Structural Aspects of the Lymphocyte Traffic in Rat Submandibular Lymph Node

Abstract

Modulation of lymphocyte flow in the lymphatic compartment of the lymph node may serve, in part, to promote lymphocyte sensitization during an antigenic challenge. This study was undertaken to show where this might occur by examining the structural relationships of the intranodal lymphatic pathways, blood vessels, and connective tissue support with respect to lymphocyte and lymph flow. Differently stained plastic resins were injected into the blood vessels and lymphatics of the submandibular lymph node and visualized with a confocal laser scanning microscope. The specimens were corroded to study the three-dimensional cast structures by scanning electron microscopy. Alkali digestion was also used to prepare the reticular fiber network in the lymph node for scanning electron microscopic examination. At the hilus of the node, two to three arteries gave off arterioles running in medullary cords towards the cortex. The medullary cords, the periphery of the deep cortex, and the perifollicular zones had dense capillary networks. In contrast, the center of the follicle and the center of the deep cortex were less highly vascularized. High-endothelial venules were restricted to the perifollicular zone and the periphery of the deep cortex. At the cortico-medullary boundary, they abruptly transformed into medullary venules with a normal endothelium. The marginal sinus of the lymph node was crossed by thick reticular fibers that arose from the inner sheets of the capsule. The lymph pathway went through the marginal sinus, into the trabecular sinus, to the cortical perifollicular sinus, the dense lymphatic sinus around the deep cortex, and finally into the medullary sinus. At present, the exact functional significance of the complex lymph node lymphatic architecture is not clear. However, the highly organized structural organization may play a significant role in regulating and directing lymphocyte flow to facilitate antigen presentation.