Lymphatic pumping: mechanics, mechanisms and malfunction

Lymphatic system identification, pathophysiology and therapy in the cardiovascular diseases

The mammalian circulatory system comprises both the cardiovascular system and the lymphatic system. In contrast to the closed, high-pressure and circular blood vascular circulation, the lymphatic system forms an open, low-pressure and unidirectional transit network from the extracellular space to the venous system. It plays a key role in regulating tissue fluid homeostasis, absorption of gastrointestinal lipids, and immune surveillance throughout the body. Despite the critical physiological functions of the lymphatic system, a complete understanding of the lymphatic vessels lags far behind that of the blood vasculatures due to the challenge of their visualization. During the last 20 years, discoveries of underlying genes responsible for lymphatic vessel biology, combined with state-of-the-art lymphatic function imaging and quantification techniques, have established the importance of the lymphatic vasculature in the pathogenesis of cardiovascular diseases including lymphedema, obesity and metabolic diseases, dyslipidemia, hypertension, inflammation, atherosclerosis and myocardial infraction. In this review, we highlight the most recent advances in the field of lymphatic vessel biology, with an emphasis on the new identification techniques of lymphatic system, pathophysiological mechanisms of atherosclerosis and myocardial infarction, and new therapeutic perspectives of lymphangiogenesis.

Lymphatic network remodeling after small bowel resection

BACKGROUND

Short gut syndrome (SGS) following massive small bowel resection (SBR) is a major cause of pediatric mortality and morbidity secondary to nutritional deficiencies and the sequelae of chronic total parenteral nutrition use, including liver steatosis. Despite the importance of lymphatic vasculature in fat absorption, lymphatic response after SBR has not been studied. We hypothesize that lymphatic vessel integrity is compromised in SGS, potentially contributing to the development of impaired lipid transport leading to liver steatosis and metabolic disease.

METHODS

Mice underwent 50% proximal SBR or sham operations. Imaging of lymphatic vasculature in the lamina propria and mesentery was compared between sham and SBR Prox1 ERCre-Rosa26^LSLTdTomato mice. mRNA expression levels of lymphangiogenic markers were performed in C57BL/6J mice.

RESULTS

Lymphatic vasculature was significantly altered after SBR. Mesenteric lymphatic collecting vessels developed new branching structures and lacked normal valves at branch points, while total mucosal lymphatic capillary area in the distal ileum decreased compared to both sham and intraoperative controls. Intestinal Vegfr3 expression also increased significantly in resected mice.

CONCLUSIONS

Intestinal lymphatics, in both the lamina propria and mesentery, dramatically remodel following SBR. This remodeling may affect lymphatic flow and function, potentially contributing to morbidities and nutritional deficiencies associated with SGS.

The Importance and Control of Low-Grade Inflammation Due to Damage of Cellular Barrier Systems That May Lead to Systemic Inflammation

Systemic low-grade inflammation can be initiated in vivo after traumatic injury or in chronic diseases such as neurodegenerative, metabolic, and autoimmune diseases. Inducers of inflammation trigger production of inflammatory mediators, which alter the functionality of tissues and organs and leads to harmful induction of different barrier systems in the body, where the blood-brain barrier, the blood-retinal barrier, blood-nerve barrier, blood-lymph barrier and the blood-cerebrospinal fluid barrier play major roles. The different barriers are unique but structured in a similar way. They are equipped with sophisticated junctional complexes where different connexins, protein subunits of gap junction channels and hemichannels, constitute important partners. The cells involved in the various barriers are coupled in networks, are excitable but do not express action potentials and may be targets for inflammation leading to changes in several biochemical cellular parameters. During any type of inflammation barrier break-down is observed where any form of injury can start with low-grade inflammation and may lead to systemic inflammation.

Call for a Multidisciplinary Effort to Map the Lymphatic System with Advanced Medical Imaging Techniques: A Review of the Literature and Suggestions for Future Anatomical Research

This review intends to rekindle efforts to map the lymphatic system by using a more modern approach, based on medical imaging. The structure, function, and pathologies associated with the lymphatic system are first discussed to highlight the need for more accurately mapping the lymphatic system. Next, the need for an interdisciplinary approach, with a central role for the anatomist, to come up with better maps of the lymphatic system is emphasized. The current approaches on lymphatic system research involving medical imaging will be discussed and suggestions will be made for an all-encompassing effort to thoroughly map the entire lymphatic system. A first-hand account of our integration as anatomists in the radiotherapy department is given as an example of interdisciplinary collaboration. From this account, it will become clear that the interdisciplinary collaboration of anatomists in the clinical disciplines involved in lymphatic system research/treatment still holds great promise in terms of improving clinical regimens that are currently being employed. As such, we hope that our fellow anatomists will join us in an interdisciplinary effort to map the lymphatic system, because this could, in a relatively short timeframe, provide improved treatment options for patients with cancer or lymphatic pathologies all over the world. Anat Rec, 302:1681-1695, 2019. © 2019 American Association for Anatomy.

