Expression of cell-specific markers for alveolar epithelium in fetal rat lung

In adults, the alveolar epithelium is composed of types I and II cells which differ structurally and functionally although they appear to belong to the same cell lineage. Using cell-specific markers (type I cells, monoclonal antibody; type II cells, Maclura pomifera lectin [MPA]), we have determined when and in what pattern their binding sites occur during development of the fetal rat lung. Rather than first appearing on days 19 to 20, when morphogenesis of type I cells occurs and lamellar bodies provide positive identification of type II cells, the markers appeared on day 15 (for type I cell marker) and day 16 (for type II cell marker). The type I cell marker was widespread by day 17 and was sufficiently abundant to be detected on a Western blot. MPA binding appeared more gradually and was often found on isolated cells. On serial sections of day 20 lung, the markers appeared to be localized to the same cells. The early appearance of cell-specific markers suggests an early onset of the developmental program that leads to full differentiation of types I and II cells. Co-expression of both cell-specific markers suggest that fetal cell lineage may differ from the scheme proposed by others that type II cells serve as type I cell precursors during development.