-3) in C; P< .001). Electron microscopic studies confirmed the presence of apoptotic nuclei in interstitial macrophages and in degenerating intraluminal type II cells. DNA analysis showed nucleosomal bands.

Normal fetal lung development is associated with a progressive increase of epithelial and interstitial apoptotic activity, a process enhanced by TL. Tracheal occlusion induces a significant increase of type II cell apoptosis, which likely contributes to the observed type II cell depletion after TL. We speculate that fetal type II cell apoptosis after TL may be induced by mechanical distension (stretch) of the airspaces.