Entrainment of Lymphatic Contraction to Oscillatory Flow

Lymphedema, a disfiguring condition characterized by an asymmetrical swelling of the limbs, is suspected to be caused by dysfunctions in the lymphatic system. A possible source of lymphatic dysfunction is the reduced mechanosensitivity of lymphangions, the spontaneously contracting units of the lymphatic system. In this study, the entrainment of lymphangions to an oscillatory wall shear stress (OWSS) is characterized in rat thoracic ducts in relation to their shear sensitivity. The critical shear stress above which the thoracic ducts show a substantial inhibition of contraction was found to be significantly negatively correlated to the diameter of the lymphangion. The entrainment of the lymphangion to an applied OWSS was found to be significantly dependent on the difference between the applied frequency and the intrinsic frequency of contraction of the lymphangion. The strength of the entrainment was also positively correlated to the applied shear stress when the applied shear was less than the critical shear stress of the vessel. The ejection fraction and fractional pump flow were also affected by the difference between the frequency of the applied OWSS and the vessel's intrinsic contraction frequency. The results suggest an adaptation of the lymphangion contractility to the existing oscillatory shear stress as a function of its intrinsic contractility and shear sensitivity. These adaptations might be crucial to ensure synchronized contraction of lymphangions through mechanosensitive means and might help explain the lymphatic dysfunctions that result from impaired mechanosensitivity.

The Visibility of the Terminal Thoracic Duct Into the Venous System Using MR Thoracic Ductography with Balanced Turbo Field Echo Sequence

RATIONALE AND OBJECTIVES

Magnetic resonance thoracic ductography (MRTD) with balanced turbo field echo (bTFE) can visualize both the thoracic duct and its surrounding vessels. This study aimed to investigate the visibility of the terminal thoracic duct into the venous system in the subclavian region using MRTD with bTFE.

MATERIALS AND METHODS

MRTD was performed with bTFE as a preoperative workup comprising respiratory gating on a 1.5-T magnetic resonance system for patients with esophageal cancer. The portion and the number of terminal thoracic ducts into the venous system and preterminal branching in the left subclavian region were assessed using MRTD in 132 patients. The confidence level of the visibility using MRTD was also evaluated.

RESULTS

The most frequent terminal portion of the thoracic duct was the jugulovenous angle (92 patients, 69.7%), followed by the subclavian vein (27 patients, 20.5%) and the internal jugular vein (8 patients, 6.1%). Four patients also exhibited double entry of the thoracic duct into the venous system. The preterminal branching was single in 96 patients (72.7%) and multiple in 36 patients (27.3%). The confidence level of the visibility of the thoracic duct using MRTD was absolutely certain in 112 patients (84.8%) and was somewhat certain in 20 patients (15.2%).

CONCLUSIONS

MRTD with bTFE is a robust imaging modality to visualize the terminal portion of the thoracic duct into the venous system in the subclavian region.

T helper 2 differentiation is necessary for development of lymphedema

T cells infiltrating lymphedematous tissues have a mixed T helper 1 (Th1) and Th2 differentiation profile. Treatment with neutralizing antibodies targeting cytokines that promote Th2 differentiation (interleukin 4 [IL-4] and IL-13) decreases the severity of lymphedema in preclinical models, suggesting that Th2 cells play a key role in the pathology of this disease. However, these previous studies do not address the contribution of Th1 cells and it remains unknown if IL-4 and IL-3 blockade acts primarily on T cells or decreases the pathological changes of lymphedema by other mechanisms. Therefore, this study sought to analyze the effect of lymphatic injury in transgenic mice with mutations that cause defects in Th1 and Th2 cell generation (T-bet knockout or T-betKO and STAT6 knockout or STAT6KO mice, respectively). Using both the mouse tail and popliteal lymph node dissection models of lymphedema, we show that Th2-deficient (STAT6KO) mice are protected from developing lymphedema, have decreased fibrosis, increased collateral vessel formation, and preserved collecting lymphatic vessel pumping function. In contrast, mice with defective Th1 cell generation (T-betKO) develop disease with the same severity as wild-type controls. Taken together, our results suggest that Th2 differentiation is necessary for development of lymphedema following lymphatic injury and that Th1 differentiation does not significantly contribute to the pathology of the disease. Such findings are important as immunotherapy directed at Th2 cells has been found to be effective in well-studied Th2-mediated diseases such as asthma and atopic dermatitis and may therefore be similarly useful for lymphedema management.

Baseline Lymphatic Dysfunction Amplifies the Negative Effects of Lymphatic Injury

BACKGROUND

Genetic mutations and obesity increase the risk of secondary lymphedema, suggesting that impaired lymphatic function before surgical injury may contribute to disease pathophysiology. Previous studies show that obesity not only decreases lymphatic function, but also markedly increases pathologic changes, such as swelling, fibroadipose deposition, and inflammation. However, although these reports provide circumstantial evidence supporting the hypothesis that baseline lymphatic defects amplify the effect of lymphatic injury, the mechanisms regulating this association remain unknown.

METHODS

Baseline lymphatic morphology, leakiness, pumping, immune cell trafficking, and local inflammation and fibroadipose deposition were assessed in wild-type and Prox1-haploinsufficient (Prox1) mice, which have previously been shown to have abnormal vasculature without overt evidence of lymphedema. In subsequent experiments, wild-type and Prox1 mice underwent popliteal lymph node dissection to evaluate the effect of lymphatic injury. Repeated testing of all variables was conducted 4 weeks postoperatively.

