Structure,blood supply,and lymphatic vessels of the sheep's visceral pleura

We investigated the morphology of the visceral pleura of 36 sheep, using macroscopic, histologic, and ultrastructural approaches to quantify regional pleural thickness, blood supply, and lymphatic drainage, including the pulmonary ligament and hilar lymphatic distributions. Pleural thickness increased caudally and dorsally, such that the costal pleura of the caudal lobes had a mean minimum pleural thickness of 83 μm. The blood supply to the entire visceral pleura came exclusively from the bronchial arteries. Lymph vessels formed an extensive plexus throughout the serous membrane of all lobes. Trunk lymphatics (> 100 μm diameter) had a density of about 2/cm of pleural length on all lobar surfaces except for the cranial and middle lobes, where their density on the costal surfaces was ≤ 1/cm. Pleural trunk lymphatics coursed to the pulmonary ligaments and to the hilum on their way to regional lymph nodes. At the hilum they anastomose with intrapulmonary lymphatic trunks. The principal lymph nodes to receive pulmonary lymph were the caudal mediastinal node and tracheobronchial nodes. The visceral pleura of sheep is thick, showing considerable regional diversity in morphology.     

Structure and distribution of the lymphatic vessels in the parietal pleura of the monkey as studied by enzyme-histochemistry and by light and electron microscopy.

The entire distribution of lymphatics in whole mount preparations of the Japanese monkey was studied using the enzyme-histochemical technique reported by KATO et al. (1990, 1991). In this staining, the lymphatic endothelium was colored dark brown by its positive 5'-nucleotidase activity, while most blood vessels (especially arterioles) were colored blue due to their positive alkaline phosphatase reaction. The whole mount preparations of the pleura treated enzyme-histochemically clearly indicated the distribution, branching patterns and running courses of lymphatic vessels. They revealed numerous short blind-ending knobs which represented the initial portions of lymphatics. These knobs were seen near the surface of the parietal pleura along its entire extent. In the costal and diaphragmatic pleura, the lymphatics ran parallel to the intercostal muscle fibers, but perpendicular to the tendinous and muscular fibers of the diaphragm; they formed ladders, independent of the courses of blood vessels. In the mediastinal pleura, lymphatic vessels showed a tree-like branching accompanying blood vessels. Under the light microscope, toluidine-blue stained semithin sections revealed the initial part of lymphatics as a small irregularly outlined cavity (7-10 microns in diameter) surrounded by a dense connective tissue. This lymphatic dilation was sometimes located close to a thin mesothelial layer. Such a structure suggesting a "stoma" was seen near the attachment of the muscular diaphragm to the sternum and along the borders of the ribs. Transmission electron microscopy revealed an occasional interruption in the mesothelium. This stoma continued to a submesothelial cavity whose base comprised an attenuated endothelium of an extended lymphatic vessel.     

Morphometric analysis of intralobular, interlobular and pleural lymphatics in normal human lung

In spite of their presumed relevance in maintaining interalveolar septal fluid homeostasis, the knowledge of the anatomy of human lung lymphatics is still incomplete. The recent discovery of reliable markers specific for lymphatic endothelium has led to the observation that, contrary to previous assumptions, human lymphatic vessels extend deep inside the pulmonary lobule in association with bronchioles, intralobular arterioles or small pulmonary veins. The aim of this study was to provide a morphometric characterization of lymphatic vessels in the periphery of the human lung. Human lung sections were immunolabelled with the lymphatic marker D2-40, followed by blood vessel staining with von Willebrand Factor. Lymphatic vessels were classified into: intralobular (including those associated with bronchovascular bundles, perivascular, peribronchiolar and interalveolar), pleural (in the connective tissue of the visceral pleura), and interlobular (in interlobular septa). The percentage area occupied by the lymphatic lumen was much greater in the interlobular septa and in the subpleural space than in the lobule. Most of the intralobular lymphatic vessels were in close contact with a blood vessel, either alone or within a bronchovascular bundle, whereas 7% were associated with a bronchiole and < 1% were not connected to blood vessels or bronchioles (interalveolar). Intralobular lymphatic size progressively decreased from bronchovascular through to peribronchiolar, perivascular and interalveolar lymphatics. Lymphatics associated with bronchovascular bundles had similar morphometric characteristics to pleural and interlobular lymphatics. Shape factors were similar across lymphatic populations, except that peribronchiolar lymphatics had a marginally increased roundness and circularity, suggesting a more regular shape due to increased filling, and interlobular lymphatics had greater elongation, due to a greater proportion of conducting lymphatics cut longitudinally. Unsupervised cluster analysis confirmed a marked heterogeneity of lymphatic vessels both within and between groups, with a cluster of smaller vessels specifically represented in perivascular and interalveolar lymphatics within the alveolar interstitium. Our data indicate that intralobular lymphatics are a heterogeneous population, including vessels surrounding the bronchovascular bundle analogous to the conducting vessels present in the pleural and interlobular septa, many small perivascular lymphatics responsible for maintaining fluid balance in the alveolar interstitium, and a minority of intermediate lymphatics draining the peripheral airways. These lymphatic populations could be differentially involved in the pathogenesis of diseases preferentially involving distinct lung compartments.     

