Histochemical visualization of lymphatic capillaries in the rat: a comparison of methods demonstrated at the posterior pharyngeal surface

A sufficient differentiation of lymphatic capillaries from blood capillaries in conventional light microscopy still eludes researches. The endothelium and media of lymphatic capillaries are characterized by a strong 5'-nucleotidase activity, whereas blood capillaries reveal no or significantly lower activity. Alkaline phosphatase activity, on the other hand, missing in the lymphatic capillaries is positive in most of the blood capillaries. For the histochemical visualization of the entire blood capillary bed, dipeptidyl peptidase IV-activity has to be used together with alkaline phosphatase. Various fixation and detection methods of 5'-nucleotidase are compared. In order to demonstrate 5'-nucleotidase activity, a method modified after Heusermann (1979) is considered to be most suitable. The results obtained are discussed with regard to their significance concerning the visualization of lymphatic capillaries. They are compared with a series of investigations in which alkaline phosphatase and dipeptidyl peptidase IV-activity are visualized in blood capillaries additional to the 5'-nucleotidase reaction. Various color reactions reveal a differentiation between blood capillaries and small lymphatics. The isolated visualization of 5'-nucleotidase activity with a simultaneous inhibition of alkaline phosphatase with L-tetramisole is considered to be the best way to histochemically demonstrate lymphatic capillaries. It was shown for the first time that only in the presence of L-tetramisole can small lymphatics be adequately visualized. A satisfactory differentiation between blood and lymphatic capillaries succeeded by means of a different color intensity of the reaction product.