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.