Morphological and Histological Studies of the Bronchial and Parabronchial System of the White Pekin Duck (Anas platyrhynchos).

The present study aimed to provide comprehensive morphological features of the bronchial and parabronchial systems using cast, histological, histochemical, and scanning electron microscopy techniques, with new insights into the parabronchial topographic distribution system on 22 white Pekin ducks. Casting illustrated that the medioventral secondary bronchi (MVSB) were the largest, but the posterior (POSB) ones were the smallest. The primary (PB) and secondary bronchi (SB) were lined with thin pseudostratified, ciliated columnar epithelium. PB contained discontinuous hyaline cartilage plates interconnected by a membrane of fibrous CT with chondrocytes, while SB had mucous glands. There were two types of hexagonal parabronchi with different lumen shapes: circular in neopulmonic and longitudinal in paleopulmonic. The parabronchi had numerous atria opened into the lumen and guarded by different directed muscles: horizontal in the neopulmonic and vertical or horizontal in the paleopulmonic. The atria were lined with squamous to cuboidal cells, forming the interatrial septum (IAS). A funnel atrial duct connecting the atrium to the infundibulum can be branched. The air capillaries were nearby, at a very short distance from the blood capillaries. Despite their small number, air capillaries, which were found in parabronchi, significantly increased in size and diameter. SEM at the 4th torus level showed a parabronchi distribution with elongated paleopulmonic on the dorsomedial part, hexagonal neopulmonic on the ventrolateral part, and some neopulmonic on the medial part. The parabronchial topography distribution exhibited their unique distribution from the 1st to the 6th torus level.