RESULTS

At baseline, Prox1 mice had dilated, leaky lymphatic vessels corresponding to low-grade inflammation and decreased pumping and transport function, compared with wild-type mice. Popliteal lymph node dissection resulted in evidence of lymphedema in both Prox1 and wild-type mice, but popliteal lymph node dissection-treated Prox1 mice had increased inflammation and decreased lymphatic pumping.

CONCLUSIONS

Subclinical lymphatic dysfunction exacerbates the pathologic changes of lymphatic injury, an effect that is multifactorial and related to increased lymphatic leakiness, perilymphatic accumulation of inflammatory cells, and impaired pumping and transport capacity. These findings suggest that preoperative testing of lymphatic function may enable clinicians to more accurately risk-stratify patients and design targeted preventative strategies.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Numerical simulations of the microvascular fluid balance with a non-linear model of the lymphatic system

Fluid homeostasis is required for life. Processes involved in fluid balance are strongly related to exchanges at the microvascular level. Computational models have been presented in the literature to analyze the microvascular-interstitial interactions. As far as we know, none of those models consider a physiological description for the lymphatic drainage-interstitial pressure relation. We develop a computational model that consists of a network of straight cylindrical vessels and an isotropic porous media with a uniformly distributed sink term acting as the lymphatic system. In order to describe the lymphatic flow rate, a non-linear function of the interstitial pressure is defined, based on literature data on the lymphatic system. The proposed model of lymphatic drainage is compared to a linear one, as is typically used in computational models. To evaluate the response of the model, the two are compared with reference to both physiological and pathological conditions. Differences in the local fluid dynamic description have been observed using the non-linear model. In particular, the distribution of interstitial pressure is heterogeneous in all the cases analyzed. The resulting averaged values of the interstitial pressure are also different, and they agree with literature data when using the non-linear model. This work highlights the key role of lymphatic drainage and its modeling when studying the fluid balance in microcirculation for both to physiological and pathological conditions, e.g. uremia.

Regulation of Chylomicron Secretion: Focus on Post-Assembly Mechanisms

Rapid and efficient digestion and absorption of dietary triglycerides and other lipids by the intestine, the packaging of those lipids into lipoprotein chylomicron (CM) particles, and their secretion via the lymphatic duct into the blood circulation are essential in maintaining whole-body lipid and energy homeostasis. Biosynthesis and assembly of CMs in enterocytes is a complex multistep process that is subject to regulation by intracellular signaling pathways as well as by hormones, nutrients, and neural factors extrinsic to the enterocyte. Dysregulation of this process has implications for health and disease, contributing to dyslipidemia and a potentially increased risk of atherosclerotic cardiovascular disease. There is increasing recognition that, besides intracellular regulation of CM assembly and secretion, regulation of postassembly pathways also plays important roles in CM secretion. This review examines recent advances in our understanding of the regulation of CM secretion in relation to mobilization of intestinal lipid stores, drawing particular attention to post-assembly regulatory mechanisms, including intracellular trafficking of triglycerides in enterocytes, CM mobilization from the lamina propria, and regulated transport of CM by intestinal lymphatics.

