Lymphatic system consists of blind-ended lymphatic capillaries, lymphatic vessels, lymph nodes, lymphatic tissues, and red bone marrow. ∼1-4 L of lymph is formed in a day, most of it derived from the gastrointestinal tract, especially after a fat-containing meal (Alexander et al., 2010). The liver is another large producer of lymph. Although physiologically very important, lymph flow rates are very low compared to blood perfusion. Lymph nodes accommodate white blood cells and filter foreign bodies from the lymph fluid. T- and B-cells interact with dendritic cells in distinct regions, facilitating adaptive immunity. Collecting lymphatic vessels have valves which ensure that lymph flows unidirectionally Lymph is collected to large lymphatic trunks, including thoracic duct and right lymphatic duct, that lead the lymph into subclavian veins.
Lymphatic capillaries consist of overlapping endothelial cells, and lack pericytes and smooth muscle cells. Basement membrane is incomplete or lacking. Lymph capillaries can still expand with the help of anchoring filaments, fibrillar structures that connect the abluminal surface of endothelial cells to the extracellular matrix. Endothelial junctions in lymphatic capillaries serve as a microvalve which allows the interstitial fluid to flow into the lumen but reduces the leakage from lumen back into the interstitium. Collecting lymphatic vessels are surrounded by smooth muscle cells, pumping lymph against pressure gradient, with contraction rate of 1-15 cycles/min. Intraluminal valves ensure unidirectional lymphatics movement in lymphatic vessels towards the lymph nodes and further into large lymphatic trunks. Lymphangion is the contractile unit, consisting of the lymphatic muscular segment bounded by two valves. Lymphatic pumping is increased by cholecystokinin (CCK), glucagon, endothelin, substance P, serotonin, dopamine, histamine, and ATP; pumping is reduced by vasopressin, vasoactive intestinal peptide (VIP), acetylcholine, and ROS (Alexander et al., 2010). Lymph nodes in the lumbar and intestinal trunk drain into cisterna chyli, from where lymph is transported into the ∼40 cm long thoracic duct.
Lymphangiogenesis is regulated by VEGF-C and VEGF-D, and indirectly by NO. MMP-9 participates in lymphatic regeneration. Inflammation and obesity induces lymphangiogenesis. Existing lymphatic networks can be expanded, venous structures can be reprogrammed, or lymphatic progenitor cells are recruited. TGF-β inhibits lymphangiogenesis (Alexander et al., 2010).
White blood cells, which have migrated from blood into interstitial space, can return to the circulation via lymphatic vessels and lymph nodes. Lymphatic endothelial cells express chemokines and adhesion molecules, and can direct the leukocytes into lymphatics or into interstitial space.
The spleen is a reticuloendothelial lymphoid organ specialized in filtering blood and producing components of complement and specific antibodies. About 25% of white blood cells and 30% of platelets in the body are located or stored in the spleen.
Chronic venous disease (CVeD) is very common, especially in the lower limbs. Valvular incompetence leads to reflux, venous hypertension, increased capillary pressure, and further to accumulation of fluid within interstitial space (oedema). Lymphatic drainage fails in the advanced stages of the disease. Increased vessel permeability leads also to extravasation of white blood cells and plasma proteins into the tissue.
Cancer cells can travel via lymphatic system to distant sites. Tumour growth can obstruct lymph flow, causing lymph reflux and lymphatic-venous shunting, which may promote metastasis formation. Tumours may release lymphangiogenic factors.
Lymphatic vessels and lymph nodes can be visualized with PET after injecting [18F]AIF-NEB into the tissue; the radioligand complexes with endogenous albumin in the interstitial fluid (Wang et al., 2015). Local injection allows the detection of sentinel lymph nodes.
Lymphatic system contains white blood cells in large numbers. Mannose receptor (CD206) is expressed on the surface of macrophages, playing a role in both innate and adaptive immune systems. Mannose receptor has been targeted by mannose-conjugated albumin, dextran, and liposomes labeled with 68Ga and 64Cu; these tracers may be useful in detecting macrophages, and have been used to localize for example lymph nodes (Choi et al, 2011; Locke et al, 2012; Eo et al, 2015; Kang et al, 2015; Lee et al., 2017). [99mTc]tilmanocept is a mannocylated dextran with FDA approval for lymphatic mapping. Li et al (2016) developed 18F-labeled mannan, and found high uptake in macrophage-rich organs in healthy rats.
Rapidly proliferating cells, including those in lymphoid organs, rely less on de novo synthesis of DNA building blocks and more on salvage pathway for DNA synthesis where deoxyribonucleosides are converted to nucleotides by deoxyribonucleoside kinases. 1-(2’-deoxy-2’-[18F]fluoroarabinofuranosyl)cytosine ([18F]FAC) has been found to be suitable radioligand for activity of the deoxyribonucleotide salvage pathway, and for imaging lymphoid organs and immune activation (Radu et al., 2008; Brewer et al., 2010).
Mononuclear phagocyte system (MPS), previously called reticuloendothelial system, is part of the immune system, consisting of phagocytic macrophages and monocytes of myeloid lineage, located in reticular connective tissue. MPS consists of macrophages in the red pulp of the spleen, Kupffer’s cells in the liver, osteoclasts in bone and bone marrow, monocytes in the blood and bone marrow, sinus histiocytes in lymph nodes, alveolar macrophages in the lungs, Langerhans cells in the skin macrophages in adipose tissue and peritoneal cavity, intraglomerular mesangial cells in the kidneys, and microglia in the central nervous system. MPS makes and destructs leukocytes and erythrocytes and plasma proteins.
SPECT imaging of MPS (or reticuloendothelial system) is based on intravenous injection of 99mTc-labelled colloids, which are phagocytosed by macrophages of the MPS. Normally about 90% of the colloid accumulates in the liver, and ∼5% in the spleen and another 5% in the bone marrow (Desai & Thakur, 1985). In acute and chronic hepatic inflammation, colloid phagocytosis in the liver is reduced, even to ∼30-35% (Krishnamurthy & Krishnamurthy, 2009).
- Glymphatic system
- Vascular system
- Vascular volume fraction
- Input function
- Perfusion (blood flow)
- Inflammation and infection
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Updated at: 2019-02-07
Created at: 2016-05-15
Written by: Vesa Oikonen