Scanning electron microscopic study of canine lymphatic vessels and their valves.

The morphology of canine thoracic duct, peripheral collecting lymphatics and their valves was determined using scanning and transmission electron microscopy (SEM and TEM). Information from surface and subsurface structures was derived simultaneously in the SEM by using a heavy metal stain combined with backscattered electron imaging. The peripheral lymphatics and thoracic duct were covered by a continuous sheet of endotheial cells, the nuclei of which were slighly raised, avoid in shape with coarsely granular chromatin and long axes oriented parallel to the direction of flow. The immediate subendothelial connective tissue in the thoracic duct was primarily smooth muscle, whereas in the peripheral lymphatics reticular and elastic fibres predominated. Valves were seen in both peripheral and central lymphatics. These were covered with endothelium similar to the collecting lymphatic vessels, were usually bicuspid and had cusps that inserted either at a single point or overlapped prior to a wide based insertion. A complex network of connective tissue and elastic fibres was seen in valves and in the vessel walls adjacent to the valves. The similarities and dissimilarities of the SEM appearances of lymphatics arteries and veins are discussed.