The contribution of ascorbic acid and dehydroascorbic acid to the protective role of pleura during inflammatory reactions

It is well-known that parapneumonic effusions lead to the formation of inflammatory exudates which contain an increasing amount of inflammatory cells, especially polymorphonuclear. At these pathological conditions characterized by oxidative stress, ascorbic acid (AA) plays an important role in quenching free radicals, so that it could protect neutrophils and mesothelial cells from oxidative damage. Besides that ascorbic acid and its metabolite dehydroascorbic acid (DHA) alters the sheep visceral and parietal pleura permeability. More specific ascorbic acid as well as dehydroascorbic acid decreases the permeability of pleura after addition on apical and basolateral side in both visceral and parietal pleurae. It seems that, AA and DHA have an opposite action upon pleura from that of the inflammatory mediators, like VEGF, which increases the permeability of pleura and causes mesothelial barrier dysfunction. The decrease of pleura permeability induced by AA and DHA suggest the hypothesis that AA and/or its metabolite DHA during inflammatory reactions not only protects mesothelial cells from oxidative damage, but also contributes to maintaining the mesothelial barrier function. Consequently, the inflammatory pleural fluid may be trapped in pleural space and the inflammation may be restricted, and have extension avoided.     

Forms of lung lymphatics: a scanning electron microscopic study of casts.

In a recent study, rats given monocrotaline underwent angiogenesis on their pleural surfaces. The rats also had novel structures in their bronchovascular bundles that were detected by scanning electron microscopy of vascular casts. These vessels could have been either new blood capillaries or dilated lymphatic capillaries. To determine if these structures were lymphatics or new blood vessels, specimens from animals that were undergoing angiogenesis were compared to those that were not. Finding similar structures in normal animals would imply that they were lymphatic. The second purpose of this work was to describe the three-dimensional anatomy of the lymphatics of the lung. Cast lymphatics were found in most lungs with edema or angiogenesis, but were rare in other conditions. The vascular structures in question were found in animals not undergoing angiogenesis and were, therefore, lymphatic. Additionally, scanning electron microscopy of casts showed several distinct forms of lymphatics in the lung. Prelymphatics are tissues planes beneath the pleura and around bronchovascular structures. They join reservoir, conduit or tubulo-saccular lymphatics. Reservoir lymphatics are broad ribbon-like structures with textured surfaces and small laterally branching pouches. They occur on the pleural surface, are closely linked with prelymphatics, and join conduit lymphatics. Conduit lymphatics are tubular structures that may contain valves, twist and go great distances without accepting tributaries. On the pleural surface, they may wind around blood vessels and vary greatly in diameter. Sacculo-tubular lymphatics surround arteries, veins and bronchioles. They have thin walls with wide saccular segments. They may be so dense that they form cylinders around the vessels or airways. Different forms of lung lymphatics suggest different function and potential.     

Ultrastructural study of pleural lymphatic drainage unit and effect of nitric oxide on the drainage capacity of pleural lymphatic stomata in the rat.

