SUBENDOTHELIAL NERVE FIBERS IN BOVINE MESENTERIC LYMPHATICS: AN ULTRASTRUCTURAL AND IMMUNomSTOCHEMICAL STUDY

Abstract

In the lymphatic vessels ofman and mostanimals the nerve fibers are confined to theadventitia. However, immunohistochemicalstudies suggest that acetylcholinesterasepositiveand monoamine-containing fibersreach as far as the endothelium in bovines.The aim of this study was to verify the presenceofsubendothelial nerve fibers by transmissionelectron microscopy (TEM) in bovine mesentericlymphatics and to determine whethertypical sensory neurotransmitters such asSubstance P (SP) and calcitonin gene relatedpeptide (CGRP) could be detected in thesefibers. TEM revealed numerous unmyelinatednerve fibers in the subendothelial connectiveenvironment in close association withendothelial cells. Their axons were devoid ofSchwann cell sheath on the endothelial sideand contained small clear vesicles and largevesicles with a dense core. Subendothelialnerve fibers were demonstrated to be SP andCGRP-immunoreactive with mousemonoclonal antibodies against SP and rabbitpolyclonal antibodies against CGRP. It ishypothesized that these fibers act as mechanoceptorscapable ofdetecting intraluminalpressure and vessel wall tension variations andoflocally releasing SP and CGRP. Since SP,potentiated by CGRP, is known to be avasoconstrictor in lymphatics, we propose thatthe contraction of bovine mesentericlymphatics may also be neurogenic.