Structural and functional properties of initial lymphatics in the rat tongue: scanning electron microscopic findings.

The luminal and outer wall morphology of the initial lymphatics in the rat tongue were demonstrated using scanning electron microscopy (SEM) after tissue perfusion with pressures up to 300 torr, topical heat and histamine administration. The findings emphasize the structural and functional importance of the reticular fiber network of the basement lamina (in contradistinction to single "anchoring filaments") as a supportive framework for lymphatic endothelium and a major regulatory role in tissue volume control. Increased tissue pressure was associated with dilatation of initial lymphatics which was maximal at approximately 60 torr. Higher pressures (up to 300 torr) did not damage the vessels. This vasodilatory response of the initial lymphatics was even more evident when tissue swelling was hindered by externally applied plaster "bandage". Under SEM, protruding and branched cells were conspicuous in otherwise flat lymphatic endothelium. These cells may have contractile properties and with pronounced dilatation, thermal injury and application of histamine these cells probably contract, thereby creating large gaps at the site of open junctions.