Localization of alpha 2,6-linked sialic acid residues in gastrointestinal tract compartments of some tetrapod's representatives: Comparative histochemical study.

Epithelial mucins composed mainly of glycoproteins and play a vital role as protective barrier against a variety of harmful molecules and microbial infection. Additionally sialic acids, like glycoproteins, are considered as a main component of epithelial mucins and play an important role in mucosal immunity. For example, alpha 2,6-linked galactose/N-acetyl-galactosamine (Gal/GalNAc) sialic acid residues can recognize and mask different biological sites in some intermolecular or intercellular interactions. In this study, the localization sites relationship between general mucins and alpha 2,6-linked Gal/GalNAc sialic acid residues in different compartments in gastrointestinal tract (GIT) of tetrapod representatives were investigated using lectin histochemistry. The toad (Bufo regularis), lizard (Trachylepis quinquetaeniata), pigeon (Columba livia domestica) and mouse (Mus musculus) were used as amphibian, reptilian, avian and mammalian representatives respectively. In general, the biodistribution sites of mucins are localized in most compartment sites and partially overlapped with the sites of sialic acid residues in some compartment in each animal representative. Additionally, the localization sites of both mucins and sialic acid in the GIT regions differ based on the tissue type in each tetrapod representative. The mucosa of oesophagus in the toad and lizard showed higher positive signal of general mucins compared with other tetrapod representatives. However, the mucosa of the oesophagus in the toad revealed a positive signal of sialic acid in the tubular glands only, whereas the lizard's mucosa showed a positive signal of sialic acid in the goblet cells. Additionally, the pigeon's oesophagus showed no localization of the sialic acid or mucins while, all layers of the mouse's oesophagus showed a positive localization of the sialic acid residues. In the stomach, all stomach mucosa compartments in all representatives showed positive signal of mucins, while the gastric glands in the toad, pigeon (proventricular glands) and mouse showed signals of sialic acid residues localization but in different trends. While the lizard showed a localization of the sialic acid in the mucosal lamina propria only. Furthermore, the mucosa of the ileum showed positive signal of mucin in the goblet cells and some absorptive cells brush borders in all tetrapod animals. While a higher signal of the sialic acid residues in the absorptive cells but not the goblet cells in the case of the toad and mouse. While the lizard's ileum showed a higher localization of sialic acid in the goblet cells only. Mucin localization in the rectum was similar to those in ileum. Specifically, the toad and lizard showed signals of the sialic acid residues in the goblet cells only, while the mouse's rectum showed a higher signal of sialic acids in the absorptive cells and lamina propria but not in the goblet cells. The present study introduces important data about the biodistribution and localization profiles of general mucins and sialic acids residues in the GIT different compartments in each representative of tetrapoda animals. Further studies are needed to investigate the important role of sialic acid residues localization in different compartments of GIT mucosa.

Comparative microanatomical and histochemical biodistribution profiles of different types of mucins in the intestinal mucosa of some tetrapod representatives.

The microanatomical features of the intestinal tract mucosa layer in different species of tetrapoda vary according to the type of species, tissue, and function of the targeted cells. In the present study, we have evaluated the histological and histochemical variations of the intestinal tract in four species representing superclass tetrapoda. Bufo regularis (toad), Trachylepis quinquetaeniata (lizard), Columba livia domestica (pigeon) and Mus musculus (mouse) were used as representatives for amphibians, reptilians, avians and mammalians respectively. Histologically, the ileum's mucosal layer of the lower tetrapods (toad and lizard) was almost similar and consists of elongated finger-like shape villi lined with simple columnar epithelium and goblet cells. Similarly, the microanatomical features in ileum of higher tetrapod representatives (pigeon and mouse) were characterized by the presence of villi lined with simple columnar epithelium and scattered goblet cells as well as intestinal glands (crypts of Lieberkühn) at the bases of the intestinal villi. In the toad rectum, the mucosal layer was similar to that of the ileum but with shorter villi and more numerous goblet cells. However, the mucosal layer of the rectum in the lizard had low numbers of absorptive columnar epithelial cells with abundant goblet basal cells. Comparatively, the pigeon's rectal mucosa had almost a similar structure to that of ileum but in leaf-like shaped villi. Finally, the rectum of the mouse has narrow rectal pits, instead of villi, lined with goblet cells and absorptive epithelial cells. Histochemically, the ileum in the four studied tetrapod representatives showed varying biodistribution profiles of neutral, sulfated and carboxylated mucins. There are variations encountered in the intestinal brush border and goblet cells of villi in all species as well as the crypts of Lieberkühn in higher tetrapods. Also, the rectum of all tetrapod species showed weak to strong positive signals for the three types of mucins in the brush border and goblet cells of villi in all species and crypts of Lieberkühn in higher tetrapods as well. In addition, the brush border of toad's rectum was lacking sulfated mucins and that of the lizard did not have any type of mucins. The data of this study will contribute to understand the relationship between the microanatomical features and mucins biodistribution profiles in the mucosal layer of tetrapod intestinal tract and their functions.

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.