This is a very low power view showing sections through three lymph nodes and some large blood vessels (BV) in the area between the hila (singular-hilum; H) of the nodes.Slide #DMS 120 [Lymph gland, monkey, H&E] A lymph node ("gland") is an encapsulated lymphoid organ. This slide has longitudinal sections of one or more nodes and is good for general histological features. Identify afferent and efferent lymph vessels (some with valves), capsule, subcapsular sinus (and the abundant macrophages therein), cortex with lymphoid nodules and medulla with medullary cords and sinuses. Depending on the plane of section, you may or may not be able to find the hilum. Note that the cortex is divided into a superficial (containing nodules) and a deep portion (paracortex). Which portions are "B cell-dependent," and which are "T cell-dependent" ? Note that lymph nodes are well supplied with both blood and lymph vessels. Look in the paracortex for high endothelial venules (HEV), the site where most circulating lymphocytes enter the node. If a small group of cells breaks off from a malignant tumor and enters a lymphatic vessel it will be filtered out by a lymph node. In what part of the node will it most likely lodge?
This is a very low power view showing sections through three lymph nodes and some large blood vessels (BV) in the area between the hila (singular-hilum; H) of the nodes.
At low power, observe that a capsule surrounds the lymph node, and that the node can be divided into a cortex and a medulla, the large lymphatic channels of the later leaving the node at its hilum.
>At somewhat higher power, again note the capsule of the lymph node and some afferent lymphatic vessels traversing it. The cortex may be divided into an outer cortex in which are found B-cell-dependent lymphoid follicles (nodules), and then an ill-defined inner cortex (paracortex) which is a T-cell dependent zone. The lymphoid follicles seen here are secondary follicles, distinguished from primary follicles by the presence of a pale-staining central region called the germinal center.
At still higher power, note the afferent lymphatic vessels penetrating the capsule of the node. The lymph entering the node through these vessels will subsequently flow into the subcapsular sinus and thence into the interstices of the lymphoid tissue of the node.
This high power view shows the subcapsular sinus in detail. Though difficult to appreciate, this channel, like all the lymphatic channels in the node, are lined by a layer of endothelial cells, albeit a leaky, sometimes discontinuous one. The meshwork of macrophages and reticular cells in the subcapsular sinus slow the flow of lymph (or lymph-borne metastatic cells) through this region, permitting opportunities for the entrapment of foreign material.
Returning to a lower power for orientation, note again the ill-defined inner- or paracortex found between the follicle-laden outer cortex and the medulla of the lymph node.
The paracortex is a T-cell dependent zone, though in the absence of specific immunochemical labeling, one cannot differentiate a T-cell from a B-cell. One feature of the paracortical region of the lymph node are the high endothelial venules (HEVs), one of which may be seen here and at higher power in the next image.
At high power, one can see the distinctive structure of a high endothelial venule (HEV). Unlike most blood vessels that are lined by a simple squamous endothelium, the HEV has almost cuboid endothelial cells. The HEVs represent an important site of lymphocyte entry into the lymph node and the site at which blood-borne lymphocytes can re-enter the lymphatic circulation.
Returning to low power, one can see the typical architecture of the medullary region of the lymph node. Here, aggregates of lymphoid tissue, the medullary cords, are found amongst dilated lymphatic channels, the medullary sinuses.
At high power, one can see portions of medullary cords bordering a medullary sinus. Cellular processes of supporting reticular cells span the spaces of the medullary and other sinuses in the node. B-cells, macrophages, and plasma cells may all be found in the medullary cords and surrounding tissues.
This is a higher power view of the hilum of the lymph node, the site where blood vessels enter and exit the node and also where the medullary sinuses coalesce into large efferent lymphatic vessels that then exit the node. One may distinguish an arteriole and blood-filled venule from the large, lymph-filled efferent lymphatic vessels. Valves are abundant in the lymphatic channels and may help to distinguish these vessels.