Effects of the adjuvant DEAE-dextran and Staphylococcus aureus on the lymph pathways and filtering capacity of popliteal lymph nodes in sheep.

The effects of subcutaneous injections of the adjuvant DEAE-dextran and, or killed Staphylococcus aureus on the structure of the lymph pathways of popliteal lymph nodes in sheep were examined using light and electron microscopy and Microfil casts. Dextran with or without killed S aureus caused significant changes in the lymph pathways both within the node and outside it. However, killed S aureus alone did not. The changes included anastomoses among afferent lymph vessels and between afferent and efferent lymph vessels; proliferation of vessels around the node; joining of parts of the capsule and trabeculae to adjacent parenchyma with loss of parts of the subcapsular and trabecular sinuses; enlargement of medullary sinuses; and reduction of the number of reticular processes in sinuses throughout the node. These changes were accompanied by a reduced ability of the node to filter chicken red blood cells labelled with chromium-51 which were injected into an afferent lymph vessel.

Fine structure of lymph pathways in nodes from the superficial inguinal lymph centre in the pig.

In the pig lymph node most lymph passes from afferent lymphatics to trabecular sinuses in centrally located dense nodular tissue. The lining of these sinuses is continuous adjacent to the trabecula but it is interrupted by numerous gaps adjacent to the parenchyma. Where the trabeculae end, their associated sinuses are continuous with the many interstitial spaces, up to 10 microns across, in the diffuse tissue. Lymph percolates through these spaces and is directly exposed to large numbers of macrophages with elaborate cytoplasmic veils and to reticular fibres which could be involved in antigen retention. Parts of the diffuse tissue are arranged into sinuses and cords in a manner similar to the medullary tissue in other species and a subcapsular sinus is also present over the diffuse tissue. There are gaps in the lining of these sinuses through which they communicate with the interstices of the parenchyma. Lymph flows from the sinuses in the diffuse tissue into efferent lymph vessels; these are usually in the capsule or along the plane of fusion of adjacent node anlagen.

Blood vessels within lymph nodes: a comparison between pigs and sheep.

In sheep, most blood vessels enter the lymph nodes at a discrete hilus, which is where the capsule overlies medullary tissue. In pigs, vessels within the node arise from an extensive network of arteries on the surface, and most vessels penetrate the capsule where it overlies diffuse tissue--the counterpart of the sheep medulla. In both species the blood vessels divide extensively as they approach and supply the dense lymphoid tissue which contains lymph nodules. The blood vessels within sheep lymph nodules are surrounded by a thick layer of amorphous material. The dense lymphoid tissue has an abundant network of venules which have smooth muscle cells in their walls. These venules are involved in lymphocyte migration and in pigs they are lined by high endothelium which is similar to that in most other species, while in sheep the endothelium is much lower. Fenestrated capillaries occur in both pig and sheep lymph nodes.

Pathways of lymph flow to and from the medulla of lymph nodes in sheep.

Medullary sinuses are continuous with penetrating afferent lymphatics, and with the trabecular and tubular sinuses which penetrate through the cortex. Tubular sinuses are often associated with blood vessels, especially in the deep cortex, and they appear to be important in the transport of lymphocytes. The subcapsular sinus is continuous over the cortex and the medulla, although trabeculae and reticular processes appear to restrict the flow of afferent lymph to the subcapsular sinus over the medulla. Lymph leaves the medulla through up to 100 or more initial efferent lymphatics, some only 60 micron across. Almost all of these arise from sinuses adjacent to the capsule lining the hilus. Some efferents remain associated with the capsule for a short distance whereas others, especially in nodes with a deep hilar depression, leave immediately at an angle of 30-90 degrees.

Afferent pathways of lymph flow within the popliteal node in sheep.

Lymph enters the popliteal node in sheep either at the subcapsular sinus, or through terminal afferent lymphatics which pass within trabeculae to medullary sinuses. Lymph from the subcapsular sinus traverses trabecular sinuses and/or a network of tubular sinuses in the cortex before entering the medullary sinuses, which are very extensive. Carbon particles, injected either into an afferent lymphatic or subcutaneously into the leg, were initially found mainly in a circumscribed area of the subcapsular sinus. Within a few hours they were present within macrophages in and around medullary sinuses in a band which extended around virtually the whole medulla. It was concluded that the pathways taken by lymph constituents in the sheep node may differ from those described for mice, rats and rabbits.

Blood vessels of lymph nodes in the pig.

Pig lymph nodes have an unusual arrangement of cortical and medullary tissue, and lack a hilus. The aim of the present study was to describe the pathways by which blood reaches these lymph nodes. Natural or synthetic latex casts were made of blood vessels of superficial inguinal, popliteal and jejunal lymph nodes. Major nodal arteries approach these lymph nodes and give rise to about five branches, most of which remain on the node surface. These branches often wrap around part or all of the node in a claw-like manner and they divide to form a network on its surface. Arteries from this network penetrate the capsule directly to supply the lymphoid parenchyma. Anastomoses are common at all levels between the arteries supplying the nodes and arteriovenous anastomoses occur within the nodes. These may help to maintain and regulate blood flow within the lymph node under different conditions of stimulation.