Peptidergic nerve terminals associated with the central lacteal lymphatics in the ileal villi of dogs.

Nerve fibers in the villi of the canine ileum were studied with special reference to their relation to the central lacteal. Immunohistochemically demonstrable nerve fibers containing substance P (SP) and calcitonin gene-related peptide (CGRP) were rather more numerous in the villi of the ileum than in those of the duodenum, as observed in our previous study (Ichikawa et al., 1991). They were distributed beneath the epithelium and associated with smooth muscle fibers. Besides these localizations, immunoreactive fibers were gathered, especially at the middle of the height of the villus, close to the endothelial cells of the central lacteal. This particular distribution of nerves was more evident in the ileum than in the duodenum. Electron microscope observation indicated beaded fibers containing large cored (peptidergic) and small clear vesicles coursing closely under the lacteal endothelium, partly intercalated by a basement membrane and partly in direct contact. The nerve fibers often penetrated the endothelial cell, being directly surrounded by its cytoplasm. Although the above-described findings essentially coincide with our previous observations in canine duodenum (Ichikawa et al., 1991), the present study in the ileum demonstrated occasional nerve fibers protruding into the lacteal lumen with a knob-like swelling. It is suggested that the SP and CGRP-containing nerves in problem might be sensory in nature, possibly monitoring mechanical information from the lumen and wall of the central lacteal. At the same time, these nerves are suggested to be secretory in nature, releasing the peptides to exert unknown effects upon the lacteal wall and its vicinity, presumably in response to luminal and mural stimuli.