In vitro branching morphogenesis of the fetal rat lung

We studied the pattern of airways branching in the fetal rat lung in vitro. Lung primordia of gestational ages 13, 14, and 15 days were allowed to grow in culture to a gestational age equivalent to 21 days. The first generation airways appear by a single new bud (monopodial budding) from the left main airway (lateral appearing before the medial). They elongate to form branches and then bud dichotomously (2 buds occurring simultaneously and adjacent to each other) at their tips. Then monopodial branching takes place along their sides. The same cycle of budding and branching seems to be repeated for the following generation of the airways. The total number of the peripheral (subpleural) buds was greatest in the day 15 explants and least in day 13 explants throughout the whole culture period, but the statistical model used indicated faster budding in the 13 day explants. Morphometric assessment showed no difference in the ratios between the lung components in the 3 age groups and that the peripheral epithelial measurements were the same in the 3 groups at an equivalent gestational age of 21 days. We have also shown that lobes do not form in the right lung, although appropriate airways do. This may indicate the importance of mesothelial covering of the lung in the process of lobe formation. The method is useful for studying the control of lung morphogenesis.