I. Bazantova

Pulmonary macrophages in birds (barn owl, Tyto tyto alba), domestic fowl (Gallus gallus f. domestica), Quail (Coturnix coturnix), and pigeons (Columbia livia)

Birds have a limited number of resident macrophages in the normal steady‐state respiratory tract. The discovery of phagocytes in lavages of lung from birds contrasts with findings that phagocytes are seldom seen in investigations in situ. An electron microscopic study was performed in the respiratory units, the parabronchi, and air capillaries in particular in several adult bird species to localize the seat of respiratory macrophages.

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

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.

Anchoring and support system of pulmonary gas-exchange tissue in four bird species.

Avian air capillaries are delicate structures compared to the mammalian pulmonary alveolus. A transmission and scanning electron microscopic study was carried out on several species of birds with the aim of determining the support structures of the avian gas-exchange mantle. Lung tissue of two bird species belonging to strong flying birds (pigeon and barn owl) and two relatively flightless species (domestic fowl and quail) was subjected to standard processing for transmission and scanning electron microscopy after intratracheal inflation. Twisted profiles of lipoproteinaceous trilaminar substance as specific secretory product of avian squamous respiratory cells can be seen in the cell body and cytoplasmic extensions that are wedged between the blood capillaries, partly surrounding them. The intracytoplasmatically located trilaminar complexes form a three-dimensional intricate spiderweb-like system between the blood capillaries and air capillaries, which presumably function as an anchoring and support structure of the gas-exchange tissue. This system is strengthened by retinacula--pairs of attenuated parallel processes of squamous respiratory cells that project to the airway lumen--expanding and bridging the opposite side of air capillaries. The trilaminar substance is discharged in the form of a 15-nm-thick acellular lining layer which is uniquely adapted to the extremely thin respiratory epithelium. The trilaminar substance arises in the cytoplasm 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 squamous respiratory cells, in which the trilaminar substance plays a paramount role. This general pattern can be observed in strong flying bird species as in the relatively flightless birds.

The development and differentiation of the parabronchial unit in quail (Coturnix coturnix).

The present study has been inspired by the conflicting data in the relevant literature concerning the embryogenesis of cell types of the parabronchial epithelium and the formation, discharge and distribution of trilaminar substance and lamellar bodies. Lung tissue from embryonic, newly hatched, immature and mature quail was subjected to standard processing for light and transmission electron microscopy. The parabronchial rudiments form shallow primitive atria on embryonic day 13. The precursors of granular cells differentiate with lamellar bodies in their cytoplasm. The residual population of non-granular epithelial cells is the common source for the differentiation of primitive squamous atrial and respiratory cells, the potential producers of trilaminar substance. The primitive squamous atrial cells sprout as branching infundibular canaliculi into the mesenchyme on embryonic day 14. The infundibular epithelium differentiates into the squamous respiratory cells that constitute with blood capillaries the blood-air barrier. Not until the time of hatching could the trilaminar substance be visualized being produced by squamous atrial and respiratory cells. In the late prehatching and early posthatching period the granular cells intensely escalate the production and discharge of lamellar bodies. The lamellar bodies form, together with sheets of trilaminar substance, mixed multilayered masses in atria. They disappear fast in the successive posthatching period. The formation of trilaminar substance in squamous atrial and respiratory cells is governed by the agranular endoplasmic reticulum, the cisternae of which take part in the formation of trilaminar units. The gas exchange tissue is predominantly represented by infundibula in immature quail. The posthatching growth of the gas exchange tissue of immature to mature quail occurs via intense multiplication of air and blood capillaries.

Lamellar inclusions and trilaminar substance in the parabronchial epithelium of the quail (**Coturnix coturnix**)

The fine structure of the epithelial cells of the parabronchus and their secretory products have been the subject of many studies and have given rise to considerable controversy about their configuration and ultrastructure. The aim of the present study was to investigate the mode of formation and discharge of lamellar bodies of granular cells and the trilaminar substance produced and discharged by the embryologically related squamous atrial and respiratory epithelial cells. The material for light and transmission electron microscopic analysis was collected from 10 mature quail and 3 individuals aged 2 days. The parabronchial atria harbour two ultrastructurally distinct types of epithelial cells. The granular cells (analogous to type II cells of the mammalian pulmonary alveolus) produce and discharge balls of lamellar bodies. The squamous atrial cells produce and discharge sheets of trilaminar substance sandwiched between long tentacle-like processes, viz. the microvilli. The infundibula and air capillaries of the gas exchange tissue are invested with squamous respiratory cells which extend very thin, long processes that cover the air capillaries and constitute, together with the blood capillaries, the blood-air barrier. The squamous respiratory cells produce and discharge trilaminar substance as an extracellularly located acellular lining layer which is found in close contact with their cell membrane. Both squamous atrial and respiratory cells hence synthetize and discharge trilaminar substance, the basic unit of which has the ultrastructural appearance of a 7.5 to 8.0 nm membrane unit. The formation of trilaminar substance originates in the agranular endoplasmic reticulum, while the origin of the lamellar bodies of granular cells is related to the granular endoplasmic reticulum, the Golgi complex and the multivesicular bodies. Their structural unit is composed of a 4.5 to 5.0 nm thread-like structure which is concentrically arranged around a spherical core of granular substance.

An SEM and TEM study of the transition of the bronchus to the parabronchus in quail (Coturnix coturnix)

The main objective was to analyse the transition of the bronchus to the parabronchus in birds and to describe its specific structure in an integrated light microscopic, transmission electron microscopic (TEM) and scanning electron microscopic (SEM) study. Lung tissue from immature and mature quail was subjected to standard processing for paraffin light microscopy, TEM and SEM after intratracheal inflation with fixative. In transverse paraffin and Durcupan semithin sections, the partition incompletely closing the broncho-parabronchial transition has the appearance of a crest-like fold delineating the entrance to the underlying parabronchial vestibulum. The core of the entrance fold is composed of loose connective tissue with free cells, and has a well-developed blood supply and innervation. Voluminous groups of smooth muscle cells are interconnected with those of neighbouring entrance folds and the interatrial septa. On the parabronchial surface and partly on the bronchial surface the entrance fold is invested with simple cuboid epithelium consisting exclusively of granular cells with lamellar inclusions. On the bronchial surface, they pass into ciliated columnar pseudostratified epithelium. At the root of the parabronchially orientated surface, they continue into the mixed population of granular and squamous atrial cells of the parabronchus. Among the granular cells of the entrance fold, scattered epithelial neuroendocrine cells are consistently present. The three-dimensional visualization demonstrated the oval form of the entrance window with a circular field of non-ciliated cells delineating the entrance to the parabronchial labyrinthine system. The general structural pattern of the entrance fold, together with the complex system of interatrial trabecles of the parabronchi underline the multifactorial function of a complex system submitted to the skeletal, regulatory and host defense of the gas exchange tissue.