Ultrastructural characteristics of inclusion bodies of type II cells in late embryonic mouse lung.

As we reported earlier, type II alveolar epithelial cells make their appearance in the early embryonic mouse lung around day 14.2, and show distinctive ultrastructural features. The present study focuses on the ultrastructural characteristics of the inclusion bodies by investigating embryos aged 17-19 days (birth on day 19), using transmission electron microscopy. Late embryonic type II cells appear also as low-columnar or cuboid cells having large, approximately round nuclei and cytoplasm displaying typical features of a differentiated cell. The inclusion bodies show a widespread distribution and are extremely variable in appearance. Schematically we discern five main types, namely cytoplasmic, granular/flocculent, multivesicular, dense, and (multi)lamellar, which occur with intermediate and composite forms. All these inclusion bodies frequently contain glycogen particles, and show a structural relation to profiles of endoplasmic reticulum which are wrapped around them. Other distinctive properties are the osmiophily of multivesicular inclusion bodies, and the presence of vesicles in many dense inclusion bodies. The possible interrelationship, and the differences in various aspects of electron density, suggest that the five main types of inclusion bodies may represent different stages in the formation of mature multilamellar bodies.