The alveolar surface is lined by a coherent liquid-crystalline phase

Abstract

The existence of a uniform phase at the alveolar surface is demonstrated by polarizing microscopy and cryogenic transmission electron microscopy (cryo-TEM). Rabbit lungs were opened and the alveolar surface film was directly deposited on optical microscope coverslips and on electron microscope grids. In the polarizing microscope, the alveolar surface material deposited exhibits a uniform birefringence, which shows that the alveolar surface is lined by a coherent liquid-crystalline phase; a surface phase. Cryo-TEM, with reduced risks of artifacts compared to conventional electron microscopy, shows the existence of a mechanically stable and continuous surface structure of regularly spaced bilayers oriented along the grid surface over large distances (microns). The periodicity observed by cryo-TEM indicates that the surface phase has the same inner organization as the ultrastructure known as tubular myelin (TM). Two alternative structures of TM are considered: the traditional crossed bilayer structure and a bilayer structure free from intersections according to the so-called crossed layers of parallels (CLP) minimal surface. Evidence presently available favors the CLP minimal surface structure alternative. The implication of the existence of a surface phase on the barrier function of the alveolar surface zone is finally considered.