The narrowing of high endothelial venules of the rat lymph node.

The lymph node contains blood vessels of a special type, termed "high endothelial venules" (HEVs), which are involved in the process of lymphocyte recirculation. In standard tissue sections, many HEVs exhibit a nearly closed or closed lumen containing small lymphocytes but few, if any, erythrocytes. The question arose as to whether the appearance of HEVs in tissue sections is influenced by the routine method of animal sacrifice and/or of tissue processing. Therefore, the present work investigated the effects on HEVs of sacrificing rats as well as of excising and fixing their nodes with various procedures. It was observed that procedures involving animal bleeding or blood loss from nodes increase the percentage of HEVs exhibiting a nearly closed or closed lumen. The results further revealed that the endothelial thickness and other morphological features of HEVs are modified by this artifactual narrowing of HEVs. The possible significance of the phenomenon is discussed.