Development of the Enteric Nervous System

11.1.1

Contribution of Postotic Hindbrain (Vagal) Neural Crest Cells to the ENS

Surgical removal of premigratory neural crest cells from different levels of the neural axis of chick embryos and chick-quail chimera studies showed that the most important source of enteric neurons is “vagal” neural crest cells, which emigrate from the caudal hindbrain adjacent to somites 1–7. Vagal neural crest cells enter the foregut ( Fig. 11.1 ), and then migrate caudally within the gut mesenchyme ( Fig. 11.2 ). In mice, when vagal neural crest-derived cells enter the midgut, the midgut and hindgut are transiently closely apposed, and a subpopulation of cells leaves the midgut, takes a short-cut through the mesentery and enters the colon without passing through the caecum. These “transmesenteric” neural crest-derived cells are then the front-runners to colonize the colon.

In human embryos, it takes around 3 weeks from when vagal enteric neural crest-derived cells (ENCCs) enter the foregut until they reach the anal end of the gastrointestinal tract , and in mice and chicks it takes 5 days.

11.1.2

Contribution of Sacral Neural Crest Cells to the ENS

Sacral level neural crest cells have been shown to give rise to some enteric neurons in mice, chick and quail. In mice, around 12% of all neural crest-derived cells in the distal hindgut are of sacral origin, while in the chick, sacral cells give rise to around 17% of neurons.

Studies in chick have shown that following surgical ablation of vagal neural crest cells, sacral crest cells cannot compensate for their loss, probably due to lower levels of expression of the receptor tyrosine kinase, RET, by sacral crest cells. In both mouse and chick, sacral neural crest cells aggregrate in the connective tissue adjacent to the hindgut, but then undergo a 2–3 day waiting period before they enter the hindgut ( Fig. 11.2 B). Although sacral neural crest cells do not enter the hindgut until after the hindgut has been colonized by vagal neural crest-derived cells, sacral crest cells do not require the presence of vagal neural crest cells to enter the hindgut, and the delay in the entry of sacral cells is due to the expression of inhibitory cues in the hindgut mesenchyme (see Section 11.2.10 ).

11.1.3

Contribution of Schwann Cell Precursors to the ENS

Sensory, visceromotor, and sympathetic neurons project their nerve fibers to the gut after the arrival of vagal ENCCs. Neural crest-derived Schwann cell precursors (SCPs) migrate along these extrinsic nerves (mesenteric nerves) and invade the gut. SCPs are derived from neural crest cells at all axial levels and associate with all nerves that project to peripheral tissues. Most SCPs in the mesenteric nerves of rodents differentiate into non-myelin-forming Schwann cells, which form bundles by surrounding several axons, and ensheath each axon in a pocket of cytoplasm without forming compact myelin. In contrast, enteric glial cells display distinct morphological features: (1) process extension over nerve fibers, (2) a partial sheath to nerve fibers, (3) long laminar processes between nerve processes and wrapping multi-axonal bundles. A fate-tracing study of SCPs revealed that a subset of SCPs generate enteric neurons postnatally, and contribute to a small proportion (< 5%) of submucosal neurons in the small intestine and < 20% of both myenteric and submucosal neurons in the large intestine. Given that SCPs are derived from all neural axial levels, SCP-derived enteric neurogenesis suggests the contribution of trunk neural crest to enteric neurons.

11.1.4

Hirschsprung’s Disease

The embryonic gut grows considerably in length while it is being colonized by ENCCs. For example, in mice, the post-caecal gut increases in length about fivefold between when ENCC first enter the post-caecal gut and when ENCC reach the anal end of the gut at E14.5. ENCC therefore migrate further than any other neural crest cell population. Failure of ENCC to colonize the entire length of the gastrointestinal tract results in distal aganglionosis, which in humans is called Hirschsprung’s disease. Because the ENS is essential for propulsive gut motility, infants with Hirschsprung’s disease suffer from intractable constipation and develop a megacolon, which is treated by surgical removal of the affected bowel region.

11.1.5

Migratory Behavior of Enteric Neural Crest-Derived Cells

After emigrating from the hindbrain neural tube, vagal neural crest cells migrate through paraxial tissues to the foregut. Retinoic acid is produced by paraxial tissues and induces the vagal cells to migrate in chains within the gut mesenchyme. ENCC chains migrate in close association with the neurites of caudally projecting, early enteric neurons.

While some vagal ENCCs must advance caudally towards the distal hindgut, a subpopulation of ENCCs must remain behind in each location to ensure that there are neurons present along the entire bowel. It had been assumed that the ENCCs that get left behind in each gut region had ceased migrating, however, time-lapse imaging studies have shown that the cells that remain behind continue to migrate for at least 24 h, but they migrate circumferentially, rather than caudally.

Sacral neural crest cells enter the hindgut along the axons of extrinsic pelvic plexus neurons and then migrate rostrally within the gut. Schwann cell precursors also enter the gut along the axons of extrinsic neurons.