THE TUNICA MEDIA OF THE THORACIC-DUCT CONTAINS A HETEROGENEOUS POPULATION OF MUSCLE-CELLS

The three-dimensional cyto-architecture and ultrastructure of cells co mposing the different regions of the tunica media of canine and feline thoracic ducts (ThD) were investigated by scanning and transmission e lectron microscopy. The tunica media of the intervalvular regions cons ists of 2-4 layers of circularly oriented ribbon-like smooth muscle ce lls (SMCs) of the contractile phenotype. Spot-like membrane contacts s imilar to gap junctions were observed between SMCs. In the parietal pa rt of the valve's sinus (PPVS), modified SMCs containing large lipid d roplets were found. Occasionally, in the upper thoracic part of the Th D in the PPVS, we observed cardiomyocyte-like cells (CMLCs) arranged a s circularly oriented bundles. Frequent interdigitation contacts were formed between SMCs and CMLCs, suggesting a possible involvement of th e latter cell type in pacemaker activity of the ThD.