The objective of this study was twofold: first to investigate the ultrastructure of the lymphatic drainage unit on the costal pleura of rats by electron microscopy, and secondly to examine the effect of nitric oxide on the pleural lymphatic stomata and fluid absorption from the pleural cavity. The lymphatic drainage unit of the rat costal pleura is composed of three special components: the lymphatic stomata between the mesothelial cells, the initial part of the lymphatic vessels and the underlying connective tissue containing many foramina. The unit is the main passage to drainage fluid, particles and cells in the pleural space. To investigate the regulator of the lymph drainage, nitric oxide synthase inhibitor and nitric oxide donor were injected into the peritoneal cavity of the rats, respectively. Trypan blue was used as tracer. The ultrastructural changes of pleural lymphatic stomata were observed under scanning electron microscope and analyzed by a computer image processing system. It turned out that the area and density of the pleural lymphatic stomata were positively correlated with the nitric oxide quantity (p < 0.05). After the tracer was injected into the pleural cavity, the nitric oxide donor group exhibited a higher trypan blue concentration than the control group (p < 0.05). The ability of the pleura to absorb trypan blue was enhanced because of the larger opening of the lymphatic stomata (p < 0.05). It is suggested that nitric oxide can increase lymphatic absorption of the pleura by opening pleural lymphatic stomata.     

Na+-glucose cotransporter is also expressed in mesothelium of species with thick visceral pleura

Molecular evidence for Na+-glucose cotransporter (SGLT1) in rabbit pleural mesothelium has been recently provided, confirming earlier functional findings on solute-coupled liquid absorption from rabbit pleural space. In this research we checked whether SGLT1 is also expressed in pleural mesothelium of species with thick visceral pleura, which receives blood from systemic circulation, but drains it into pulmonary veins. To this end immunoblot assays were performed on total protein extract of scraped visceral and parietal mesothelium of lambs and adult sheep, and of a human mesothelial cell line. All of them showed SGLT1 specific bands. Moreover, confocal immunofluorescence images of lamb pleural mesothelium showed that SGLT1 is located in apical membrane. Therefore, a solute-coupled liquid absorption should also occur from pleural space of species with thick visceral pleura. Because of this protein-free liquid entering interstitium between visceral mesothelium and capillaries, inherent Starling forces should be different than hitherto considered, and visceral pleura capillaries could absorb liquid even in these species.     

Forms of lung lymphatics: A scanning electron microscopic study of casts

In a recent study, rats given monocrotaline underwent angiogenesis on their pleural surfaces. The rats also had novel structures in their bronchovascular bundles that were detected by scanning electron microscopy of vascular casts. These vessels could have been either new blood capillaries or dilated lymphatic capillaries. To determine if these structures were lymphatics or new blood vessels, specimens from animals that were undergoing angiogenesis were compared to those that were not. Finding similar structures in normal animals would imply that they were lymphatic. The second purpose of this work was to describe the three-dimensional anatomy of the lymphatics of the lung. Cast lymphatics were found in most lungs with edema or angiogenesis, but were rare in other conditions. The vascular structures in question were found in animals not undergoing angiogenesis and were, therefore, lymphatic. Additionally, scanning electron angiogenesis and were, therefore, lymphatic. Additionally, scanning electron microscopy of casts showed several distinct forms of lymphatics in the lung. Prelymphatics are tissues planes beneath the pleura and around bronchovascular structures. They join reservoir, conduit or tubulo-saccular lymphatics. Reservoir lymphatics are broad ribbon-like structures with textured surfaces and small laterally branching pouches. They occur on the pleural surface, are closely linked with prelymphatics, and join conduit lymphatics. Conduit lymphatics are tubular structures that may contain valves, twist and go great distances without accepting tributaries. On the pleural surface, they may wind around blood vessels and vary greatly in diamater. Sacculo-tubular lymphatics surround arteries, veins and bronchioles. They have thin walls with wide saccular segments. They may be so dense that they form cylinders around the vessels or airways. Different forms of lung lymphatics suggest different function and potential. © 1992 Wiley-Liss, Inc.     

