Morphology of alveolar lining layer

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Abstract
The extracellular alveolar lining layer (ALL), composed of both surface film and hypophase, is described under different conditions and fixatives.With primary osmium fixation of small blocks the ALL is not preserved. Lamellar and homogeneous osmiophilic bodies are scattered at random within alveoli. Occasionally the surface film remains within an alveolus but the hypophase is not seen. Poor preservation of the ALL is thought to be due to the limited ability of osmium to cross link proteins and to the mechanical deformation of the alveoli which inevitably accompanies the cutting of fresh lung into small blocks. This leads to the escape of alveolar air and a loss of the air‐liquid interface.With primary glutaraldehyde fixation of the whole lung, the ALL is well preserved in many alveoli. The hypophase is mostly homogeneous but occasionally contains osmiophilic bodies and lattices. There are no lamellar and homogeneous osmiophilic bodies within the air space of the alveoli when the ALL is preserved, which suggests that primary osmium fixation of tissue leads to the dislodgement of these osmiophilic bodies from the hypophase.The electron densities of the surface film, the hypophase and the capillary fluid are alike in a given preparation but vary considerably with the concentration of the glutaraldehyde solution and the buffer vehicles used. It is pointed out that in phosphate‐buffered preparations the hypophase, the basement membrane and the capillary fluid are highly electron‐dense which makes difficult the localization of an electrondense tracers.