Development of the adrenergic innervation and chromaffin cells in the human fetal gut

Abstract

Developing adrenergic nerves have been examined in human fetal (5–19 weeks gestational age) gut by fluorescence histochemistry. No specific fluorescence was detected within the gut before 9–10 weeks, although fluorescent nerves were always observed in the mesentery. After incubation of the intestine with norepinephrine, however, fluorescent nerves were detectable in Auerbach's plexus at 8 weeks, indicating that developing adrenergic nerves can take up norepinephrine before their endogenous transmitter stores are demonstrable histochemically. The first fluorescent nerves in the gut (in Auerbach's plexus near the mesenteric border) are partly varicose and some are already arranged about intramural (nonfluorescent) ganglion cells in a manner suggestive of the pericellular networks involved in modulation of reflex activity in adult gut. Their appearance is discussed in relation to the onset of motility. In later fetuses fluorescent nerves are distributed more evenly around the gut perimeter, are more clearly varicose, and their pericellular networks are more extensive. In the inner muscle coat of the stomach of fetuses of 13 weeks or more, fluorescent varicose nerve fibers run close to the muscle bundles, indicative of direct adrenergic innervation. Elsewhere all fluorescent nerves in the gut muscle are associated with blood vessels. In the submucosa fluorescent nerves first occur in perivascular plexuses at 10 weeks. Pericellular networks about the submucosal ganglion cells appear at 13 weeks but remain sparse. The few fluorescent nerves in the mucosa of the oldest fetuses are probably associated with blood vessels in the lamina propria. The distribution of enterochromaffin and small intensely fluorescent cells in the gut has also been described.