The lymphatic vessels develop secondary to the blood vessels either phylogenically or ontogenically. Amphibians and reptiles posses enormous lymph sacs or sinuses encircling the aorta and almost all other arterial and venous blood vessels in order to store water and prevent loss of heat through use of solar radiation. The lymphatics of birds and mammals, animals which can intrinsically maintain a constant body temperature, become abruptly narrow and tubular, and tend to run independently from the blood vessels. Communications between the blood and lymphatic vessels are also decreased. Ontogenically, the main lymphatic vessels and ducts of mammals arise directly by sprouting of venous endothelial cells on both sides of two junctions between the cervical and subclavian veins and between the inferior vena cava and the renal vein. The openings at the latter communication site remain very rarely as lymphatico-venous anastomoses.
The lymphatic vessels are not distributed in all tissues and organs. Neither blood vessels nor lymphatic vessels are present in the epithelium, sclera, cartilage or intima of blood vessels. In bone and muscle, blood vessels are present but lymphatic veessels are absent. The central and peripheral nervous systems are rich in blood vessels but also do not contain internal lymphatics. The lymphatic capillaries are usually distributed at a distance from the blood capillaries in order to absorb tissue fluid that filters out from the latter, and form a fine network. The neighboring endothelial cells of lymphatic capillaries are separated by a wide gap to facilitate the passage of erythrocytes during periods of vigorous absorption. After closure of the gaps, the lymphatic capillaries continue to absorb tissue fluid through intracellular pinocytotic vesicles. Complete or almost complete absence of a basement membrane makes such absorption easier. Furthermore, pericytes are absent. Anchoring filaments prevent the collapse of lymphatic capillaries.
The collecting lymphatics and lymphatic ducts possess various amounts of smooth muscle fibers in rich connective tissue, contracting intrinsically. However, the major mode of lymph propulsion is passive movement, such as that occurring during massage or respiration. The presence of many valves is important for regulation of the lymph flow.
An extravascular reticular network leads tissue fluid toward the lymphatic capillaries. This kind of prelymphovascular passage is present, at sites of absorption from the peritoneal, pericardial and pleural cavities. These prelymphovascular extravascular passages are also present at sites where cerebrospinal fluid escapes outward and enters the lymphatic capillaries outside the central and peripheral nervous systems. The network of fine reticular fibers in lymphoid tissues plays an important role not only in conducting lymph-born antigens to immunocompetent cells, but also in determining the direction of cell migration.
As well as the absorption of tissue fluid, the lymphatics serve for the transport of cells. About 95-99% of cells in the thoracic duct lymph are lymphocytes, of which 80-85% are T lymphocytes and 10-20% are B lymphocytes. The output of thoracic duct lymphocytes, which is controlled to a large extent by the adrenal glands, is 2-5 times the total number of blood lymphocytes. In other words, blood lymphocytes are replaced 2-5 times per day by thoracic duct lymphocytes. About 70% of lymphocytes in the thoracic duct lymph are recirculating lymphocytes. B lymphocytes are also present among recirculating lymphocytes. T lymphocytes produced in the thymus and B lymphocytes produced in the bursa of Fabricius are discharged via the lymphatic routes into the general circulation. Macrophages or monocytes are present in considerable numbers (5-20%) in the peripheral lymph, but disappear after passing through the regional lymph nodes. They preferentially enter the germinal centers of lymph nodes, suggesting the possibility o

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In order to discriminate rat lymphatics from blood vessels on tissue cryosections by immunohistochemistry, a mouse monoclonal antibody (B27) was produced by immunization of mice with a homogenate of rat thoracic duct. B27 strongly recognized endothelial cells of almost all rat lymphatics, such as lacteals, lymphatic capillaries in the diaphragm at sites of absorption from the peritoneal cavity, collecting lymphatics and the thoracic duct. Besides the lymphatics, B27 reacted with the endothelium of some types of blood vessel, the mesothelium of the peritoneal cavity and substances between smooth muscle fibers. A new double immunostaining technique was then developed to distinguish the lymphatic capillaries, having no or only scanty basement membranes more clearly from the blood vessels. Cryosections were stained first with an anti-type IV collagen antibody for basement membranes, secondly with B27 for the endothelium, and then examined by either light microscopy or fluorescence microscopy. With this technique, the lymphatic capillaries were easily distinguished from other vessels by their positive reaction with B27 alone. B27 appears to be very useful for the simple and reliable identification of rat lymphatics, particularly lymphatic capillaries, in various tissues when applied for double immunostaining.

