ABSTRACT
The formation and development of cytoplasmic inclusion bodies of type II pneumocyte were investigated using 7 cases of human fetal lungs from 9 to 20 weeks of gestation by transmission electron micropscopy. The results obtained were as follows: 1. The multilamellar bodies, the characteristic inclusion body of type II pneumocyte, have developed in developing epithelium of lung at 9 week of gestation. Another inclusion bodies specific to type II pneumocyte also have developed at that time. 2. The inclusion bodies were formed in association with cytoplasmic reticulum, outer membrane of nuclear envelope, and mitochondria. 3. The inclusion bodies were distributed in cluster at the apical cytoplasm, and classified schematically with the contents as multilamellar, cytoplasmic, granular/f locculent, multivesicular, dense, and multilamellar. But the intermediate and composite forms of inclusion bodies appeared at the time toward 20 week of gestation. In summary, it is suggested that the differentiation of type II pneumocyte starts before 9 week of gestation and 4 main types of inclusion bodies considered as the precursor of multilamellar body were found. Although the inclusion bodies were formed at endoplasmic reticulum or etc, it is likely that they form multilamellar body through the complex process such as fusion of inclusion bodies.