INTRAALVEOLAR BUBBLES AND BUBBLE-FILMS .2. 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 a irway free gas and alveolar bubbles at the level of respiratory bronch ioles is established within 1 hour after birth. We now examine the lun g through the rest of development, namely, 2 weeks, 1, 2, and 3 months , and adulthood. Methods: In quick succession in anesthetized spontane ously breathing rabbits, the abdominal aorta was transected and trache a was occluded either after an end-tidal exhalation at functional resi dual capacity (FRC) or after volume expansion in vivo by a single infl ation from FRC to 20 or 25 cm H2O pressure (V-20, V-25). Immediately t he thorax was opened and lungs were examined (anterior, anterolateral) through a dissecting stereomicroscope while still in the chest, unper turbed (pleural surface temperature 34 degrees C). Heart and lungs wer e then removed en bloc and re-examined (anterior, lateral, posterior) to confirm that architecture had not changed (22-27 degrees C). After these immediate examinations, lungs were entered into one of the proto cols enumerated in Results.Results: Immediate examination revealed bub bles in all aerated subpleural and deep (''central'') alveoli from ape x to base at all ages and temperatures. Bubbles were confirmed from tw o views (top and tangential) and from their individual mobility in res ponse to gentle microprobe pressure. A ''common bubble'' (>30 mu m to similar to 120 mu m inside diameter at FRC) appeared to occupy a singl e alveolus, sometimes arranged in clusters and collectively accounting for similar to 84% of the total bubble population. Few ''large bubble s'' appeared to be intraductal. We concluded that ''small bubbles'' (l ess than or equal to 30 mu m; similar to 16% of the total population) were contracted common bubbles. The free gas-bubble film boundary of t he airways was at the level of respiratory bronchioles. Subsequent pro tocols: (1) Common bubbles moved out of adjoining tissue following sub pleural incision. Adjacent bubbles either moved into vacated spaces or into the outside liquid medium. Large bubble(s) followed common bubbl es out of the tissue. Small bubbles were less mobile and distal common bubbles did not move. The sequence of bubble movement at V-25 was the same. Isolated bubbles had normal surfactant content and surface tens ion according to ''Pattle's stability ratio.'' Transection revealed an alogous conditions in central alveoli. (2) Bubble size increased durin g inflation from FRC to V-25. Airless spaces were aerated with bubbles during inflation. (3) The bubble surface was compressed during deflat ion to 81% of maximal volume (Vmax) and below, including deflation to minimal volume (Vmin). (4) Bubble/alveolar shape changed from spherica l-oval to polygonal when the pleural surface dried at FRC and V-25. Th e 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 bu bbles. Conclusions: Bubbles and bubble films are fundamental to normal architecture of aerated alveoli at all lung volumes from birth throug h adulthood, As infrastructure, they sustain aeration and resist defor mation, With ductal films, they may be expected to form an alveolar su rface liquid (foam film) network (Scarpelli, 1988, Surfactants and the Lining of the Lung) that modulates liquid balance principally at Plat eau borders. They expand and contract respectively during inflation an d deflation, maintaining their closed film integrity. Films are compre ssed to ''film collapse'' in situ during deflation from volumes well a bove FRC to Vmin. At these volumes, intact films sustain aeration; som e may disperse into the hypophase liquid. New bubbles form during infl ation. With the septal tissue, bubble films determine alveolar size, s hape and surface area, namely the spherical-oval ''bubble form'' when ''wet'' and the polygonal ''septum form'' when ''dry.'' (C) 1996 Wiley -Liss, Inc.