Determination of alveolar epithelial cell phenotypes in fetal sheep: evidence for the involvement of basal lung expansion.

The factors that control the differentiation of alveolar epithelial cells (AECs) into type-I and type-II cells in vivo are largely unknown. As sustained increases in fetal lung expansion induce type-II AECs to differentiate into type-I cells, our aim was to determine whether reduced fetal lung expansion can induce type-I AECs to trans-differentiate into type-II AECs. Chronically catheterised fetal sheep were divided into two age-matched control groups and three experimental groups (n = 5 for each). The experimental groups were exposed to either: (1) 10 days of increased lung expansion induced by tracheal obstruction (TO), (2) 10 days of TO followed by 5 days of reduced lung expansion induced by lung liquid drainage (LLD), or (3) 10 days of TO followed by 10 days of LLD. Following 10 days of TO, 5 days of LLD reduced the proportion of type-I AECs from 89.4 +/- 0.9 % to 68.4 +/- 2.8 %, which was similar to control values (64.8 +/- 0.5 %), and increased the proportion of type-II AECs from 1.9 +/- 0.3 % to 21.9 +/- 2.8 %, which remained below control values (33.4 +/- 1.7 %). The same treatment increased surfactant protein (SP)-A, SP-B and SP-C mRNA levels (expressed as a percentage of control values) from 26.7 +/- 6.0 %, 40.0 +/- 7.3 % and 10.3 +/- 1.8 % to 78.1 +/- 10.3 %, 105.8 +/- 12.7 % and 121.0 +/- 14.1 %, respectively. Similar results were obtained after 10 days of LLD, which followed 10 days of TO. These results indicate that the phenotypes of type-I and type-II AECs are strongly influenced by the basal degree of lung expansion in fetal sheep. Furthermore, the coincident increase in type-II AEC proportions and SP mRNA levels in response to LLD suggests that type-I AECs can trans-differentiate into functional type-II cells, and hence are not terminally differentiated.