Ultrastructural Dynamics of Transendothelial Migration of Lymphocytes Through High Endothelial Venules (HEVs) of the Mucosa Associated Peyer's Patches

Background and Purpose: Although the pivotal role of high endothelial venules (HEVs) in the migration of leukocytes from the blood into the secondary lymphoid parenchyma is well established, conflicting ideas concerning the cellular dynamics both of leukocytes and endothelial cells throughout the migratory processes have been present. Here we focused specifically on the cellular dynamics of HEVs from Peyer's patches in an ultrastructural perspective. Materials and Methods: In order to determine the microstructural organization of transendothelial migration we used conventional methods for transmission electron microscopy. Results: Our results indicate that both lymphocytes and endothelial cells are highly active in the processes of transmigration steps, and a series of morphological and cellular alterations can occur depending upon their activity. Various types of cellular protrusions provide a direct contact between luminal lymphocytes and the endothelial cells at the initial phases of the migration. The endothelial protrusions subsequently embrace the lymphocytes and guide them into lymphoid stroma during the transcellular migration. Meanwhile, different sizes of vesicles show different cellular localizations according to their roles. The vesicles which are clustered near the lateral borders and the stand alone ones found only in the abluminal surfaces of endothelial cells might be involved in the paracellular migration. Concurrently, the other types of vesicles were smaller and appeared in the lateral border of the endothelial cells. Differ from the clustered and abluminally localized vesicles, they were closely related with plasma membranes. Conclusions: These results indicated that not only adhesion molecules, but also cellular dynamics of leukocytes and endothelial cells regulate the leukocyte traffic into lymphoid stroma or vice versa.