Intermediate filaments in the developing type II cell of fetal monkey lung.

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Abstract
Anticytokeratin monoclonal antibody was used to study epithelial cell development in fetal monkey lungs taken from animals of different ages. It is well established that the overall maturity of fetal lung depends greatly on the maturation of type II epithelial cells in the alveolus. In this study, we have correlated the cytokeratin phenotype of mammalian epithelial cells with pneumocyte maturation. We show that differentiation and maturation of the type II cell is related to intermediate filament expression. Twenty-four fetal monkeys (Macaca nemestrina) were delivered by cesarean section at a gestational age of 135-140 days (term = 168 days) and divided into two groups. One group of animals was sacrificed during the first 3 hr of life, and the other group was maintained in incubators for 92-120 hr. Anticytokeratin monoclonal antibody recognizes only alveolar type I and type II epithelial cells. In the first 3 hr of life, the cytokeratin was localized only at the alveolar surface and at the cytoplasmic periphery of the type II cells of these premature animals. However, at the age of 92-120 hr, the epithelia in the lungs reacted more intensely than they did during the first 3 hr. Electron microscopy revealed and confirmed that the type II cells were matured and abundant intermediate filaments appeared in the cytoplasm. The filaments appeared to form either aggregates or parallel filament bundles and few were closely associated with the lamellar bodies. In the immature type II cells at 0-3 hr of life, few intermediate filaments could be localized in the cytoplasm, and no parallel filament bundle was observed, though many appeared in the 92-120 hr lungs. This suggests that the intermediate filaments have a functional significance in the development and maturation of the type II cell. The location and stability of keratin filaments in type II cells may confer the structural strength necessary for cells covering a free surface in the alveoli during lung maturation.