Lymphatic vessels of the eye - old questions - new insights

Due to its accessible position and tissue heterogeneity, the eye is ideally suited for studying the lymphatic system. As early as the 19th century, questions about the origin and function of this system were discussed. For example, whether Schlemm's canal, which is of particular importance in the pathogenesis of glaucoma, is a lymphatic vessel, or does this vascular system begin with finger-shaped protuberances? Despite the discovery of lymphatic endothelial molecules and the use of molecular imaging technologies, these questions are still discussed controversially today. Leber demonstrated in 1873 with a solution consisting of two dyes of different particle size that only the smaller particles from the anterior chamber of the eye filled the episcleral and conjunctival veins around the corneal margin. He believed to have proven - to be read in the historical review of our article - that the Canalis Schlemmii in humans is a venous circular vessel and not a lymphatic vessel. In our own investigations, we reduced the rather contradictory and complex question of whether there are lymphatic vessels in the eye to the question of whether there are drainage connections between the different sections of the eye and the lymphatic system or not. With different radioactive tracers and combined with unilateral ligation of cervical lymph vessels, we observed outflow from the subconjunctival and retrobulbar space, from the anterior chamber and the vitreous body. The rate of discharge of the radioactive tracer was determined by the radiopharmaceutical and injection site. In analogy to the lymphatic drainage of the head we found a segmental drainage of lymphatic substances on the eye. Vitreous humour and retrobulbar space were drained by lymphatic vessels, predominantly to the deep cervical lymph nodes, while anterior chamber and subconjunctival space drains predominated over the superficial cervical lymph nodes. Eyeball tattoos - as loved by some fan communities - should therefore cause a coloured staining of the superficial cervical lymph nodes. The boundary of the drained segments would be in the area of the eyeball's equator. According to the textbooks, the lymph is actively removed from finger-shaped initial segments via pre-collectors and collectors with properly functioning intraluminal valves and smooth muscle cells in the vessels' media. In patients with spontaneous conjunctival bleeding, however, we observed phenomena in the conjunctival lymph vessels, which ca not be explained with old familiar ideas. At nozzle-shaped vessel constrictions separation of blood components occurred. The erythrocytes formed partially a so-called fluidic "resting bulk layer". Parallel vessel parts caused a retrograde filling of already emptied segments. These observations led our experimental investigations. In the literature, there are different scanning electron microscopy (SEM) images of lymphatic endothelial surfaces; nevertheless they are unassigned to a particular vessel segment. In the conjunctiva, we studied the question whether there is a dependence between vessel diameter and the surface characteristics of endothelial cells (after unfolding by lymphography). A constantly applied photo-mathematical procedure for all specimens allowed determining the size of the cross sections. The specimens were randomized into seven groups with diameters of 0.1-1.0mm and above and examined by SEM. In the smallest vessels (diameter=0.11mm), the impressions of the occasionally occurring nuclei in the lumen were clearly impressive. With increasing diameter, these impressions were lost and the individual endothelial nuclei could no longer be identified. Rather, one recognized only wall-like structures. In vessels of intermediate diameter (0.3-0.4mm), structures could be seen on the surface similar to reticular fibres. With increasing diameters, their prominent character weakened. In the group with diameters above 0.5mm, wavy surface structures were shown. Finally, in vessels of diameters over 1.0mm, a uniform, flat surface was observed. Regardless of the collection site of the specimens, we found certain surface characteristics related to the vessels' calibre. In further investigations by means of interstitial dye lymphography, we were able to demonstrate in the conjunctiva that under increasing injection pressure, additional vessels stained from finger-shaped processes. At least in the conjunctiva, the existence of so-called "blind-ending initial segments" seems doubtful (despite the fact that initial segments or "initial lymphatics" would begin in periphery, not end). Rather, these are likely to be temporary filling states. SEM investigations were carried out on the internal structure of these dome-shaped vessel parts by means of a specially developed preparation technique. Despite numerous variants in the lymphographic design of the blind bags - in the form of finger, balloon, dome, piston, pyramidal, double-humped and spearhead-like endings - slot-shaped, lip-shaped and saw blade-like structures were repeatedly found, similar to a zipper. These findings suggest preformed connections to the next segment and may control lymphatic flow. To clarify the retrograde fluid movements, we examined the lymph vessels' valves or those structures that were previously interpreted as valves. The different structures found could be subdivided into three groups. The lack of common bicuspid structures provides an explanation for retrograde fluid movement. That nevertheless a directional flow is possible, is explained by the flow model developed by Gerhart Liebau. Conjunctival lymphatics show intraluminal structures by double contrast injection, which we divided into four groups due to anatomical differences: An accurate statement about the occurrence of certain intraluminal vascular structures in certain vascular calibres was possible only conditionally. However, complex and extended structures (group d) were found almost exclusively in larger vessel calibres (diameter>0.9mm). The structures are reminiscent of published findings in the "collector channel orifices of Schlemm's canal". They should play an important role in the regulation of the intraocular pressure, or the balance between production and outflow of the aqueous humour. The influence of such structures on the function of the lymphatic vessels is not yet known. As an approach models could be used, which for instance are applied in the water industry for the drainage, the degradation of introduced substances, or the detention pond. The latter serves for the retention and purification of drainage water (storage, treatment and reuse of drainage water). Dead zones, barriers, short-circuit currents and swirling are further hydraulic terms. Can intraluminal vascular structures, for example, affect the lymphatic flow and thus the mechano-sensitivity of lymphatic endothelial cells? Whatever interpretation model we use, the warning of the Swiss anatomist His from 1862 is still true today that all theories about the formation and movement of lymph should be based on precise anatomical basics. This review article therefore tries to make a contribution therefore. Despite knowing of lymphatic endothelial molecules, despite the discovery of the role of lymphangiogenic growth factors in diseases and the use of molecular imaging technologies, we still know too little about the anatomy and function of the lymphatic system.

Electrical Communication in Lymphangions

Contractions of lymphangions, i.e., the segment between two one-way lymphatic valves, generate the pressure gradients that propel lymph back to the circulation. Each lymphangion is comprised of an inner sheet of lymphatic endothelial cells circumscribed by one or more layers of lymphatic muscle cells (LMCs). Each contraction is produced by an LMC action potential (AP) that propagates via gap junctions along the lymphangion. Yet, electrical coupling within and between cell layers and the impact on AP waves is poorly understood. Here, we combine studies in rat and mouse lymphatic vessels with mathematical modeling to show that initiation of AP waves depends on high input resistance (low current drain), whereas propagation depends on morphology and sufficient LMC:LMC coupling. Simulations show that 1) myoendothelial coupling is insignificant to facilitate AP generation and sustain an experimentally measured cross-junctional potential difference of 25 mV, i.e., AP waves propagate along the LMC layer only; 2) LMC:LMC resistance is estimated around 2-10 MΩ but depends on vessel structure and cell-cell coupling, e.g., some degree of LMC overlap protects AP waves against LMC decoupling; 3) the propensity of AP wave initiation is highest around the valves, where the density of LMCs is low; and 4) a single pacemaker cell embedded in the LMC layer must be able to generate very large currents to overcome the current drain from the layer. However, the required current generation to initiate an AP wave is reduced upon stimulation of multiple adjacent LMCs. With stimulation of all LMCs, AP waves can also arise from heterogeneity in the electrical activity of LMCs. The findings advance our understanding of the electrical constraints that underlie initiation of APs in the LMC layer and make testable predictions about how morphology, LMC excitability, and LMC:LMC electrical coupling interact to determine the ability to initiate and propagate AP waves in small lymphatic vessels.

