Comparative microanatomical and histochemical biodistribution profiles of different types of mucins in oesophageal gastric tract mucosa of some tetrapod representatives.

The microanatomical features of the oesophageal gastric tract in tetrapod representatives and their function, especially those related to the mucosal layer, have not yet been fully investigated. The mucosal layer cells and their function in the oesophageal gastric tract differ structurally and functionally in tetrapod representatives based on interspecies difference and the type of food and feeding habits. The present study was, therefore, postulated to compare the mucosal microanatomical structure and histochemical biodistribution of different mucin types in oesophageal gastric tract tissues of four tetrapod species. A representative of each tetrapod class was selected, as follows: the Egyptian toad Bufo regularis, the lizard Trachylepis quinquetaeniata, the domestic pigeon Columba livia domestica and the albino mouse Mus musculus for Amphibia, Reptilia, Aves and Mammalia, respectively. Microanatomically, in lower tetrapods (toad and lizard), the mucosal layer of the oesophagus was composed of simple ciliated columnar epithelium with goblet cells, whereas in higher tetrapods (pigeon and mouse) it was composed of stratified squamous epithelium, with non-keratinised epithelium in the pigeon but keratinised epithelium in the mouse. However, the gastric mucosal layer of the stomach in lower tetrapods consists of simple columnar epithelium and gastric glands. Similarly, the mucosa of the pigeon's proventriculus consists of simple columnar epithelium with proventricular glands opened into the lumen, whereas mouse mucosa consists of simple columnar epithelium which folds and forms gastric glands with gastric pits having a variety of cell types. Histochemically, the neutral mucin profile biodistribution in the oesophagus mucosal layer was variable. It was strongly positive in the toad and lizard, but was weak in the pigeon and completely negative in the mouse. In contrast it was strongly positive in the gastric mucosa of the toad, lizard and pigeon, but was weak in the mouse's gastric mucosa. On the other hand, the signals of carboxylated and sulfated mucins were found to be different. They were strong in the mucosa of the lizard oesophagus. In contrast, the carboxylated mucins in the gastric mucosa were positive in all representatives except the mouse. The sulfated mucins were, however, seen localised in the mucosal layer cells of the lizard and pigeon only. The study revealed that the microanatomical structures and functions as well as mucin distribution profiles in the oesophageal gastric tract are in line with interspecies difference and the type of food and feeding habits. However, this may need further investigations including more tetrapod representatives.