Ontogeny of surfactant proteins and lipid-synthesizing enzymes in cultured fetal lung epithelial cells.

Fetal rat lung epithelial cells were isolated on gestational day 17 (term is 22), separated from fibroblasts, and cultured up to 6 days in a serum-free medium on a basement membrane matrix. Surfactant protein (SP) A, barely detectable by immunostaining at the beginning of the culture, considerably increased in cells and subsequently in the lumen of the epithelial cell clusters. SP-A mRNA, already detectable at culture initiation, progressively increased. By contrast, SP-B and its mRNA appeared after 2-3 days. SP-C mRNA appeared only after 4 days of culture. Cells cultured 6 days had a phospholipid composition similar to that of freshly isolated adult rat type II cells. The enhancement of lipid synthesis between the first and the sixth culture days, reported earlier to occur in these cells, was found to be accompanied by a two- to fivefold increase in amount of mRNAs of lipogenic enzymes and choline phosphate cytidylyltransferase. In conclusion, alveolar epithelial type II cells appear to be capable of full differentiation in vitro, and components of the surfactant system are all regulated developmentally at a pretranslational level.