Experimental Models Used to Assess Lymphatic Contractile Function

Recent years have seen a renewed interest in studies of the lymphatic system. This review addresses the differences between in vivo and ex vivo methods for visualization and functional studies of lymphatic networks, with an emphasis on studies of collecting lymphatic vessels. We begin with a brief summary of the historical uses of both approaches. For the purpose of detailed comparisons, we subdivide in vivo methods into those visualizing lymphatic networks through the intact skin and those using surgically opened skin. We subdivide ex vivo methods into isobaric studies (using a pressure myograph) or isometric studies (using a wire myograph). For all four categories, we compile a comprehensive list of the advantages, disadvantages, and limitations of each preparation, with the goal of informing the research community as to the appropriate kinds of experiments best suited, and ill suited, for each.

Contraction of collecting lymphatics: organization of pressure-dependent rate for multiple lymphangions

The paper describes the extension of a previously developed model of pressure-dependent contraction rate to the case of multiple lymphangions. Mechanical factors are key modulators of active lymphatic pumping. As part of the evolution of our lumped-parameter model to match experimental findings, we have designed an algorithm whereby the time until the next contraction depends on lymphangion transmural pressure in the contraction just completed. The functional dependence of frequency on pressure is quantitatively matched to isobaric contraction experiments on isolated lymphatic segments. When each of several lymphangions is given this ability, a scheme for their coordination must be instituted to match the observed synchronization. Accordingly, and in line with an experiment on an isolated lymphatic vessel segment in which we measured contraction sequence and conduction delay, we took the fundamental principle to be that local timing can be overridden by signals to initiate contraction that start in adjacent lymphangions, conducted with a short delay. The scheme leads to retrograde conduction when the lymphangion chain is pumping against an adverse pressure difference, but antegrade conduction when contractions occur with no or a favourable pressure difference. Abolition of these conducted signals leads to chaotic variation of cycle-mean flow-rate from the chain, diastolic duration in each lymphangion, and inter-lymphangion delays. Chaotic rhythm is also seen under other circumstances. Because the model responds to increasing adverse pressure difference by increasing the repetition rate of contractions, it maintains time-average output flow-rate better than one with fixed repetition rate.

CD4+ T cells are activated in regional lymph nodes and migrate to skin to initiate lymphedema

T cell-mediated responses have been implicated in the development of fibrosis, impaired lymphangiogenesis, and lymphatic dysfunction in secondary lymphedema. Here we show that CD4+ T cells are necessary for lymphedema pathogenesis by utilizing adoptive transfer techniques in CD4 knockout mice that have undergone tail skin and lymphatic excision or popliteal lymph node dissection. We also demonstrate that T cell activation following lymphatic injury occurs in regional skin-draining lymph nodes after interaction with antigen-presenting cells such as dendritic cells. CD4+ T cell activation is associated with differentiation into a mixed T helper type 1 and 2 phenotype, as well as upregulation of adhesion molecules and chemokines that promote migration to the skin. Most importantly, we find that blocking T cell release from lymph nodes using a sphingosine-1-phosphate receptor modulator prevents lymphedema, suggesting that this approach may have clinical utility.

Mesenteric organ lymphatics and inflammatory bowel disease

Inflammatory bowel disease (IBD) is a complex gastrointestinal disorder and its etiology is unclear yet. Current theory in IBD is focused on genetics, immunity and intestinal microbes. Emerging clinical evidence and experimental results suggest that morphologic abnormalities and dysfunction of mesenteric lymphatics may have potential roles in the pathogenesis and disease course of IBD. In this review, we summarize the findings of specific investigations of the lymphatics and explore its role in IBD.

First Aid and Pre-Hospital Management of Venomous Snakebites

BACKGROUND

Antivenom is the definitive treatment for venomous snakebites, but is expensive and not available in many rural and poorly developed regions. Timely transportation to facilities that stock and administer antivenom may not be available in rural areas with poorly developed emergency medical services. These factors have led to consideration of measures to delay onset of toxicity or alternatives to antivenom therapy.

METHODS

PubMed searches were conducted for articles on snakebite treatment, or that contained first aid, emergency medical services, tourniquets, pressure immobilization bandages, suction devices, and lymphatic flow inhibitors.

RESULTS

The reviewed articles describe how venoms spread after a venomous snakebite on an extremity, list the proposed first aid measures for delaying the spread of venoms, and evaluate the scientific studies that support or refute methods of snakebite first aid. The recommendations for field treatment of venomous snakebites will be discussed.

CONCLUSIONS

The evidence suggests that pressure immobilization bandages and related strategies are the best interventions to delay onset of systemic toxicity from venomous snakebites but may increase local toxicity for venoms that destroy tissue at the site of the bite, so their use should be individualized to the circumstances and nature of the venom.