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Intralobular lymphatic vessels in the mouse thymus were demonstrated enzyme-histochemically by combined light and electron microscopy. In sections reacting to both 5′-nucleotidase (5′-Nase) and alkaline phosphatase (ALPase), the intralobular lymphatic vessels were identified as irregularly shaped spaces with strong 5′-Nase activity. The lymphatic vessels were closely associated with the branches of ALPase-positive intralobular arteries and veins. The initial lymphatics, which presumably originate from the perivascular spaces, were 5′-Nase positive. The distribution and intensity of the 5′-Nase activity in the lymphatic vessels revealed by light microscopy correlated well with those by backscattered image electron microscopy. The backscattered image scanning electron microscopy of the same area as observed under a light microscope more clearly highlighted the peculiar contours of lymphatic endothelial cells. Transmission electron microscopy showed that specific reaction product of the 5′-Nase after incubation in a medium containing L-tetramisole was predominantly localized on the outer surface of the lymphatic endothelial cell membranes.

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Several immunohistochemical methods using Factor VIII-Related antigen (FVIIIR: Ag), laminin, Type IV collagen and fibronectin antisera were applied for the purpose of differentiating rat lymphatics from blood vessels by light and electron microscopy. Weibel-Palade bodies (WPB) were demonstrated in both types of vessels by conventional electron microscopy. The immunoreactivity to laminin and Type IV collagen in blood vessels showed a strong, continuous, linear subendothelial staining pattern in contrast to lymphatic vessels in which immunoreactivity was absent or weak in paraffin-embedded sections stained with the indirect immunoperoxidase technique. A positive reaction for fibronectin was observed in all extra-vascular tissue spaces as well as in lymphatics and blood vessels. FVIIIR: Ag and WPB were present in both lymphatic and blood endothelial cells. FVIIIR: Ag antiserum labeled with gold particles was observed only in the vacuoles which were assumed to be identical with WPB as demonstrated by our conventional electron microscopy.
We conclude that the immunohistochemical method using laminin and Type IV collagen antisera is a reliable and practical way to differentiate lymphatic vessels from blood vessels by light microscopy.

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Microlymphatics can be devided into two segments, initial lymphatics which are made up of irregular tissue crevices lined by a continuous attenuated endothelium, and collecting lymphatics with a smooth muscle media and the ability for spontaneous contractility. Virtually the entire array of mammalian organs with lymphatic drainage have initial lymphatics which are drained by collecting lymphatics, but in organs like skeletal muscle and intestine almost all lymphatics are of the initial type, and the muscular collecting lymphatics arise only outside the organs per se. How can interstitial fluid find its way into the sparely positioned initial lymphatics? Initial lymphatics exhibit no detectable contractile activity. Their endothelium shows incomplete attachment between neighbouring cells, providing a mechanism to open and close lymphatic endothelial microvalves along the walls of the initial lymphatics. Current evidence suggests that lymph fluid formation in the initial lymphatics requires periodic expansion and compression of the initial lymphatics. Expansion of the initial lymphatics causes filling by percolation of interstitial fluid across the open endothelial microvalves. Compression causes closure of the endothelial microvalves and outflow along the lumen of the microlymphatics with eventual transport into collecting lymphatics, towards the nodes and into the thoracic ducts. Reflow towards the initial Lymphatics is prevented by valves. Expansion and compression of the initial lymphatics depend on deformation of the tissue in which they are embedded. In skeletal muscle, lymphatics are usually paired with arterioles so that vasomotion and arterial pulsations as well as muscle contraction may lead to periodic expansion and compression. In other organs alternative mechanisms are expected to operate. In light of multiple pump mechanisms, lymph flow may be adjusted according to physiological activity of the host organ.

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The distribution and structure of the mammalian cardiac lymphatic system have been investigated by puncture injection, intra-arterial injection of silver nitrate, hydrogen peroxide immersion, and light and electron microscopy.
The cardiac lymphatic system consists of drainage vessels and lymphatic capillaries. The drainage vessels contain many valves and are mainly situated subepicardially following branches of the coronary artery. The lymphatic capillaries are composed of a thin layer of endothelial cells, and form relatively dense networks in a fishnet arrangement. These lymphatic networks are richer in the ventricles than in the atria, being present in the subepicardial myocardial and subendocardial regions. In addition, networks are found in all cusps of the atrioventricular valves, and in the sinuatrial node and atrioventricular system.
The lymphatic system maintains cardiac homeostasis by receiving proteins, electrolytes and excess fluid from the interstitial tissue and returning them to the venous system.

