Cellular elements organization in the trachea of mallard (Anas platyrhynchos) with a special reference to its local immunological role.

Many studies have been carried out to investigate the histological structure of the trachea in many species of birds. However, the cellular organization of the trachea in the mallard duck is still unclear. This study was performed on 12 sexually mature male Mallard duck to demonstrate the cellular organization of the trachea using light and electron microscopy. The tracheal epithelium is considered the first line of defense against airborne pathogens. The mallard trachea was lined by a pseudostratified ciliated columnar epithelium that contained many morphologically distinct cell types: ciliated, non-ciliated, basal cells that encircled by a population of sub-epithelial immune cells, fibroblasts, and telocytes (TCs). Telocytes were first recorded in duck trachea in this study and showed a wide variety of staining affinity. They presented two long telopodes that made up frequent close contacts with epithelium, tracheal cartilages, and other neighboring TCs, immune cells, blood capillaries, and nerve fibers. TCs express VEGF and S-100 protein. The immune cells include mast cells, eosinophils, basophils, lymphocytes, plasma cells, and dendritic reticular cells. The ciliated tracheal epithelium was interrupted by numerous intraepithelial mucous glands and solitary goblet cells. This mucociliary apparatus constitutes the major defense mechanism against inhaled foreign materials. The cellular organization of the duck trachea and its relation to the immunity was discussed.