Electron microscopic alterations of the rat's pleura after experimental haemothorax.

The initial signs of pleural reactivity and the subsequent mechanisms of pleural healing still remain unsolved. The visceral and parietal (costal and diaphragmatic) pleura were investigated following an experimental haemothorax (EH) by transmission electron microscopy. Young-adult Wistar rats were divided in five groups and survived 6 hours, 1, 3, 8 and 15 days respectively after EH. Six hours after EH the mesothelial cells had a more prominent lysosomal system and electron-dense material in the vesicles, as in the dilatated intercellular spaces. On the 1st day of the EH the mesothelial cytoplasm formed a thin interrupted band. The extravasal cells built multiple layers over the basal lamina, leading to a thicker submesothelial layer, occupying the superficial position toward the pleural cavity. The activated mesothelial cells covered both pleural sheets on the 3rd day after EH. Eight days after EH different membrane bodies, large apical evaginations, elastic-like formations, an extensive vesicular and cytofilamentous systems characterized the mesothelium. The wider elastic membrane showed thickenings, protrusions, bifurcations and double course. Fifteen days after EH larger zones in both pleural sheets displayed thinner basal lamina, remnants of elastic membrane and a thicker submesothelial layer. In conclusion, different newly formed structures (reversible and stable) retain the tendency of enlargement of the pleural surface in all investigated periods. Simultaneous intercellular and transcellular transport, as an increase of the lysosomal system characterize the passing of the electron-dense material through the mesothelium. The early period (until 3rd day after EH) is characterized by more prominent mesothelial changes, involving activated cells. The initiation of the late period (on the 8th day after EH) begins with the appearance of lamellar bodies and newly formed elastic membrane. The following late changes (on the 15th day after EH) concern predominantly the components of the connective tissue layer, such as collagen accumulations and blood capillaries. The present data suggest that the alterations over the entire pleura are irregular and asynchronous, showing significant morphological differences in both pleura sheets, some of them are diffuse in character, the final ones appear to be stable and ensure incomplete pleural restoration.     

The yellow nail syndrome. Light and electron microscopic aspects of the pleura

The yellow nail syndrome is a rare cause of recurrent pleural effusions. We studied a case of this entity, placing special emphasis on the microscopic and ultrastructural aspects of the pleural lymphatics. The patient had the classic symptoms of recurrent bilateral pleural effusions, yellow, dystrophic fingernails and toenails, and lower-limb edema. To control the pleural effusions, a left parietal pleurectomy was performed. Histologic study showed both pleura to be thickened with fibrosis and chronic inflammatory infiltration. The lymphatic capillaries in the visceral pleura were dilated. Electron microscopy confirmed the lymphatic nature of these capillaries. We believe that these ectatic lymphatic capillaries suggest a downstream obstruction to the lymph drainage.     

Light and electron microscope observations of the lymphatic drainage units of the peritoneal cavity of rodents

Fluid, particles, and cells are taken up from the peritoneal cavity by lymphatic drainage units, which, in the mouse and rat, are located along the peritoneal surface of the muscular portion of the diaphragm. The drainage units are composed of three specifically differentiated components: a lymphatic lacuna, a covering of lacunar mesothelium, and intervening submesothelial connective tissue. The units are drained by connecting lymphatic vessels that cross the diaphragm to empty into collecting lymphatic vessels running along the pleural surface of the diaphragm. The collecting lymphatics empty into parasternal lymphatic trunks. In this report, we briefly review critical features of the drainage apparatus and describe new observations, summarized below, about their structure. Around the rim of stomata, the mesothelial openings that lead into the lymphatic lacunae, plasma membranes of lacunar mesothelial cells and of lacunar enidothelial cells abut but are not linked to one another by recognizable junctional specializations. Lacunarendothelial cells often extend valve-like processes that bridge the distal end of the channel beneath the stoma. The configuration of the endothelial processes may be complex. Occasionally, processes from fibroblasts in the submesothelial connective tissue adjacent to stomata make contact with the interstitial surface of lacunar endothelial cells. A discontinuous elastic layer in the submesothelial connective tissue spans the roof of each lacuna. Connecting and collecting lymphatics, which drain lymphatic lacunae, possess endothelial valves. Possible functions for each of these newly described structural features are discussed.     

