The timing of enteric neural crest cell colonisation of the chick embryo cloaca

Retinoic acid upregulates ret and induces chain migration and population expansion in vagal neural crest cells to colonise the embryonic gut

Vagal neural crest cells (VNCCs) arise in the hindbrain, and at (avian) embryonic day (E) 1.5 commence migration through paraxial tissues to reach the foregut as chains of cells 1–2 days later. They then colonise the rest of the gut in a rostrocaudal wave. The chains of migrating cells later resolve into the ganglia of the enteric nervous system. In organ culture, E4.5 VNCCs resident in the gut (termed enteric or ENCC) which have previously encountered vagal paraxial tissues, rapidly colonised aneural gut tissue in large numbers as chains of cells. Within the same timeframe, E1.5 VNCCs not previously exposed to paraxial tissues provided very few cells that entered the gut mesenchyme, and these never formed chains, despite their ability to migrate in paraxial tissue and in conventional cell culture. Exposing VNCCs in vitro to paraxial tissue normally encountered en route to the foregut conferred enteric migratory ability. VNCC after passage through paraxial tissue developed elements of retinoic acid signalling such as Retinoic Acid Binding Protein 1 expression. The paraxial tissue's ability to promote gut colonisation was reproduced by the addition of retinoic acid, or the synthetic retinoid Am80, to VNCCs (but not to trunk NCCs) in organ culture. The retinoic acid receptor antagonist CD 2665 strongly reduced enteric colonisation by E1.5 VNCC and E4.5 ENCCs, at a concentration suggesting RARα signalling. By FACS analysis, retinoic acid application to vagal neural tube and NCCs in vitro upregulated Ret; a Glial-derived-neurotrophic-factor receptor expressed by ENCCs which is necessary for normal enteric colonisation. This shows that early VNCC, although migratory, are incapable of migrating in appropriate chains in gut mesenchyme, but can be primed for this by retinoic acid. This is the first instance of the characteristic form of NCC migration, chain migration, being attributed to the application of a morphogen.

Cholinergic innervation in the developing chick cloaca and colorectum

PURPOSE

Acetylcholine is the major excitatory neurotransmitter involved in intestinal contraction and therefore plays a pivotal role in normal gastrointestinal motility. Acetylcholinesterase (AchE) is an enzyme involved in hydrolysing acetylcholine during its metabolism. The AchE histochemistry can therefore be used to label acetylcholine-positive nerve cells and fibres, giving an overview of cholinergic innervation in the gut. Persistent cloaca is the most common anorectal malformation seen in female infants. The normal avian embryo contains a cloaca, reminiscent to the human malformation, which acts as a collecting chamber into which the digestive, urinary, and reproductive tracts merge. The aim of our study was to investigate cholinergic innervation in the chick embryo cloaca and colorectum at various stages of development.

METHODS

Chick embryos were harvested at embryonic days 12 (E12), E14, E16, and E18. The colorectum and cloaca were removed from each embryo and frozen. Frozen transverse sections were then cut and AchE histochemistry was performed. The AchE staining was evaluated and graded using light microscopy.

RESULTS

Results of our study show that acetylcholine is increasingly expressed in the chick embryo cloaca and colorectum from E12 to E18, with a stronger expression in the colorectum at each stage. The AchE-positive ganglia and fibres were evident in the submucosal and myenteric plexuses at all stages in both the cloaca and the colorectum. Ganglia size was seen to increase with age, and there was a notable increase in the amount of AchE-positive nerve fibres.

CONCLUSION

Results show an increase in cholinergic innervation in both the cloaca and colorectum during development. As acetylcholine is the major excitatory neurotransmitter in the enteric nervous system, this study provides us with valuable information regarding the regional differences in the development of cholinergic innervation of the embryonic hindgut.