ANCHORING AND SUPPORT SYSTEM OF PULMONARY GAS-EXCHANGE TISSUE IN 4 BIRD SPECIES

Avian air capillaries are delicate structures compared to the mammalia n pulmonary alveolus. A transmission and scanning electron microscopic study was carried out on several species of birds with the aim of det ermining the support structures of the avian gas-exchange mantle. Lung tissue of two bird species belonging to strong flying birds (pigeon a nd barn owl) and two relatively flightless species (domestic fowl and quail) was subjected to standard processing for transmission and scann ing electron microscopy after intratracheal inflation, Twisted profile s of lipoproteinaceous trilaminar substance as specific secretory prod uct of avian squamous respiratory cells can be seen in the cell body a nd cytoplasmic extensions that are wedged between the blood capillarie s, partly surrounding them, The intracytoplasmatically located trilami nar complexes from a three-dimensional intricate spiderweb-like system between the blood capillaries and air capillaries, which presumably f unction as an anchoring and support structure of the gas-exchange tiss ue. This system is strengthened by retinacula -pairs of attenuated par allel processes of squamous respiratory cells that project to the airw ay lumen -expanding and bridging the opposite side of air capillaries, The trilaminar substance is discharged in the form of a 15-nm-thick a cellular lining layer which is uniquely adapted to the extremely thin respiratory epithelium. The trilaminar substance arises in the cytopla sm of squamous respiratory cells from profiles of granular and smooth endoplasmic reticulum. The integrity and stability of the gas-exchange tissue is likely to be guaranteed by a specific arrangement of the sq uamous respiratory cells, in which the trilaminar substance plays a pa ramount role, This general pattern can be observed in strong flying bi rd species as in the relatively flightless birds.