Ultrastructure of tracheal surface liquid: low-temperature scanning electron microscopy.

ABSTRACT

A layer of liquid lines the airways in the lung. Previous microscopic studies have suggested that it is in two phases, with a mucous gel lying above a periciliary sol. However, shrinkage artifacts due to chemical fixation, dehydration, and drying have prevented reliable estimates of the depth of these layers. To avoid such problems, we have studied the surface liquid of bovine trachea by low-temperature scanning electron microscopy (LTSEM). A polished copper probe cooled to liquid nitrogen temperature was applied to the mucosal surface of sheets of excised tracheal epithelium to effect rapid freezing of surface liquid. Tissue sheets were then mounted in an LTSEM (AMRay 1000A with Biochamber) which maintains samples at -180 degrees C with a Joule-Thompson refrigerator built into the stage. Tissues were fractured at right angles to the epithelial surface, coated with gold, and viewed, all at 10(-5) to 10(-6) torr without transfer through air. The sample was stable under the electron beam at accelerating voltages up to 20 kV. Epithelial features (nuclei, cilia, microvilli, mucous granules) were well preserved. The mucosal surface of the cells was covered with material on the order of 8 microns in depth. The mucous gel and periciliary sol could be seen as distinct layers and could be distinguished by the size and pattern of ice crystal voids generated by radiant-etching of the fractured surface of the sample.