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The three-dimensional organization and ultrastructure of lymphatics in the rat intestinal mucosa were studied by scanning electron microscopy using the KOH-collagenase digestion method (to remove extracellular connective tissue matrices) and by conventional transmission electron microscopy.
Each villus possessed three or more lymphatics in the deeper layer of the lamina propria. These initial lymphatics were 5-15μm in diameter and ran parallel to each other in the longitudinal direction of the villus. These lymphatics were free from adhering periendothelial cells, but had numerous cytoplasmic folds and projections of the endothelium on their abluminal surface. At the villous base, the lymphatics emptied into several thicker (about 70-80μm) lymphatics, which further connected together to form a flat and wide sinus (intra-villous lymphatic sinus). This sinus was relatively smooth in abluminal surface. From the bottom of each sinus, several lymphatics descended perpendicularly to drain into lymphatics in the submucosa. The villous lymphatics described above seemed to be partially accompanied by bundles of smooth muscle cells, which ran longitudinally in the villous core.
The submucosal lymphatics were smooth in abluminal surface and possessed no periendothelial cells. These lymphatics ran in a transverse direction just beneath the muscularis mucosae to form a two-dimensional meshwork by anastomosing with one another. This submucosal lymphatic network was often closely associated with the muscle cells of the muscularis mucosae, but was not accompanied by submucosal arteries.

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A hitherto undescribed intimate association of the nerve fibers with the central lacteal endothelium in the canine duodenum was demonstrated by transmission electron microscopy of ultrathin serial sections. Around the proximal half of the entire extent of the central lacteal, there exist three to ten unmyelinated nerve fibers 0.1-0.15μm in diameter. Some of these exhibit bulbar swellings (0.7-1.0μm in diameter) which contain small clear vesicles (30-35nm in diameter) and/or rather large vesicles (60-90nm) with electron-dense cores (40-60nm). Most of the bulbar structures of the nerve fibers are in contact with or surrounded by the central lacteal endothelial cells. These results suggest the existence of a neuronal control over the activity of the lacteal endothelial cells.

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The lymphatic sinus of the rabbit appendix was examined by scanning and transmission electron microscopy. Luminal surfaces of the sinus were lined by squamous endothelial cells, some of which, however, were blebby in surface relief due to the presence of electron dense granules in the cytoplasm. Many clusters of lymphocytes were present on the endothelial surface, mainly along the blood vessels of the trabeculae and of the sinus wall. Regional differences in collagen fibrillar arrangement of the sinus wall were demonstrated after the removal of the endothelial cells by the NaOH maceration method (OHTANI, 1987). In the upper region of the sinus and the lymphatic plexus of the inter-nodular thymus dependent area (TDA), the fibrillar layer was a delicate network of interwoven thin fibrils. Furthermore, transmission electron microscopy revealed frequent transmural passages of lymphocytes, which suggest the existence of pores or fenestrations in the fibrillar layer of these regions. The fibrillar layer in the lateral and basal sinus was formed by densely inter-woven thick bundles or sheets which consisted of thin collagen fibrils. No obvious space was seen between fibrils. These observations suggest that the upper region of the sinus and the lymphatics in the TDA may be active sites of cell migration from the reticular tissue.

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The three-dimensional organization of the lymphatic vessels and their relationship to blood vessels in rabbit Peyer's patches were demonstrated by scanning electron microscopy (SEM) of corrosion casts and of tissues. The interconnected central lacteals in the villi overlying the interfollicular area were connected with the lymphatic plexus in the area. There were many blind-ending lymphatic vessels in the upper part of the interfollicular area. These lymphatics gradually fused and formed perifollicular lymphatic sinuses which surrounded the lateral surfaces and bottoms of the follicles. There were no lymphatic vessels within the dome and the follicle. The perifollicular lymphatic network surrounded the capillary network of the follicle. Between the perifollicular lymphatic networks in the interfollicular area were many high endothelial venules (HEVs) which collected the capillaries in the dome and the follicle. The voluminous perifollicular lymphatic sinuses seemed to have a great potential capacity as both reservoirs and as drainage routes for fluid and lymphocytes. The close association of HEVs with the perifollicular lymphatic vessels seemed to facilitate the prompt drainage of fluid and macromolecules leaking out of HEVs during lymphocyte migration into the lymphatics. That the HEVs are downstream of the capillaries in both the dome and the follicle suggests that substances such as cytokines may be involved in the induction of the post capillary venules into HEVs.

