Strategic localization of toll-like receptor 4 in the digestive tract of blunt snout bream (Megalobrama amblycephala).

This study was performed to determine the localization strategies of Toll-like Receptor 4 (TLR4) in digestive tract (oesophagus, bulbodium, foregut, midgut and hindgut) of Blunt snout bream (Megalobrama amblycephala) using immunohistochemical staining method. TLR4 positive cells were observed throughout the digestive tract. In the oesophagus, some positive reactions in lamina propria were found around small blood vessels and there were also some positive cells within the stratified squamous epithelium. Lots of positive cells were observed in the muscular layer of the oesophagus. In bulbodium, foregut and hindgut, the expression of TLR4 was mainly restricted to the apical surface of epithelial cells located at the bottom of the mucosal folds and the mesenchymal cells in lamina propria. It was very interesting that epithelial cells in the midgut, but none in other parts, had many TLR4 positive cytoplasmic granular structures which were also periodic acid Schiff positive. These findings suggested that TLR4 was expressed in a compartmentalized manner in the Blunt snout bream (M. amblycephala) digestive tract and provided novel information about the in vivo localization of pattern recognition receptors.