Microanatomy of the rat diaphragm with special reference to the lymphatics and mesothelial stomata

The three-dimensional microstructure of the rat diaphragm was studied in order to reveal morphological bases which permit peritoneal fluids to pass across the diaphragm to enter the pleural cavity. The methods used include scanning electron microscopy of either intact or alkali-treated tissues, enzyme-histochemistry, and confocal laser scanning microscopy (CLSM). The peritoneal and pleural surfaces of the diaphragm are covered with mesothelial cells studded with numerous microvilli. There are many round gaps between mesothelial cells on the peritoneal side of the diaphragm. The subperitoneal connective tissue contains voluminous, irregularly shaped lymphatics which extended many funnel-shaped projections of the endothelia towards the pored region of the mesothelium. On coming into contact with the mesothelium, many of the lymphatic projections are perforated at their ends, thus giving rise to stomata connecting the peritoneal cavity and lymphatic lumen. Some projections ended blindly while plugging the mesothelial pores, thereby making visible some intercellular gaps in this contact. The subperitoneal sheet of collagen fiber network possesses clusters of pores which tightly fit the passage of the lymphatic projections. CLSM of the diaphragm after intraperitoneal injection of FITC-dextran has demonstrated the tracer both in the lymphatic lumen and in the connective tissue spaces. The tracer has also been detected in the lymphatics located in the subpleural connective tissue space. These results indicate that peritoneal fluid is allowed to flow into the lymphatics directly through the stomata and indirectly through the intercellular gaps between endothelia and mesothelial cells, and then drain into the subpleural lymphatics. Discussions were made on the probable mechanisms by which a hydrothorax may occur during continuous ambulatory peritoneal dialysis.     

Hyperplastic mesothelial cells in mediastinal lymph node sinuses with extranodal lymphatic involvement

We describe a patient with hyperplastic mesothelial cells localized to mediastinal lymph node sinuses. These mesothelial cells were originally misdiagnosed as metastatic carcinoma, and the patient received radiotherapy. Histologic review, immunohistochemistry, and ultrastructural studies confirmed mesothelial cell origin. These nodal mesothelial cells were associated with pericardial and pleural effusions. Extranodal lymphatics also contained hyperplastic mesothelial cells, confirming their mode of lymphatic transport to node sinuses. This finding supports the theory that hyperplastic mesothelial cells derive from reactive serosal mesothelium and are dislodged into draining lymphatics. This is the first report, to our knowledge, that demonstrates the pathogenetic significance of this lymphatic transport mechanism. Awareness of intralymphatic and nodal benign hyperplastic mesothelial cells and their mimicry of invasive malignant neoplasms is important for accurate diagnoses and appropriate therapy.     

Scanning and transmission electron microscopic study of visceral and parietal peritoneal regions in the rat

The visceral peritoneum of intraabdominal organs (spleen, stomach, liver, small intestine), omentum majus and the parietal peritoneum of the anterior abdominal wall and the diaphragm were studied in adult Wistar rats by combined scanning and transmission electron microscopy (SEM, TEM). In general, the peritoneal surface consisted of a mesothelium composed of cubic, flat or intermediate cell types delimited by a basal lamina. Cubic mesothelial cells predominated in parenchymal organs (spleen, liver) and were characterized by prominent and indentated nuclei, a cytoplasm richly supplied with organelles, a dense microvillous coat, basal invaginations and elaborate intercellular contacts. Flat mesothelial cells were observed in the intestinal, omental and parietal peritoneum (tendinous diaphragm, abdominal wall) and showed elongated nuclei, scant cytoplasm, a poorly developed organelle apparatus and sparsely distributed microvilli. An intermediate mesothelial cell type was described within the gastric peritoneum characterized by a central cytoplasmic protrusion at the nuclear region containing most of the cytoplasmic organelles and by thin finger-like cytoplasmic processes. The submesothelial connective tissue layer was composed of collagen fiber bundles, fibroblasts and free cells (macrophages, granulocytes, mast cells) and contained blood and lymphatic vessels. In the spleen, elastic fibers formed a membranous structure with intercalated smooth muscle cells. Mesothelial openings were observed as tunnel-like invaginations within the hepatic peritoneum and as clusters of peritoneal stomata within the parietal peritoneum of the anterior abdominal wall and the muscular diaphragm. The round or oval openings of the peritoneal stomata were frequently occluded by overlapping adjacent mesothelial cells and their microvillous coat or obstructed by cellular material. At the side of the peritoneal stomata the mesothelial cell layer was interrupted to allow a direct access to the underlying submesothelial lymphatic system. The mesothelium and lymphatic endothelium shared a common basal lamina. The endothelial cells were discontinuous and displayed valve-like plasmalemmatic interdigitations facilitating an intercellular transport of fluids and corpuscular elements from the peritoneal cavity to the submesothelial lymphatic lacunae. The findings underline the morphological heterogeneity of the peritoneum in visceral and parietal regions, suggesting different functional implications, and further support the presence of extra-diaphragmatic peritoneal stomata.     

