Expression of surfactant proteins in embryonic rat lung

Because surfactant proteins A, B, and C (SP-A, SP-B, and SP-C) are putative markers for alveolar epithelial type II cells, we investigated their expression in embryonic rat lung (12 to 15 days, term = 22 days). The expression of the messages for SP-A, SP-B, and SP-C was assessed by the reverse-transcriptase polymerase chain reaction (RT-PCR). Embryonic rat lung at 12 days' gestation lacked detectable mRNAs for all three surfactant proteins. Messages for SP-A, SP-B, and SP-C were, however, present in embryonic rat lung at 13 days' gestation. Expression of SP-A mRNA increased in embryonic rat lung with advancing gestation. Surfactant protein mRNA expression during the embryonic period of lung development was limited to the epithelial cells. The expression of SP-A mRNA, while limited to the lung, did not appear to be a marker for cells destined to become type II cells, since it was detected in the trachea and the presumptive main bronchial ducts of embryonic rat lung at 13 days' gestation, as well as in the distal lung prealveolar region. Expression of both SP-B and SP-C mRNAs in embryonic lung was confined to the distal portion of the ductal system, although message of SP-C was also found in brain and kidney. These results suggest that none of the three surfactant protein mRNAs studied are, at this early stage of lung development, specific for cells destined to become type II pneumocytes.