A study of modified lymphatics in the deep cortex of ruminant lymph nodes.

Ruminant lymph nodes, except when very small, were found to have a system of smooth-walled channels in the periphery of the 'deep cortical units' defined by Bélisle & Sainte-Marie (1981 a,b). Each channel originated with many 'blind' branches in the subnodular layer of the cortex and ended by joining a medullary sinus. The wall consisted of a continuous endothelial lining, a sometimes thin or discontinuous basement membrane without a basal lamina, and at least one layer of flattened reticular fibroblasts. The endothelium was higher than in most typical lymphatics, with a cytoplasmic fine structure similar to that of sinus-lining cells in the medullary sinuses. The intercellular junctions were generally long and elaborate. The lumen often contained opaque material, especially in the branches, as for initial lymphatics, as well as a few lymphocytes and an occasional nonlymphoid cell, but sinus macrophages were never seen. In some lymph nodes the lumen was crowded with lymphocytes. When small ferripolymaltose particles arrived in the node with the afferent lymph, many of them rapidly passed through the outer cortex and reached the lumen of the smooth-walled channels by way of the intercellular junctions of the endothelium. When colloidal carbon was introduced the same way, some of it also reached the channels where it accumulated in the basement membrane and in vesicles and vacuoles of the endothelium. These channels are interpreted as initial lymphatics of the same type as in other lymphoid organs rather than lymph node sinuses. They seem to play an important role for the exit of lymphocytes from the nodes and also for the passage of particulate material, including antigens, through those areas where recirculating lymphocytes arrive in the cortex.