AN ELECTRON-MICROSCOPIC STUDY OF THE PARABRONCHIAL EPITHELIUM IN THE MATURE LUNG OF 4 BIRD SPECIES

Background: No integrated comprehensive description of the ultrastruct ure of the parabronchial epithelium is available, The origin, discharg e, and occurrence of the trilaminar substance have not yet been suffic iently studied. Therefore, the main objectives were to classify the ce ll types of the parabronchial epithelium and to describe their role in manufacturing the trilaminar substance,Methods: Lung tissue of mature quail, domestic fowl, town pigeon, homing pigeon, and barn owl was su bjected to standard processing for transmission electron microscopy, b oth after intratracheal inflation and intravascular perfusion, Results : The atrial epithelium is constituted by granular and squamous atrial cells. Granular cells (1) are confined to the atrial wall; they produ ce and discharge osmiophilic lamellar bodies. Squamous atrial cells (2 ) manufacture and discharge a trilaminar substance in sheets sandwiche d between the long microvilli emerging from the apical cytoplasm, Thei r attenuating cell outgrowths overlap granular cells, At the bases of atria, they pass as intermediate squamous atrial cells to the infundib ula, contacting the extensions of squamous respiratory cells, The squa mous atrial cells undergo distinct structural variations depending on age and environment, Squamous respiratory cells (3) (cellulae squamosa e) continuously line the air capillaries and neighboring infundibula, They constitute the epithelial compartment of the blood-gas barrier. T he cell bodies extend long, very thin cell outgrowths, The apical surf ace is smooth and the basal part is invested with a very thin basement membrane, The trilaminar substance originates from granular and agran ular endoplasmic reticulum in the form of convoluted profiles which ar e discharged as an acellular lining layer on the air surface of squamo us respiratory cells, Conclusions: Granular cells are analogous to the type ii cells of mammalian pulmonary alveolus, Squamous atrial and re spiratory cells, of a common embryonic origin, do not meet any counter part in epithelial cell populations of lung terminal airways in verteb rates. The specific trilaminar substance-lipoproteinaceous in nature-i s a constant compound of atria and air capillaries. (C) 1997 Wiley-Lis s, Inc.