Differential Muscle Cell Recruitments and Functions in Mouse Lymphatic Tissue Beds

The molecular mechanisms that regulate the lymphatic vessel maturation, particularly the recruitments of lymphatic muscle cells are completely unknown. We propose that muscle cell recruitment to different lymphatic beds is dependent upon its functional requirements. While mesenteric lymphatic vessels consistently stain strongly positive for LYVE1 and podoplanin, and weakly positive for CD31 in all ages of mice, muscle staining for neither vascular nor enteric actin is observed even in 4 month old mice. Iliac and thoracic duct lymphatics show actin positive cells on the walls of the vessels. In situ measurements of contractile activity of mesenteric lymphatics show phasic contractions are either absent or very limited. Thoracic duct exhibits comparatively strong basal tone and shows contractile responses to substance P. Iliac lymphatics exhibit some phasic contractions. The diastolic diameters for the iliac preparations were 245±6 μm; the systolic diameter was 225±5 μm (n=5). The contractile frequency was 19±3 per minute. Acetylcholine at 10 μM completely blocked the phasic contraction; the diastolic diameter increased to 280±6 μm. Our data suggest that the mouse mesenteric lymphatics lack muscle cells and therefore may serve merely as passive conduits for lymph flow without active influencing it, whereas, iliac and thoracic duct lymphatics might actively control lymph flow.