Intrinsic innervation patterns of the smooth muscle in the rumen and reticulum of lambs.

The rumen and reticulum of sheep serve as a fermentation chamber. Both compartments exhibit specific motility patterns. With developmental changes, the size of the reticulorumen dramatically increases when newborn lambs mature to adult sheep. This makes it possible to investigate the intrinsic innervation of the reticuloruminal muscles in lambs by taking the entire reticulum and rumen into account. The aim of the study was to analyse the projections and neurochemistry of myenteric neurons in the rumen and reticulum, which project to the inner or outer muscle layer, respectively. Therefore, we applied retrograde tracing with the fluorescent dye 1,1'-didodecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) and subsequent immunohistochemical detection of choline acetyltransferase (ChAT), substance P (SP) and vasoactive intestinal peptide (VIP). In both compartments innervation of both the inner and the outer muscle layer consisted mainly of cholinergic neurons (65-70%). The majority of them co-localized SP. The non-cholinergic neurons projecting to the muscle expressed immunoreactivity for VIP. Polarized innervation of the muscle layers was found neither in the rumen nor in the reticulum. Consequently, intrinsic innervation patterns for the smooth muscle layers in the rumen and reticulum differ from all gastrointestinal regions examined thus far.