Ontogeny of pulmonary alveolar epithelial markers of differentiation.

We studied differentiation of the pulmonary epithelium in the periphery of fetal rat lung in vivo and in vitro by comparing the ontogeny of cell-surface glycoconjugates with that of surfactant phospholipids. Apical surface binding of the lectin Maclura pomifera agglutinin (MPA) and expression of a 200-kDa MPA-binding glycoprotein (MPA-gp200) was evident at 20 days gestation in type 2 cells, but did not correlate with ultrastructural features of type 2 cell differentiation. Epithelial cells isolated from peripheral lung of 18-day gestation fetal rats displayed hormone-sensitive surfactant synthesis prior to the hormone-insensitive expression of MPA-gp200. Expression of MPA-gp200 occurred in association with the appearance of many new apical surface proteins suggesting a hormone-independent process of polar membrane differentiation. Thus membrane and secretory differentiation are discordant and can be dissociated. In vivo binding of Ricinus communis 1 agglutinin (RCA1), an apical marker of the differentiated alveolar type 1 cell occurred in undifferentiated peripheral lung epithelial cells as early as 18 days gestation, disappeared from differentiating type 2 cells and appeared in differentiated type 1 cells. Both undifferentiated fetal epithelial cells at 18 days gestation and fully differentiated type 1 cells express multiple glycoproteins with terminal beta-linked galactose residues which bind RCA1. Some of these RCA1-binding glycoproteins appear to be similar. These observations suggest that alveolar epithelial type 1 cells may derive directly from undifferentiated peripheral lung epithelial cells as well as from fully differentiated type 2 cells. In addition, terminal differentiation of fetal lung peripheral epithelium into type 1 and type 2 cells may involve repression as well as induction of differentiation-related genes.