Ultrastructural features of type II alveolar epithelial cells in early embryonic mouse lung.

Immunofluorescence studies of type II alveolar epithelial cells indicate that they first appear in the pseudoglandular period of mouse lung development (around day 14.2). They are the only cell type to line the prospective pulmonary acinus at this time. The ultrastructural characteristics of this cell are defined by investigating embryos aged 13-16 days with transmission and scanning electron microscopy. Early embryonic type II cells appear as low-columnar or cuboid cells having large, approximately round nuclei and distinct ultrastructural features, including a well-developed Golgi apparatus with many associated vesicles, multivesicular bodies, dense bodies, and large apical and basal glycogen fields. These fields represent a distinctive property of the cell. Frequently, they show compartmentalization due to the presence of membrane systems, and association with dense bodies of various sizes.