The fine structure of the lymphatic capillary and the surrounding tissue areas was investigated. Instead of a continuous basal lamina (basement membrane) surrounding the capillary wall, these observations revealed the occurrence of numerous fine filaments that insert on the outer leaflet of the trilaminar unit membrane of the lymphatic endothelium. These filaments appear as individual units, or they are aggregated into bundles that are disposed parallel to the long axis of the lymphatic capillary wall and extend for long distances into the adjoining connective tissue area among the collagen fibers and connective tissue cells. The filaments measure about 100 A in width and have a hollow profile. They exhibit an irregular beaded pattern along their long axis and are densely stained with uranyl and lead. These filaments are similar to the microfibrils of the extracellular space and the filaments observed in the peripheral mantle of the elastic fibers. Infrequently, connections between these various elements are observed, suggesting that the lymphatic anchoring filaments may also contribute to the filamentous units of the extracellular space. It is suggested that these lymphatic anchoring filaments connect the small lymphatics to the surrounding tissues and represent the binding mechansim that is responsible for maintaining the firm attachment of the lymphatic capillary wall to the adjoining collagen fibers and cells of the connective tissue area.

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