Distribution of serotonin-immunoreactive gut endocrine cells in chicks at hatching. Examination of possible co-localisation with peptides reveals unexpected cross-reactivity of substance P antiserum with serotonin.

Serotonin-immunoreactive, i.e. enterochromaffin (EC) cells were found to be widely distributed in the intestine of the newly hatched chick but sparse in the stomach, and being particularly abundant in the duodenum, upper ileum and rectum. Although in birds, as in mammals, EC cells are most abundant in the intestine, in the stomach they are far sparser than in mammals. Comparison of adjacent sections immunostained for serotonin and a peptide provided no evidence that EC cells in the hatching chick contain motilin or substance P, and that at least the great majority of bombesin-immunoreactive cells contain no serotonin: it is apparent that the mammalian pattern of distribution of peptides in EC cells does not occur in the chick, at least at hatching. Cross reaction of an antiserum to substance P with serotonin was discovered, suggesting the need for a review of existing evidence for co-localisation of this peptide with serotonin.