Functions and imaging of mast cell and neural axis of the gut

Abstract

Close association between nerves and mast cells in the gut wall provides the microanatomic basis for functional interactions between these elements, supporting the hypothesis that a mast cell-nerve axis influences gut functions in health and disease. Advanced morphology and imaging techniques are now available to assess structural and functional relationships of the mast cell-nerve axis in human gut tissues. Morphologic techniques including co-labeling of mast cells and nerves serve to evaluate changes in their densities and anatomic proximity. Calcium (Ca(++)) and potentiometric dye imaging provide novel insights into functions such as mast cell-nerve signaling in the human gut tissues. Such imaging promises to reveal new ionic or molecular targets to normalize nerve sensitization induced by mast cell hyperactivity or mast cell sensitization by neurogenic inflammatory pathways. These targets include proteinase-activated receptor (PAR) 1 or histamine receptors. In patients, optical imaging in the gut in vivo has the potential to identify neural structures and inflammation in vivo. The latter has some risks and potential of sampling error with a single biopsy. Techniques that image nerve fibers in the retina without the need for contrast agents (optical coherence tomography and full-field optical coherence microscopy) may be applied to study submucous neural plexus. Moreover, the combination of submucosal dissection, use of a fluorescent marker, and endoscopic confocal microscopy provides detailed imaging of myenteric neurons and smooth muscle cells in the muscularis propria. Studies of motility and functional gastrointestinal disorders would be feasible without the need for full-thickness biopsy.

Figure 1

Close association between nerves and mast cells in human sections. Tissues were stained for the pan-neuronal marker protein gene product 9.5 (PGP9.5), mast cell tryptase (MC tryptase), or c-Kit as mast cell markers and transient receptor potential vanilloid 1 (TRPV1) as a marker for extrinsic sensory nerve terminals. (A) Low-power image of a cross-section of the human colonic wall with the epithelium, submucous plexus, circular muscle, myenteric plexus, and longitudinal muscle layers. Note that the highest density of mast cells is in the epithelial and submucous layers. (B) High-power image of the inner submucous plexus showing a ganglion (marked by arrow head) and a mast cell nearby. (C) High-power image of the epithelial layer illustrating numerous mast cells in close vicinity to nerve fibers.

Figure 2

Close association between nerves and mast cells in human whole mount tissue. (A) Whole-mount preparation with the exposed submucous plexus as an intact network. Note the intimate relation between mast cells, nerve fibers, and enteric ganglia (one is marked by arrowhead). (B) Paucity of mast cells in a whole mount preparation of the myenteric plexus. This image shows only 12 mast cells, most of them far away from the ganglia. (C) Contact between enteric ganglia (one is marked by arrowhead), mast cells, and transient receptor potential vanilloid 1 (TRPV1) terminals. Numerous TRPV1 terminals (in light blue because of co-localization with PGP.9.5) are present in submucous plexus ganglia. Note that close proximity between mast cells and TRPV1 fibers only occurs nearby an enteric ganglion. In the extra-ganglionic regions there are hardly any TRPV1 fibers that run close by a mast cell. (D) Almost all mast cells in the submucous plexus layer are co-labeled by MC tryptase and c-kit (purple; one ganglion marked by arrowhead). E, Whole-mount preparation of the submucous plexus region containing blood vessels. Note that mast cells are located along blood vessel. In addition, this image shows that submucous plexus ganglia (2 are marked by arrowheads) are often close to blood vessels.