Distribution of peptide-immunoreactive nerves in the foetal and newborn guinea-pig caecum

The distribution of vasoactive intestinal polypeptide-, substance P-, [met]enkephalin- and somatostatin-like immunoreactive nerves was studied in the caecum from foetal guinea-pigs of 6-9 weeks gestation (i.e., approximately 1-4 weeks before birth) and 4-5-day-old guinea-pigs. Peptide-immunoreactive nerves were first detected in the myenteric and submucous plexuses and circular muscle layer at 6 weeks of gestation and in the mucosa at 7 weeks of gestation. The density of fibres in these layers increased during prenatal development until, by 9 weeks of gestation, their distribution resembled that seen in the postnatal animals. This distribution was similar to that described previously in adult animals. A different pattern of development was observed in the caecal taenia coli muscle. Peptide-immunoreactive fibres were not detected until 8 weeks of gestation in this tissue layer, and were then only sparsely distributed. A dramatic increase in the number of labelled fibres, however, occurred between 8 and 9 weeks of gestation. Further, vasoactive intestinal polypeptide- and substance P-immunoreactive fibres were more numerous in the taeniae coli of 9-week-old embryos than in those of postnatal animals. Thus, the guinea-pig enteric nervous system, which in many respects is well-developed at the time of birth, may still be undergoing developmental changes at this time.