Distribution and structure of glandular tissue in the oropharynx and proximal esophagus of the emu (Dromaius novaehollandiae)

The glandular regions of the upper digestive tract in the emu were non-pigmented (except for the tongue in most specimens) and invested by a non-keratinised stratified squamous epithelium. The glands found in these regions were exclusively simple in nature and composed of tubular secretory units lined by Periodic Acid Schiff Stain-positive mucus-secreting cells. The naming of the various glandular fields was based on previously identified anatomical features and on nomenclature modified from previous studies on birds. The glands were classified into two main types, namely, simple tubular and simple branched tubular mucus-secreting glands. Simple branched tubular glands were a feature of the regions exposed to the greatest amount of friction during feeding, whereas simple tubular glands were a feature of regions exposed to less friction. The saliva produced by the salivary glands in birds functions to moisturize and lubricate food boli. Mucins in saliva also protect mucosal surfaces from desiccation and mechanical damage, assist in maintaining cellular water balance, provide lubrication and have an antimicrobial action. This study suggests that, in addition to the role of specific gross anatomical features, the type and distribution of glandular tissue in the emu upper digestive tract supports the cranioinertial feeding method employed by this species.