The type 1 alveolar lining cells of the mammalian lung. II. In vitro identification via the cell surface and ultrastructure of isolated cells from adult rabbit lung.

Using a newly described dissociation and isolation technique, Type 1 alveolar lining cells were obtained from adult rabbit lung within a heterogeneous population. Identification of many lung cell types in this mixed population was by a) comparison of isolated cells with in situ lung cells in lung sections using identical parallel staining, b) stepwise ultrastructural examination of cells during all stages of lung dissociation so that intercellular associations were monitored throughout, and c) Type 1 cell surface changes following collagenase treatment. This phenomenon was studied with both electron and light microscopy, the latter employing tetrachrome staining of basophilic blebs as well as characteristic staining of nucleus and cytoplasm. Following their isolation, most Type 1 cells lost their surface blebs and assumed a "relaxed" state. In this condition, Type 1 cells were exposed to cytochalasin D (CD) for various times and at several concentrations. Surface knobs, having all the characteristics of zeiotic knobs produced in a number of cultured cell lines by exposure to CD, were produced in isolated Type 1 epithelial cells within 45 minutes. The reaction to CD was temperature-dependent, proceeding maximally at 37 C with inhibition at lower temperatures and was inhibited by antimetabolites such as dinitrophenol and 2-deoxyglucose in the presence of CD. As with established cell lines, formation of zeiotic knobs at the isolated Type 1 cell surface appeared closely related to microfilamentous nets located beneath the plasmalemma. The density of this net appeared to vary as isolated Type 1 cells underwent expansion and contraction in response to CD. Zeiotic knobs were formed as the result of herniation of endoplasm through the cell cortex. The significance of such a labile cortical zone is considered in relation to the deformation changes Type 1 cells undergo during inflation-deflation of alveoli and the folding-unfolding of alveolar lining cells as a result of lung volume changes.