Objective To investigate the role of CCAAT enhancer binding protein A (C/EBPα) in lung development by analyzing the relationship between dynamic expression of C/EBPα protein and cell differentiation in rat lung tissue.Methods According to the histological stages of rat lung development,lung tissues were collected on 15.5 d (the late pseudoglandular period),17.5 d (the canalicular period),19.5 d (the early saccular period) of embryonic age and at 12 h (the middle saccular period),on 4 d (the late saccular period),7 d (the alveolar period,the alveolar stage),14 d (the alveolar period,the equilibrium stage) of postnatal age.The lung morphologic appearance was observed by using HE staining.Western blot was used to detect the expressions of C/EBPα and surfactant Protein (SP)-A,SP-B,SP-C,SP-D.Phosphatidylcholine (PC) assay kit was utilized to analyze the secretion of PC.Periodic acid-schiff (PAS) staining was used to evaluate the amcunt of glycogen in lung tissue.Results (1) C/EBPαt and SP-A began to express on 15.5 d of embryonic age (0.36 ±0.02,0.01 ±0.01),while SP-B,SP-C and SP-D started to express on 17.5 d of embryonic age (0.33 ±0.06,0.01 ±0.01,0.11 ±0.08).All of them increased with the development of lung,and C/EBPα,SP-A,SP-C reached the highest level at 12 h of postnatal age (3.48 ±0.05,3.24 ± 0.19,1.26 ± 0.21),and SP-D on the postnatal 4 d (1.48 ± 0.10),then gradually decreased,while the expression of SP-B continued to rise.The levels of C/EBPα and SPs maintained stable on postnatal 14 d.The C/EBPα protein level was positively correlated with SPs at embryonic age of 15.5 d,17.5 d,19.5 d and postnatal age 12 h (r =0.999,0.991,0.982,0.951,all P ＜ 0.05).(2) The level of PC was very low at embryonic age of day 15.5 [(60.50 ± 1.30) μg/g].With the development of lung,the secretion of PC increased gradually,but there was no significant correlation between the expression of PC and C/EBPoα(all P ＞ 0.05).(3) The level of glycogen was high in the late pseudoglandular stage (15.5 d) (585.50 ± 2.20),the content of glycogen decreased with the development of lung,especially on the canalicular (embryonic day 17.5) and during early saccular period (embryonic day 19.5),and then it became stable during the alveolar period (postnatal age 7 d).The expression of C/EBPα had negative correlation with the content of glycogen in fetal lung(r =-1.000,P ＜ 0.01).Conclusion C/EBPα plays an important role in rat lung development,as it may promote lung maturation by regulating the synthesis and secretionof SPs and PC.