The gastrointestinal tract is richly supplied with lymphatic vessels. Lymphatic vessels are lined by flattened endothelium with an incomplete basement membrane. Smooth muscle and connective tissue also surround the larger lymph vessels. The contractile activity of lymphatic vessels is most likely to be related to the amount of associated smooth muscle. The valves found in the larger vessels assist with the propulsion of lymph.
In the oesophagus, stomach and intestine, there is a plexus of lymph vessels present in the mucosal, submucosal and muscular layers with short vessels linking the networks together, and passing through lymph nodes which act as filters for lymph directed into larger vessels. The major lymphatic trunks are found on the left side of the body with the bulk of the lymph entering the circulation at the left jugulo-sub-clavian tap which is located at the base of the neck.
The lymphatic vessels of the small intestine are called the lacteals. The central lymphatic vessel is a blind-ending tube. The walls consist of a single layer of thin endothelium and resemble blood capillaries, however the small fenestrations seen in the blood vessel walls are not found in the lymphatics. The intestinal villi rhythmically contract and relax which probably serves to pump lymph into the lacteals of the submucosa. The flow of lymph in the thoracic duct is about 1-2 ml min-1 between meals but this can increase by 5 to 10 fold during absorption and digestion of a meal.
In addition to its main function of absorption, the gastrointestinal tract is a lymphoid organ. The lymphoid tissue is referred to as the gut-associated lymphoid tissue or GALT. The number of lymphocytes in the GALT is roughly equivalent to those in the spleen.
Lymphatic tissue can be seen in certain areas of the gastrointestinal tract close to epithelial surfaces, or as large aggregates e.g. pharyngeal tonsils and Peyer's patches in the ileum. Peyer's patches are lymphoid follicles located in the mucosa and extending into the submucosa of the small intestine, especially the ileum. Peyer's patches are usually situated on the ante-mesenteric border. Each patch typically comprises of 40 to 50 nodules which are separated from the gut lumen by a layer of epithelial cells, the M-cells or micro fold cells. There is a thin layer of vascularised connective tissue between the nodules and the serosa. The patches have their own blood supply. M-cells lack fully developed microvilli, are pinocytic and contain numerous vesicles. These cells may play an essential role in the intestinal immune response since they transport macromolecules and therefore have a specific role in antigen uptake. At this point in the intestine the mucosal barrier may be breached by pathogens. M cells do not digest proteins, but transport them into the underlying tissue, where they are taken up by macrophages. The macrophages which receive antigens from M cells present them to T cells in the GALT, leading ultimately to appearance of immunoglobulin A-secreting plasma cells in the mucosa. The secretory immunoglobulin A is transported through the epithelial cells into the lumen, where, for example, it interferes with adhesion and invasion of bacteria.
In adults, B lymphocytes predominate in Peyer's patches. Smaller lymphoid nodules can be found throughout the intestinal tract. Lamina propria lymphocytes are lymphocytes scattered in the lamina propria of the mucosa. A majority of these cells are IgA-secreting B cells. Intraepithelial lymphocytes are lymphocytes positioned in the basolateral spaces between luminal epithelial cells, beneath the tight junctions. These are inside the epithelium, not inside epithelial cells as the name may suggest.
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