Features of the peritoneal covering of the lesser pelvis with special reference to stomata regions

Occasional reports describe various aspects of the fine morphology of the pelvic peritoneum, but its complete organ characteristics remain undefined. The peritoneal covering of the urinary bladder, rectum, uterus, uterine tube, ovary, broad ligament (BL) and testis in Wistar rats was examined by means of transmission and scanning electron microscopy (TEM, SEM). Unusually complicated relief and stomata between the cubic mesothelial cells characterized the surface of the BL. Deep, parallel furrows separated the wide longitudinal folds over the entire length of the uterine tube. The uterus and the ovary formed less numerous, shallow or extremely deep crypt-like invaginations, as well as serous villus-like or papilla-like evaginations. The flat cells were the predominant cell type over the BL, while the cubic mesothelium was the basic covering of the organs. Most of the cubic cells were located in the invagination of the submesothelial layer (SML). Such cells formed an almost smooth surface over the urinary bladder or formed larger areas of the rectum and the testis surfaces. Numerous microvilli, ciliae, round evaginations and complex lamellar bodies characterized their apical plasmalemma. In conclusion, the mesothelial heterogeneity is a stable feature of the lesser pelvis peritoneum, confirmed by TEM and SEM. The cubic mesothelium characterizes the organ peritoneum, while the BL plays the role of the parietal sheet, involving lymphatic units in the SML. The different types of contacts between the mesothelio-endothelial cells, large lymphatic vessels and occasional stomata are the usual components of the lymphatic units in norm, visible by TEM. Images of stomata, seen by SEM, demonstrate oval-shaped deep channel-like gaps surrounded by cubic mesothelium. The last data extend the evidence on stomata regions, which resemble the diaphragmatic ones. Clusters of cells (macrophages, mastocytes and Lymphocytes), small vessels (blood or lymphatic) and nerve fibers (unmyelinated and rare myelinated) form highly specialized complexes in the SML of the ovary, the uterus and the testis.     

大鼠膈腹膜间皮孔的电镜观察

用扫描电镜和透射电镜观察了正常大鼠膈腹膜间皮，并观察了腹膜腔内注射中国墨汁和兔血液后大鼠膈腹膜间皮的变化以及腹膜腔和间皮下毛细淋巴管的关系。     

Pleural MALT lymphoma diagnosed on thoracoscopic resection under local anesthesia using an insulation-tipped diathermic knife

