An electron microscopic study of the parabronchial epithelium in the mature lung of four bird species

Abstract

Background: No integrated comprehensive description of the ultrastructure of the parabronchial epithelium is available. The origin, discharge, and occurrence of the trilaminar substance have not yet been sufficiently studied. Therefore, the main objectives were to classify the cell 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 subjected to standard processing for transmission electron microscopy, both 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 produce and discharge osmiophilic lamellar bodies. Squamous atrial cells (2) manufacture and discharge a trilaminar substance in sheets sandwiched between the long microvilli emerging from the apical cytoplasm. Their attenuating cell outgrowths overlap granular cells. At the bases of atria, they pass as intermediate squamous atrial cells to the infundibula, contacting the extensions of squamous respiratory cells. The squamous atrial cells undergo distinct structural variations depending on age and environment. Squamous respiratory cells (3) (cellulae squamosae) continuously line the air capillaries and neighboring infundibula. They constitute the epithelial compartment of the blood-gas barrier. The cell bodies extend long, very thin cell outgrowths. The apical surface is smooth and the basal part is invested with a very thin basement membrane. The trilaminar substance originates from granular and agranular endoplasmic reticulum in the form of convoluted profiles which are discharged as an acellular lining layer on the air surface of squamous respiratory cells. Conclusions: Granular cells are analogous to the type II cells of mammalian pulmonary alveolus. Squamous atrial and respiratory cells, of a common embryonic origin, do not meet any counterpart in epithelial cell populations of lung terminal airways in vertebrates. The specific trilaminar substance-lipoproteinaceous in nature-is a constant compound of atria and air capillaries.