Protection from chronic stress- and depressive symptom-induced vascular endothelial dysfunction in female rats is abolished by preexisting metabolic disease

While it is known that chronic stress and clinical depression are powerful predictors of poor cardiovascular outcomes, recent clinical evidence has identified correlations between the development of metabolic disease and depressive symptoms, creating a combined condition of severely elevated cardiovascular disease risk. In this study, we used the obese Zucker rat (OZRs) and the unpredictable chronic mild stress (UCMS) model to determine the impact of preexisting metabolic disease on the relationship between chronic stress/depressive symptoms and vascular function. Additionally, we determined the impact of metabolic syndrome on sex-based protection from chronic stress/depressive effects on vascular function in female lean Zucker rats (LZRs). In general, vasodilator reactivity was attenuated under control conditions in OZRs compared with LZRs. Although still impaired, conduit arterial and resistance arteriolar dilator reactivity under control conditions in female OZRs was superior to that in male or ovariectomized (OVX) female OZRs, largely because of better maintenance of vascular nitric oxide and prostacyclin levels. However, imposition of metabolic syndrome in combination with UCMS in OZRs further impaired dilator reactivity in both vessel subtypes to a similarly severe extent and abolished any protective effect in female rats compared with male or OVX female rats. The loss of vascular protection in female OZRs with UCMS was reflected in vasodilator metabolite levels, which closely matched those in male and OVX female OZRs subjected to UCMS. These results suggest that presentation of metabolic disease in combination with depressive symptoms can overwhelm the vasoprotection identified in female rats and, thereby, may reflect a severe impairment to normal endothelial function. NEW & NOTEWORTHY This study addresses the protection from chronic stress- and depression-induced vascular dysfunction identified in female compared with male or ovariectomized female rats. We determined the impact of preexisting metabolic disease, a frequent comorbidity of clinical depression in humans, on that vascular protection. With preexisting metabolic syndrome, female rats lost all protection from chronic stress/depressive symptoms and became phenotypically similar to male and ovariectomized female rats, with comparably poor vasoactive dilator metabolite profiles.

New and Emerging Treatments for Lymphedema

Although nonoperative and operative treatments for lymphedema (LE) are well established, these procedures typically provide only partial relief from limb swelling, functional impairment, and the risk of cellulitis. The lack of a cure for LE, however, is due to an incomplete understanding of the underlying pathophysiological mechanisms, and current research efforts are focusing on elucidating these processes to provide new, targeted therapies for this prevalent disease for which there is no cure. This article reviews the current literature regarding the pathophysiological mechanisms that underlie LE, as well as new and emerging therapies for the condition.

Differences in L-type Ca2+ channel activity partially underlie the regional dichotomy in pumping behavior by murine peripheral and visceral lymphatic vessels

We identified a regional dichotomy in murine lymphatic contractile function with regard to vessel location within the periphery or visceral cavity. All vessels isolated from peripheral regions [cervical, popliteal, inguinal, axillary, and internodal inguinal axillary (Ing-Ax)] developed robust contractions with maximal ejection fractions (EFs) of 50-80% in our ex vivo isobaric myograph experiments. Conversely, vessels isolated from the visceral cavity (mesenteric, thoracic duct, and iliac) demonstrated maximal EFs of ≤10%. Using pressure myography, sharp electrode membrane potential recordings, and Ca²⁺ imaging, we assessed the role of L-type Ca²⁺ channels in this contractile dichotomy. Ing-Ax membrane potential revealed a ~2-s action potential (AP) cycle (resting -35 mV, spike -5 mV, and plateau -11 mV) with a plateau phase that was significantly lengthened by the L-type Ca²⁺ channel agonist Bay K8644 (BayK; 100 nM). APs recorded from mesenteric vessels, however, displayed a slower upstroke and an elongated time over threshold. BayK (100 nM) increased the mesenteric AP upstroke velocity and plateau duration but also significantly hyperpolarized the vessel. Contractions of vessels from both regions were preceded by Ca²⁺ flashes, detected with a smooth muscle-specific endogenous Ca²⁺ reporter, that typically were coordinated over the length of the vessel. Similar to the membrane potential recordings, Ca²⁺ flashes in mesenteric vessels were weaker and had a slower rise time but were longer lasting than those in Ing-Ax vessels. BayK (100 nM) significantly increased the Ca²⁺ transient amplitude and duration in both vessels and decreased time to peak Ca²⁺ in mesenteric vessels. However, a higher concentration (1 μM) of BayK was required to produce even a modest increase in EF in visceral lymphatics, which remained at <20%. NEW & NOTEWORTHY Lymphatic collecting vessels isolated from murine peripheral tissues, but not from the visceral cavities, display robust contractile behavior similar to lymphatic vessels from other animal models and humans. These differences are partially explained by L-type Ca²⁺ channel activity as revealed by the first measurements of murine lymphatic action potentials and contraction-associated Ca²⁺ transients.

