MPs contained in the supernatant of a processed CF sputum were subjected to a single centrifugation at 14,000 × g for 45 min. MP pellet was fixed in 4% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.4) for 24 hours. The sample was then dehydrated in solutions of ethanol of increasing strength from 50%, 70%, 95% and 100% for 10 minutes in each solution. The sample was finally dehydrated in propylene oxide for 15 minutes. Finally, the sample was embedded in epoxy resin (Epon 12). After overnight polymerization, ultrathin sections (70 nm) were cut and examined in a JEOL (Tokyo, Japan) transmission electron microscope.