Fine structure of the lymphatic capillary and the adjoining connective tissue area.

The lymphatic capillaries possess salient morphological features which differentiate them from the blood capillaries. Instead of a definitive basement lamina surrounding the lymphatic wall, there are numerous fine filaments (about 60 A in diameter) that are attached to the lymphatic endothelia at areas of increased electron density that are similar to half desmosomes. The filaments extend for varying vessel to the surrounding connective tissue. Endothelial projections occur along both the luminal and abluminal surfaces of the cell. The intercellular junctions are varied in structure along the lymphatic wall. Terminal processes of adjacent endothelial apposed without overlapping. At various points along the intercellular junction the apposed surface membranes are held in close apposition by adhesion devices, (a) zonula occludens, (b) zonula adhaerens, and (c) macula adhaerens (desmosome). Frequently open junctions are observed along the lymphatic wall. It is suggested that the areas between apposing membrane surfaces which lack attachment devices may represent specialized regions along the lymphatic wall that are easily separated and would permit the passage of excess tissue fluids and large molecules into the lumen of the lymphatic vessel. The transport of colloidal carbon particles across the endothelium in vesicles of various sizes and via intercellular junctions is demonstrated.