A cation-retaining layer in the alveolar-capillary membrane.

The ultrastructural cytochemistry of the alveolar-capillary "membrane" has been investigated. Dialyzed iron and high iron diamine staining demonstrated a layer containing sulfated mucosubstance at the basal surface of the membranous pneumocytes. In the same layer, fixation with an osmium tetroxide-potassium pyroantimonate solution yielded abundant fine precipitates which were abolished by inclusion of ehtylene glycol bis-N,N'-tetraacetic acid (EGTA) in the fixative. This cation-retaining layer could be interpreted as providing a mechanism for retarding cation transport from the basement membrane to the alveolar space. High iron diamine staining also demonstrated a layer of sulfated mucosubstance on the microvilli of the granular pneumocytes. This layer corresponded with a sialidase-resistant, dialyzed iron-reactive stratum which overlaid the sialomucin-rich cell coat.