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The topography and ultrastructure of the omentum in normal and stimulated mice were investigated with combined transmission (TEM) and scanning electron microscopy (SEM). The present study demonstrated that lymphocytes and monocytes were the principle cell types in the non-stimulated milky spot. Following stimulation with bacterial toxin and adjuvant there was an increased microvascular permeability to fluid, neutrophils, monocytes and fibrin deposits within the connective tissue matrix of milky spots, and a subsequent increased cellular migration across the mesothelial lining into the peritoneal cavity. Cellular migration from the milky spot to the peritoneal cavity is facilitated by the absence of a basal lamina from the submesothelial connective tissue layer, therefore, cells can migrate from the interstitial spaces of the milky spot to intercellular gaps between mesothelial cells without having to penetrate a fibrous barrier.

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A migratory pathway of macrophages as well as lymphatic communications from the liver to the celiac lymph nodes were studied both macroscopically and histologically. The injection of gelatinized carbon into the porta hepatis revealed a new pathway of the liver lymphatics running independently of the portal vein in addition to the ordinary periportal lymphatics. By obstruction of the efferent lymph flow of the celiac nodes and immunostaining with a monoclonal antibody to lymphatics, perilobular lymphatic vessels in the portal tract was readily demonstrated. It was suggested that heavily carbon-laden (HC) macrophages had migrated from the sinusoid into the interlobular connective tissue by 6h after an intravenous injection of carbon, and then entered the lymphatic vessels of the portal tract. By 9h to 12h after carbon injection, HC macrophages started to migrate into the celiac nodes via the two lymphatic pathways. From the marginal sinus in the celiac nodes, they moved into the inter-follicular area of the superficial cortex, then accumulated in the paracortex by 12h to 24h. They finally ended up in the corticomedullary junction. Migrating HC macrophages showed morphological homogeneity. The liver lymphatic pathway in the rat and a significance for the translocation and function of migrating macrophages were discussed.

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Branches of the hepatic veins in the dog are equipped with peculiar, periodically arranged sphincter muscles which are known to constrict in response to hematogenous shock agents, causing the severe hepatic congestion characteristic of this species. As was confirmed in this study, the sphincters are more strongly and effectively disposed in the peripheral portion of the veins, including the sublobular and central veins.
Mast cells were numerous around the sublobular branches, being specifically gathered beneath the endothelium as recorded by FUJITA (1964). The present observation light-microscopically extended his findings, particularly with regard to the distribution of the mast cells along the entire course of the hepatic vein branches—from the proximal trunks through the sublobular veins to the central veins. In addition, mast cell condensation was especially pronounced in the peripheral branches, apparently in accordance with the development of the sphincters. Electron microscope observation confirmed the subendothelial location of the mast cells and revealed that, through an endothelial gap, the cells may extend a microprocess into the venous lumen, thus enabling the direct detection of hematogenous agents.
A suspension of a bacterium, Listeria monocytogenes, was injected into the dog livers to induce hepatitis, and the resulting pathologically altered parts of the organ were examined light-microscopically. A heavy infiltration of inflammatory cells was found around the peripheral branches of the hepatic veins. The lymphatics accompanying the veins often contained lymphocytes and macrophages at two days after the injection. At five days, the lymphatics were extremely distended and twisted. The subendothelial mast cells were not encountered at the sites of severe cell infiltration.
It is suggested that the hepatic vein branches, together with their subendothelial mast cell sheath and adventitial lymphatics, form a functional and defensive unit in the canine liver. The mast cells, with their histamine and other bioactive agents, not only induce the constriction of the venous sphincters and the outflow of the fluid component of the blood into the lymphatics, but also presumably attract infiltrating cells around the veins and thereafter regulate their functions. The possibility was proposed that the flowing fluid may effectively carry the immune-related substances released from the infiltrating cells into the lymphatics and the systemic circulation.

