An unusual structure of venules in tonsils of the soft palate of young pigs.

In tonsils of the soft palate of pigs high endothelial venules (HEV) occur throughout the parafollicular region. In some regions of the venules the endothelial lining is uniformly high; in other areas, mainly the nuclear region of the endothelial cell protrudes into the lumen and the cytoplasmic region is attenuated. In this portion of the venule endothelial cells may traverse the lumen of the vessel to form intravascular bridging processes. The endothelial cells are characterised by pinocytotic vesicles, many surface microvilli, cytoplasmic processes and local cell-cell contacts with adjacent endothelial cells. These specialised HEV may play a role in the unusual pattern of lymphocytes from tonsillar tissue into blood. In the tonsils of the soft palate of pigs, HEV may mediate both lymphocyte migration to and from the lymphoid tissue. Further investigation is required to determine the importance of these 2 migration pathways.