Lymphatic system of the rat pancreas.

Light and electron microscopy combined with morphometric analysis were used toinvestigate the distribution, extent and structure of lymphatic vessels in the head, bodyand tail of the rat pancreas. Serial sections 3-4 m in thickness were cut from tissue fixedby perfusion. Alternate sections were processed for light microscopy. Interveningsections were left uncovered to be re-embedded and sectioned for electron microscopy asneeded. Vessels with valves were tentatively identified as lymphatics using the lightmicroscope, with final identification being made on adjacent sections by electronmicroscopy. The ultrastructure of the pancreatic lymphatic vessels was typical oflymphatics generally. Interlobular lymphatic vessels were present throughout thepancreas and were found to be associated primarily with blood vessels lying inconnective tissue septa. Intralobular lymphatics were also seen but were comparativelyrare. Only about 19% of the wall of the lymphatic system of the pancreas was in closerelationship to acinar cells%none was closely related to the endocrine islets. The meanvolume density of the system was 0.0012 m3/ m3 and the profile density of lymphaticswas 3.24/mm2. Special attention was paid to the areas of contact between adjacentendothelial cells. Open gaps of more than 30 nm in width were rare. Dilatations andassociated cytoplasmic processes, suggestive of a type of intercellular transport, wereseen in addition to the intracellular cytoplasmic vesicular system. The findings areconsistent with the view that the lymphatic system of the pancreas does not have aspecific role in the transport of pancreatic secretions other than the removal ofmacromolecules that may escape to the interstitium in small amounts under normalcircumstances. The fine structure of the endothelial wall suggests that the mechanism oflymph formation in the pancreas is more comparable to that in other encapsulatedorgans such as the kidney and liver than to that in the dermis or diaphragm where fluidappears to enter lymphatics primarily by way of gaps between adjacent cells.