Abstract 1816 Poster Session IV, Tuesday, 5/4 (poster 315)

Explants of fetal rat lung have been a useful model to study fetal rat lung development. The alveolar phase of rat lung development begins just prior to birth and continues for the first 3 weeks of life. The cellular and molecular mechanisms of alveolar formation have not been studied. We have previously shown that there is an abrupt increase in apoptosis of lung mesenchyme around the time of birth in rat lung. Our hypothesis is that the regulated balance between apoptosis and cell proliferation is a key mechanism by which the lung remodels during the alveolar phase of lung development. The goal of this study was to determine the usefulness of the organ culture model for the study of early alveolar lung development. Explants of 21 days' gestation (term=22 days) fetal rat lung were placed in culture for 0-7 days. At each day tissue was harvested for protein extraction, total RNA extraction and histologic analysis. Apoptosis was quantified using the TUNEL assay in histologic sections. Immunoblots using appropriate primary antibodies were used to measure changes in p21 (marker of cell cycle arrest) and SP-A receptor (specific marker of type II cells). Northern blots measured changes in steady state levels of mRNA for α-1 procollagen and tropoelastin (markers of alveolar septal formation). Similar experiments were done in vivo using lungs from newborn and 1-14d old rat pups. We found increased apoptosis in explants after 0-2d in culture compared to 5-7d in culture, corresponding to the increased levels of apoptosis measured in vivo. These changes in culture occurred concomitantly with increased levels of p21. Similarly, there were increased p21 levels at postnatal d3 compared to d1 in vivo. We also found increased levels of SP-A receptor after 1-2 days in culture followed by a decline in SP-A receptor from 2-5d in culture which parallels the changes seen in vivo. Levels of α-1 procollagen mRNA increased after 5 days in culture, and levels of α-1 procollagen mRNA increased at 4-5d postnatal age in vivo. Changes in steady state levels of tropoelastin were similar to those of α-1 procollagen. We conclude that the explant culture is a useful model for study of the early events during the alveolar phase of lung development since the changes in culture recapitulate the changes in vivo: 1) increased apoptosis and cell cycle arrest; 2) increased collagen and elastin expression (markers of alveolar septum formation); 3) increased specific markers of type II cells.