A 79-year-old man presented with back pain. Chest CT scan showed elevated nodular lesions in the right parietal pleurae with pleural effusion. There were no intrapulmonary or mediastinal abnormalities. Under local anesthesia, right thoracoscopy and subsequent thoracoscopic pleural resection were performed using an insulation-tipped diathermic knife (IT-knife). The resected pleura, 2.2 cm in diameter, had a rough granular surface. Lymphoid cells histologically infiltrated diffusely into the pleura. They were composed of centrocyte-like and monocytoid cells. On immunohistochemistry they were found to be positive for Bcl2, CD20, CD45RB and CD79a, but negative for CD3, CD5, CD10 and cyclin D1. EBV-encoded small RNA-1 (EBER-1) in situ hybridization was negative. A diagnosis of extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) arising in the pleura was therefore made. To the authors' knowledge this is the first case in which IT-knife was used for diagnosis of a pleural lesion. This large, single-piece, only slightly crushed pleural specimen, enabled study of histopathological findings (listed here) that could not have been obtained on conventional biopsy: (i) lack of apparent evidence of plasmacytic differentiation; (ii) no recognition of lymphoid follicles; (iii) mesothelial cells not infiltrated by lymphoma cell clusters; (iv) thin layer of hyperplastic mesothelial cells continuously covering the surface; and (v) no proliferation of fibroblast-like submesothelial cells.     

Lymphatic and blood vessels of the popliteal node in sheep

Our aim was to describe the lymphatic and blood vascular pathways to and from the popliteal lymph node in sheep. The blood vessels and lymphatics were filled with Microfil, and were cleared in methyl salicylate. Afferent lymphatics divide and anastomose as they pass dorsally along the lateral saphenous vein, and 6–12 lymphatics reach the node. Each branches extensively on the surface of the node giving rise to 20–50 terminal afferents which enter the node over a roughly circular area. Most enter the subcapsular sinus, but some penetrate deeply into the node. Lymph leaves the node through numerous initial efferent lymphatics, many of which contain valves. These join forming progressively larger vessels, and 2–4 efferent trunks emerge from the hilus. The hilus varies considerably in shape, depth and location, and it is filled with fat. Either a single artery, or up to 10–12 arteries derived from an anastomotic network or circle, enter the node from the hilar fat pad. Arteries may also enter at other sites. The arteries originate from the caudal femoral, or the medial circumflex femoral artery; a single node may receive blood from both arteries. This arrangement may help to maintain blood flow especially during an immune response, and despite external pressures applied to the arteries and node during movements of the animal.     

Influence of the softness of the parietal pleura on respiratory sliding mechanisms

The pleural surfaces of the lung and chest wall slide against each other with low friction. Normal load support can be effected either by a combination of quasi-static fluid pressure and solid-solid contacts of relatively stiff asperities, or by shear-induced hydrodynamic pressures in the pleural fluid layer. To distinguish between these mechanisms, we measured surface topography and spatial distribution of stiffness of rat parietal pleura using atomic force microscopy. The topography of the pleural surface has unevenness at length scales smaller than the thickness of pleural fluid, similar to mesothelial cell diameters. The estimated maximum normal contact pressure that could be borne by asperities of the soft pleura is much less than that required to support a substantial difference between pleural fluid pressure and the pleural surface pressure. These results suggest that during sliding motion, unevenness of the pleural surface is smoothed by local hydrodynamic pressure, preventing any significant contribution of solid-solid contacts.     

Embolization of mesothelial cells in lymphatics: the route to mesothelial inclusions in lymph nodes?

Aims : To provide evidence that lymphatic embolization is the mechanism for mesothelial inclusions in lymph nodes.  

Methods and results

A 60-year-old man with alcoholic cirrhosis and ascites had an umbilical hernia resected. The herniorrhaphy specimen contained numerous dermal and submesothelial lymphatic vessels filled by cells similar to the cells that lined the hernia sac. Most of the cells in lymphatics were submesothelial reactive cells, whose cytoplasm stained with antibodies against cytokeratins (AE1–AE3; 8,18), smooth muscle actin, vimentin, desmin and tissue polypeptide antigen (TPA). Some cells seemed to be superficial mesothelial cells, being positive with high molecular weight anticytokeratin antibody 34βE12. On ultrastructural study submesothelial cells with intermediate cytoplasmic filaments, rough endoplasmic reticulum and primitive cell junctions, and scanty superficial mesothelial cells with microvilli, tonofilaments and desmosomes were found in the lymphatics.  

Conclusions

Lymphatic dissemination of mesothelial and submesothelial cells is an uncommon and not well known phenomenon. Lymphatic dissemination is probably the route by which the mesothelial cells reach the lymphatic nodes. These cells may be mistaken for malignant cells.