Electron microscopic observations of developing albino rat lung provide further evidence for the endodermal origin of the type I and II pulmonary epithelial cells, and for the mesodermal origin of the interstitial pulmonary cells.

Cytoplasmic glycogen increased in the fetal endodermal cells from day 16 to the twentieth day of gestation. Further development revealed a decrease of this substance in the differentiating pulmonary epithelial cells, and a concurrent increase in the mesodermal interstitial pulmonary cells, these being cells formerly characterized by their high lipid content. A continuous basement membrane delineated the mesodermal and endodermal components of the developing rat lung from day 16 to the third postnatal day. Tight junctions between adjacent endodermal epithelial cells were present throughout this same gestational period. Lamellar bodies, which are characteristic of the type II pulmonary epithelial cell, may be found simultaneously with large quantities of cytoplasmic glycogen. This feature is considered characteristic of fetal endodermal components.

A definite continuum of ultrastructural changes may be traced from the endodermal, columnar epithelial cells to the definitive type I and II pulmonary epithelial cells. Comparable observations for the mesodermal components reveal a progression leading to mature interstitial pulmonary cells.