Intraalveolar bubbles and bubble films: II. Formation in vivo through adulthood.

Background Intraalveolar bubbles and bubble films have been shown to be part of the normal alveolar architecture in vivo from birth through the first 2 days of extrauterine life of rabbit pups (Scarpelli et al., 1996a. Anat. Rec. 244:344–357). The intraluminal boundary between airway free gas and alveolar bubbles at the level of respiratory bronchioles is established within 1 hour after birth. We now examine the lung through the rest of development, namely, 2 weeks, 1, 2, and 3 months, and adulthood. Methods In quick succession in anesthetized spontaneously breathing rabbits, the abdominal aorta was transected and trachea was occluded either after an end-tidal exhalation at functional residual capacity (FRC) or after volume expansion in vivo by a single inflation from FRC to 20 or 25 cm H2O pressure (V20, V25). Immediately the thorax was opened and lungs were examined (anterior, anterolateral) through a dissecting stereomicroscope while still in the chest, unperturbed (pleural surface temperature 34°C). Heart and lungs were then removed en bloc and re-examined (anterior, lateral, posterior) to confirm that architecture had not changed (22–27°C). After these immediate examinations, lungs were entered into one of the protocols enumerated in Results. Results Immediate examination revealed bubbles in all aerated subpleural and deep (“central”) alveoli from apex to base at all ages and temperatures. Bubbles were confirmed from two views (top and tangential) and from their individual mobility in response to gentle microprobe pressure. A “common bubble” (>30 μm to ∼120 μm inside diameter at FRC) appeared to occupy a single alveolus, sometimes arranged in clusters and collectively accounting for ∼84% of the total bubble population. Few “large bubbles” appeared to be intraductal. We concluded that “small bubbles” (≤30 μm; ∼16% of the total population) were contracted common bubbles. The free gas-bubble film boundary of the airways was at the level of respiratory bronchioles. Subsequent protocols: (1) Common bubbles moved out of adjoining tissue following subpleural incision. Adjacent bubbles either moved into vacated spaces or into the outside liquid medium. Large bubble(s) followed common bubbles out of the tissue. Small bubbles were less mobile and distal common bubbles did not move. The sequence of bubble movement at V25 was the same. Isolated bubbles had normal surfactant content and surface tension according to “Pattle's stability ratio.” Transection revealed analogous conditions in central alveoli. (2) Bubble size increased during inflation from FRC to V25. Airless spaces were aerated with bubbles during inflation. (3) The bubble surface was compressed during deflation to 81% of maximal volume (Vmax) and below, including deflation to minimal volume (Vmin). (4) Bubble/alveolar shape changed from spherical-oval to polygonal when the pleural surface dried at FRC and V25. The original shape was restored when the surface was re-wet. Dry tissue showed but did not emit bubbles when cut; re-wet tissue did. (5) Lung liquid content and volume-pressure were normal at FRC. (6) As expected, conventionally fixed, dehydrated, and embedded sections showed no bubbles. Conclusions Bubbles and bubble films are fundamental to normal architecture of aerated alveoli at all lung volumes from birth through adulthood. As infrastructure, they sustain aeration and resist deformation. With ductal films, they may be expected to form an alveolar surface liquid (foam film) network (Scarpelli, 1988. Surfactants and the Lining of the Lung) that modulates liquid balance principally at Plateau borders. They expand and contract respectively during inflation and deflation, maintaining their closed film integrity. Films are compressed to “film collapse” in situ during deflation from volumes well above FRC to Vmin. At these volumes, intact films sustain aeration; some may disperse into the hypophase liquid. New bubbles form during inflation. With the septal tissue, bubble films determine alveolar size, shape and surface area, named the spherical-oval “bubble form” when “wet” and the polygonal “septum form” when “dry.” © 1996 Wiley-Liss, Inc.