Development of the vagal innervation of the gut: steering the wandering nerve

Abstract

Background: The vagus nerve is the major neural connection between the gastrointestinal tract and the central nervous system. During fetal development, axons from the cell bodies of the nodose ganglia and the dorsal motor nucleus grow into the gut to find their enteric targets, providing the vagal sensory and motor innervations respectively. Vagal sensory and motor axons innervate selective targets, suggesting a role for guidance cues in the establishment of the normal pattern of enteric vagal innervation.

Purpose: This review explores known molecular mechanisms that guide vagal innervation in the gastrointestinal tract. Guidance and growth factors, such as netrin-1 and its receptor, deleted in colorectal cancer, extracellular matrix molecules, such as laminin-111, and members of the neurotrophin family of molecules, such as brain-derived neurotrophic factor have been identified as mediating the guidance of vagal axons to the fetal mouse gut. In addition to increasing our understanding of the development of enteric innervation, studies of vagal development may also reveal clinically relevant insights into the underlying mechanisms of vago-vagal communication with the gastrointestinal tract.

Figure 1

Schematic diagram illustrating the locations of vagal sensory and motor endings in the gastrointestinal tract. Intramuscular arrays (blue) are found in the longitudinal and circular smooth muscle layers, intraganglionic laminar endings (red) are predominantly located in the myenteric plexus, and mucosal projections (green) extend into the lamina propria. Vagal motor endings (brown) synapse directly on neurons in the myenteric and submucosal plexi.

Figure 2

Growth cone responses to chemical guidance cues. Upon sensing a gradient of netrin-1, growth cones expressing DCC are attracted towards the attractive cue (green shading). This is first seen as an elongation of filopodia towards the source followed by the reorientation of the growth cone towards the gradient. In contrast, upon sensing a repulsive cue such as Slit2, growth cones expressing Robo receptors collapse, thereby drawing back from the repulsive gradient (red shading).

Figure 3

Three stages of IGLE development are illustrated with confocal images of DiI-labeled vagal profiles in the myenteric plexus of the mouse forestomach. A. The initial stage of IGLE development. This example is a putative IGLE precursor shown at P0 that consisted of a small number of vagal fiber terminals arising from a bifurcating axon bundle in the myenteric plexus. The terminals had the appearance of growth cone-like structures or puncta that straddled the top or bottom surface of a myenteric ganglion. B. The second stage of IGLE development is illustrated by this putative IGLE precursor shown at P3 (at center of image). Maturation to this stage consisted of increased numbers of terminal puncta. C. The third stage of putative IGLE precursor maturation is illustrated here at P8. The continued development of putative IGLE terminals consisted of a further increase in numbers of terminal puncta, an increase in their packing density, and their arrangement in several leaf-like patterns or groups. Some IGLEs at this stage exhibited one or more groups of terminal puncta lying below the surface of a myenteric ganglion, and one more above it. Scale bars = 10 μm. This figure was previously published as Fig. 5B, D, F (48), copyright Elsevier (2007).

Figure 4

Four stages of IMA development are illustrated with confocal images of DiI-labeled vagal profiles in the muscle layers of the mouse forestomach. A. The initial stage of IMA formation is neurite formation, shown here by a putative IMA precursor at P0. Small vagal axon bundles in the myenteric plexus gave off a single axon (lower left), which entered the smooth muscle layer (as it continued diagonally to the upper right) and gave rise to three neurites (arrows; neurite furthest to the right is enlarged in inset). Some neurites extended small growth cone-like structures. These neurites may represent the initial formation of an IMA or a telodendrion. B. The second stage of IMA maturation is telodendria formation, illustrated here by a putative IMA precursor at P0. Two axons that exited from the myenteric plexus are shown here coursing within the longitudinal muscle layer where they branched and distributed short rectilinear fibers to begin formation of IMA telodendria (arrows) that ran parallel to the muscle fibers, including one pair that was interconnected by a crossbridge fiber (arrowheads). C. The third stage of IMA development is telodendria extension, demonstrated here by a putative IMA precursor at P0 within the circular muscle layer. The main features of this stage were lengthening of the telodendria and an increase in the numbers of telodendria connected by cross-bridge fibers. D. The final stage of IMA maturation is shown here in a montage of confocal images of putative IMA precursors at P4. This stage was dominated by further lengthening of IMA telodendria as compared with earlier stages (e.g., A-C) as well as by continued formation and growth of telodendria. Scale bars = 10 μm (A-D) and 25 μm (inset, A). This figure was previously published as Fig. 5A, C, E (48) and Fig. 3D (52), copyright Elsevier (2007, 2008).

Figure 5

DCC immunoreactivity is found in fetal mouse paraesophageal vagal nerve trunks at E16. (A-C) Esophagus. Only fasciculated vagal nerve trunks are DCC immunoreactive. The same section of esophagus has been labeled by DiI applied to the nodose ganglia (A; red) and with antibodies to DCC (B; green). (C) The images in panels A and B are merged. Note the absence of co-localization in the ramifying fibers, suggesting that other molecules may be involved in guiding vagal endings to their correct enteric targets. Bar = 50 m. This figure was previously published at Fig. 4F-H (44), copyright John Wiley & Sons, Inc. (2006).