Ultrastructure of congenital cystic adenomatoid malformation of the lung.

Four cases of congenital cystic adenomatoid malformation of the lung are presented with ultrastructural analysis. In some of the cases the ultrastructure is similar to that described in normal fetal airways during the early embryonic phase of lung development. Varying degrees of differentiation are noted, confirming what has previously been observed ultrastructurally in this lesion. One new finding is the presence of tonofilaments in one of our cases, which may represent a metaplastic change. Although the exact nature of the insult is unknown, there may be a developmental defect affecting epithelial differentiation as well as the interaction between the developing pulmonary mesenchyme and distal respiratory units.