Immunohistochemical demonstration of peptidergic nerve fibers associated with the central lacteal lymphatics in the duodenal villi of dogs.

Immunohistochemical demonstration was made of the peptidergic nerves distributed in the central lacteal lymphatics of the canine duodenal villi. The central lacteal-associating nerve fibers were predominantly immunoreactive for both substance P (SP) and calcitonin gene-related peptide (CGRP). Observation of doubly immunostained sections evidenced that both peptides were located in one and the same nerve fibers. The SP/CGRP-immunoreactive fibers were concentrated in the intermediate portion of the villus height. Ultrastructurally, the SP/CGRP-immunoreactive nerve fibers ran closely beneath the endothelial cells of the lacteal, some of them penetrating into the cytoplasm with knob-like swellings. The immunoreactive products were localized to large-cored vesicles in the sub- and intra-endothelial nerves. The occurrence of SP and CGRP in the nerve fibers distributed in the central lacteals which lack smooth muscles implies that these nerves may be sensory in nature. A mechanoreceptive function of the nerves is proposed on the basis of their peculiar knob-like projection into the lacteal endothelium.