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The localization of neuropeptide Y (NPY)-, substance P (SP)- and calcitonin gene-related peptide (CGRP)-containing nerve fibers around lymphatic capillaries (initial lymphatics) in the interlobular connective tissue of the rat liver was investigated by preembedding immunoelectron-microscopy. Nerve terminals with NPY were frequently seen in close apposition to the abluminal surface of lymphatic endothelium. A small number of NPY fibers without a glial (Schwann cell) covering at the tip ran toward lymphatic capillaries in the interlobular connective tissue. Nerve fibers immunoreactive for SP were present within unmyelinated fiber bundles that ran close to lymphatic capillaries in the interlobular connective tissue. Besides these immunoreactive nerve fibers, many of which appeared to pass through the subendothelial regions of lymphatic capillaries, scattered SP nerve endings were seen in areas contiguous to lymphatic endothelium. CGRP terminals were rarely found around lymphatic capillaries, although nerve fiber bundles containing CGRP components traversed close to some lymphatic capillaries. These findings suggest that NPY and SP, if released from nerve terminals into the subendothelial areas of adjacent lymphatic capillaries, are more likely to affect the metabolic activity of lymphatic endothelium and the flow (or formation) of lymph than CGRP. SP and CGRP, as possible mediators of sensory transmission, might be involved in the conveyance of information on the hydrostatic pressures of hepatic lymphatics and surrounding tissue fluid to the central nervous system.

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The normal role that the macrophage plays in tissue homeostasis is presented along with the morphological and functional changes that occur to the macrophage population as the lymphoedema progresses from the latent to the chronic phase and then with the treatment with a representative benzopyrone called coumarin.
Underlying the lymphoedema, there is a chronic inflammation. It is this, in association with the accumulating protein and the subsequent alterations it produces in the tissues that attract monocytes and macrophages to the affected area. Despite the fact that macrophages are facultative anerobes, and that larger numbers than normal accumulate, the tissue conditions result in a depression in their activity levels. Apart from these tissue conditions there is the possible production of deactivating proteins such as transforming growth factor beta 1 and 2. Evidence for this deactivation comes from enzymatic studies in which levels of typical macrophage enzymes are reduced and from morphological work which has shown a reduction in pseudopods and a tendency to accumulate large amounts of lipid in their vacuoles.
As a consequence of this deactivation further protein accumulation occurs thereby osmotically atracting fluid. Also there is a tendency for the tissues to become fibrotic as the balance between collagen lysis and deposition shifts towards the latter since it has been shown that macrophages have an important role in collagen lysis.
The administration of coumarin stimulates the macrophages resulting to their return to normal or supranormal activity levels within the lymphoedematous tissues. As well as this there is an increase in macrophage numbers. The reasons for stimulation are uncertain, however, alterations in the fine structure of the proteins and complement which make these more attractive for phagocytosis seem the most likely. The end result is an rapid enhanced breakup of the excess interstitial protein and the removal of the osmotically attracted fluid together with a more gradual removal of the deposits of fibrotic tissue by the non-stimulated macrophage. Clinically this manifests itself as a softening of the tissues, a reduction in circumference of the lymphoedematous extremity, a return to normal tissue remodelling processes and a range of subjective improvements for the patient.

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This article aims to clarify the topographical relationships of the bronchomediastinal collecting lymph vessels to other structures, in particular the great vessels, the trachea, the esophagus and the mediastinal pleura. Minute dissection was performed on eight cadavers with special reference to the converging collecting lymph vessels which form the bronchomediastinal trunks.
On the right side, the trunks were consistently observed on both the right brachiocephalic vein and the subserous surface of the mediastinal pleura (anterior and posterior mediastinal trunks). The pathway from the right recurrent chain nodes ran laterally behind the carotid sheath and led either into the deep cervical nodes situated on the scalenus anterior or directly into the right venous angle.
On the left side, the trunks showed varying courses. The nodes from which the trunks arose were constant, and classifiable into three groups: the uppermost paratracheal nodes near the recurrent chain nodes, the anterior mediastinal nodes (the left phrenic nodes) surrounding the phrenic nerve in front of and inferior to the aortic arch (the origin of the superior mediastinal trunk), and the left tracheobronchial nodes (the origin of the inferior mediastinal trunk).
The large transverse superficial communicating vessel between the right and left sides was usually found in front of the trachea above the aortic arch; it was often connected to the nodes of the brachiocephalic angle. Deep communications were also found in front of the carina and behind the trachea.
These findings allow the collecting vessels from the thoracic viscera to be divided into two pathways on each side: the anterior and posterior mediastinal trunks on the right side, and the superior and inferior mediastinal trunks on the left side. In addition to the four trunks, the superficial communicating vessel between the right and left sides is also drained from the superior mediastinum. The internal mammary lymph chain, which often emptied directly into the venous angle or into the deep cervical nodes, occasionally joined with the right anterior mediastinal trunk or the left superior mediastinal trunk.

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