Lymphatic System Flows

The supply of oxygen and nutrients to tissues is performed by the blood system, and involves a net leakage of fluid outward at the capillary level. One of the principal functions of the lymphatic system is to gather this fluid and return it to the blood system to maintain overall fluid balance. Fluid in the interstitial spaces is often at subatmospheric pressure, and the return points into the venous system are at pressures of approximately 20 cmH₂O. This adverse pressure difference is overcome by the active pumping of collecting lymphatic vessels, which feature closely spaced one-way valves and contractile muscle cells in their walls. Passive vessel squeezing causes further pumping. The dynamics of lymphatic pumping have been investigated experimentally and mathematically, revealing complex behaviours indicating that the system performance is robust against minor perturbations in pressure and flow. More serious disruptions can lead to incurable swelling of tissues called lymphœdema.

Lymphatic System Flows

The supply of oxygen and nutrients to tissues is performed by the blood system, and involves a net leakage of fluid outward at the capillary level. One of the principal functions of the lymphatic system is to gather this fluid and return it to the blood system to maintain overall fluid balance. Fluid in the interstitial spaces is often at subatmospheric pressure, and the return points into the venous system are at pressures of approximately 20 cmH₂O. This adverse pressure difference is overcome by the active pumping of collecting lymphatic vessels, which feature closely spaced one-way valves and contractile muscle cells in their walls. Passive vessel squeezing causes further pumping. The dynamics of lymphatic pumping have been investigated experimentally and mathematically, revealing complex behaviours indicating that the system performance is robust against minor perturbations in pressure and flow. More serious disruptions can lead to incurable swelling of tissues called lymphœdema.

Magnetic resonance thoracic ductography assessment of serial changes in the thoracic duct after the intake of a fatty meal

The thoracic duct, a terminal lymph vessel, is thought to dilate after the intake of a fatty meal. However, this physiological change has not been well explored in vivo. Therefore, the present study aimed to assess serial changes in the thoracic duct after the intake of a fatty meal using magnetic resonance thoracic ductography (MRTD). Eight healthy volunteers were subjected to one MRTD scan before a fatty meal and eight serial MRTD scans every hour thereafter. The cross-sectional areas of the thoracic duct were estimated using MRTD measurements of the diameters of the thoracic duct at the upper edge of the aortic arch, the tracheal bifurcation, the mid-point between the tracheal bifurcation and the left part of the diaphragm and the left part of the diaphragm. The change-rates in these areas were calculated before and after the fatty meal intake, and the maximal change-rate and timing of its achievement were determined for each subject. The summed change-rates in the four portions of the thoracic duct ranged from -40.1 to 81.3%, with maximal change-rates for each subject ranging from 22.8 to 81.3% (mean, 50.4%). Although individual variations were observed, most subjects (88.9%) exhibited a maximal change-rate at 4-6 h after meal intake, with subsequent decreases at 7-8 h. In conclusion, MRTD revealed a tendency toward thoracic duct enlargement at 4-6 h after the intake of a fatty meal, followed by contraction.

Lymphangiogenesis Facilitates Initial Lymph Formation and Enhances the Dendritic Cell Mobilizing Chemokine CCL21 Without Affecting Migration

Objective—

Lymphatic vessels play an important role in body fluid, as well as immune system homeostasis. Although the role of malfunctioning or missing lymphatics has been studied extensively, less is known on the functional consequences of a chronically expanded lymphatic network or lymphangiogenesis.

Approach and Results—

To this end, we used K14-VEGF-C (keratin-14 vascular endothelial growth factor-C) transgenic mice overexpressing the vascular endothelial growth factor C in skin and investigated the responses to inflammatory and fluid volume challenges. We also recorded interstitial fluid pressure, a major determinant of lymph flow. Transgenic mice had a strongly enhanced lymph vessel area in skin. Acute inflammation induced by lipopolysaccharide and chronic inflammation by delayed-type hypersensitivity both resulted in increased interstitial fluid pressure and reduced lymph flow, both to the same extent in wild-type and transgenic mice. Hyperplastic lymphatic vessels, however, demonstrated enhanced transport capacity after local fluid overload not induced by inflammation. In this situation, interstitial fluid pressure was increased to a similar extent in the 2 strains, thus, suggesting that the enhanced lymph vessel area facilitated initial lymph formation. The increased lymph vessel area resulted in an enhanced production of the chemoattractant CCL21 that, however, did not result in augmented dendritic cell migration after induction of local skin inflammation by fluorescein isothiocyanate.

Conclusions—

An expanded lymphatic network is capable of enhanced chemoattractant production, and lymphangiogenesis will facilitate initial lymph formation favoring increased clearance of fluid in situations of augmented fluid filtration.

A vicious circle in chronic lymphoedema pathophysiology? An adipocentric view

Chronic lymphoedema is a disease caused by a congenital or acquired damage to the lymphatic system and characterized by complex chains of pathophysiologic events such as lymphatic fluid stasis, chronic inflammation, lymphatic vessels impairment, adipose tissue deposition and fibrosis. These events seem to maintain and reinforce themselves through a positive feedback loop: regardless of the initial cause of lymphatic stasis, the dysfunctional adipose tissue and its secretion products can worsen lymphatic vessels' function, aggravating lymph leakage and stagnation, which can promote further adipose tissue deposition and fibrosis, similar to what may happen in obesity. In addition to the current knowledge about the tight and ancestral interrelation between immunity system and metabolism, there is evidence for similarities between obesity-related and lymphatic damage-induced lymphoedema. Together, these observations indicate strong reciprocal relationship between lymphatics and adipose tissue and suggest a possible key role of the adipocyte in the pathophysiology of chronic lymphoedema's vicious circle.

