VIP-immunoreactive nerve structures of the gastrointestinal tract in the developing and adult domestic duck.

The first appearance and distribution of VIP-containing nerve structures were studied in the gut of embryonal, newly hatched and adult domestic ducks. Immunoreactive structures appeared in the mesenchyme of the gizzard-duodenum junction (antrum) at 5 days of incubation, and soon after diffused both cranially and caudally. Early in development, VIP-positive cell bodies were more numerous than positive fibres, appearing 3 days before the latter. Later on, the positive cell bodies decreased in number while the fibres increased. In the foregut, VIP-positive structures firstly appeared in the myenteric plexus while in the mid- and hindgut they appeared simultaneously in the myenteric and submucous-plexuses. The gastrointestinal VIP-innervation in newly hatched ducks differed from that of the adult because of the incomplete development of its mucosal component. The findings obtained indicated following: The time interval from 13 to 17 days of incubation seems critical for the growth of the gastrointestinal VIP-containing neurons. The molecular factors regulating the appearance of neuronal phenotypes are probably synthesized early in the avian gizzard development. VIP is widely diffused in the rectum where it may play important roles in regulating the onset and synchronism of peristaltic and anti-peristaltic movements.