Lymphocytes enter lymph nodes directly from the blood by migrating across the walls of specialized postcapillary venules which are located in the paracortex (T lymphocyte areas) of the node. These vessels have a characteristic morphology which distinguishes them from other types of blood vessel. The endothelial cells which line these vessels have a plump or cuboidal morphology, which contrasts to the flattened morphology of endothelial cells lining other types of vessel (Figure 1). It is the peculiar endothelial morphology that has engendered their common name - high endothelial venules (HEVs). The endothelial lining is surrounded by a thickened basement membrane, which is more pronounced than in other types of blood vessel. In immunocompetent adults, the localization of lymphocytes, but not other leukocytes, either attached to the inner (luminal) surface or within the vessel wall and the distinct morphological appearancc are the most recognizable features of HEV. However, these vessels are also readily distinguishable from other blood vessels by histochemical, biochemical and immunological markers. The lining endothelial cells, which will be referred to as high endothelial cells (HECs),
stain more intensely with RNA-binding dyes such as pyronin and contain a prominent Golgi apparatus and more secretory vesicles than other types of endo-thelia. Early histochemical studies also distinguished HEC from 'flat' endothelial cells lining other types of vessel. HEC express higher levels of nonspecific esterase, lactate dehydrogenase and NADH reductase. Biochemical studies identified a unique biosynthetic pathway for inorganic sulfate in HECs which is crucial for the generation of the peripheral addressin, an important adhesion molecule which mediates L-selectin-dependent binding of lymphocytes to HECs (see below). Several monoclonal antibodies have been described that specifically stain HEVs and not other types of blood vessel. These include MECA 79 and MECA 367, which identify the peripheral and mucosal addressins respectively, HECA 452, which identifies part of the functional carbohydrate epitope in the peripheral addressin, the sialyl Lewis* saccharide and MECA 325, for which the antigen has not yet been identified.
The actual height of the endothelial cells lining HEVs varies between species and according to the method of tissue collection; therefore these vessels cannot be defined solely by endothelial height. For example, endothelial cells (ECs) which line equivalent vessels in lymph nodes of sheep, rabbits and guinea pigs are not as 'high' as those in rodents. Additional markers such as the expression of the vascular addressins are therefore important to identify these specialized blood vessels. Collection of rodent lymph nodes under reduced arterial pressure results in a constriction of the vessel lumen and plumping up of the endothelium to 15-20 |xm in height. If the patency of HEVs is preserved by perfusion during fixation, then the endothelium is only 5-7 |j,m in height, although this is still greater than that lining
Figure 1 Distinct morphological appearance of HEVs. Transmission electron micrographs of an HEV in peripheral lymph node (left) and a capillary in the vasa vasorum of aorta (right) of the rat. Note numerous lymphocytes (Ly) trapped in the walls of the HEV and increased height of lymph node high endothelial cells (HEC) in comparison with capillary endothelial cells (EC).
non-HEV blood vessels, which is 1-2 |xm thick (Figure 1). The relationship between vessel patency and endothelial height suggest that HEVs are contractile vessels. Both the endothelial cell and the underlying basement membrane could contribute to HEV contractility. Dynamic changes in HEV morphology in vivo may enhance the trapping and passage of lymphocytes from the blood. The term HEV will be used to describe those postcapillary venules within lymph nodes which support the migration of naive lymphocytes from the blood irrespective of exact endothelial height.
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