Photoacoustic imaging of lymphatic pumping

The lymphatic system is responsible for fluid homeostasis and immune cell trafficking and has been implicated in several diseases, including obesity, diabetes, and cancer metastasis. Despite its importance, the lack of suitable in vivo imaging techniques has hampered our understanding of the lymphatic system. This is, in part, due to the limited contrast of lymphatic fluids and structures. Photoacoustic imaging, in combination with optically absorbing dyes or nanoparticles, has great potential for noninvasively visualizing the lymphatic vessels deep in tissues. Multispectral photoacoustic imaging is capable of separating the components; however, the slow wavelength switching speed of most laser systems is inadequate for imaging lymphatic pumping without motion artifacts being introduced into the processed images. We investigate two approaches for visualizing lymphatic processes in vivo. First, single-wavelength differential photoacoustic imaging is used to visualize lymphatic pumping in the hindlimb of a mouse in real time. Second, a fast-switching multiwavelength photoacoustic imaging system was used to assess the propulsion profile of dyes through the lymphatics in real time. These approaches may have profound impacts in noninvasively characterizing and investigating the lymphatic system.

Intestinal lymphatic vasculature: structure, mechanisms and functions

The mammalian intestine is richly supplied with lymphatic vasculature, which has functions ranging from maintenance of interstitial fluid balance to transport of antigens, antigen-presenting cells, dietary lipids and fat-soluble vitamins. In this Review, we provide in-depth information concerning the organization and structure of intestinal lymphatics, the current view of their developmental origins, as well as molecular mechanisms of intestinal lymphatic patterning and maintenance. We will also discuss physiological aspects of intestinal lymph flow regulation and the known and emerging roles of intestinal lymphatic vessels in human diseases, such as IBD, infection and cancer.

The passive biomechanics of human pelvic collecting lymphatic vessels

The lymphatic system has a major significance in the metastatic pathways in women's cancers. Lymphatic pumping depends on both extrinsic and intrinsic mechanisms, and the mechanical behavior of lymphatic vessels regulates the function of the system. However, data on the mechanical properties and function of human lymphatics are lacking. Our aim is to characterize, for the first time, the passive biomechanical behavior of human collecting lymphatic vessels removed at pelvic lymph node dissection during primary debulking surgeries for epithelial ovarian cancer. Isolated vessels were cannulated and then pressurized at varying levels of applied axial stretch in a calcium-free Krebs buffer. Pressurized vessels were then imaged using multi-photon microscopy for collagen-elastin structural composition and fiber orientation. Both pressure-diameter and force-elongation responses were highly nonlinear, and axial stretching of the vessel served to decrease diameter at constant pressure. Pressure-diameter behavior for the human vessels is very similar to data from rat mesenteric vessels, though the human vessels were approximately 10× larger than those from rats. Multiphoton microscopy revealed the vessels to be composed of an inner layer of elastin with an outer layer of aligned collagen fibers. This is the first study that successfully described the passive biomechanical response and composition of human lymphatic vessels in patients with ovarian cancer. Future work should expand on this knowledge base with investigations of vessels from other anatomical locations, contractile behavior, and the implications on metastatic cell transport.

Inflammatory Manifestations of Lymphedema

Lymphedema results from lymphatic insufficiency leading to a progressive inflammatory process that ultimately manifests as discomfort, recurrent infections, and, at times, secondary malignancy. Collectively, these morbidities contribute to an overall poor quality of life. Although there have been recent advances in microsurgical interventions, a conservative palliative approach remains the mainstay of treatment for this disabling disease. The absence of a cure is due to an incomplete understanding of the pathophysiological changes that result in lymphedema. A histological hallmark of lymphedema is inflammatory cell infiltration and recent studies with animal models and clinical biopsy specimens have suggested that this response plays a key role in the pathology of the disease. The purpose of this report is to provide an overview of the ongoing research in and the current understanding of the inflammatory manifestations of lymphedema.

Edema and pain reduction using transcutaneous electrical nerve stimulation treatment

[Purpose] The purpose of this study was to investigate the impact on the edema and pain when applying transcutaneous electrical nerve stimulation. [Subjects and Methods] Eleven patients who were diagnosed with lymphedema were selected as the subjects of the study. The experimental group received transcutaneous electrical nerve stimulation treatment on edema regions three times per week for four weeks. Surface tape measurement was used to measure changes in lower extremity edema. Pain intensity was measured using the visual analog scale. [Results] The edema decrements in the experimental group were significantly larger than those in the control group. The pain decrements in the experimental group were significantly larger than those in the control group. [Conclusion] In conclusion, application of transcutaneous electrical nerve stimulation was confirmed to be effective in reducing edema and pain.

The Lymphatic System: Integral Roles in Immunity

The lymphatic vasculature is not considered a formal part of the immune system, but it is critical to immunity. One of its major roles is in the coordination of the trafficking of antigen and immune cells. However, other roles in immunity are emerging. Lymphatic endothelial cells, for example, directly present antigen or express factors that greatly influence the local environment. We cover these topics herein and discuss how other properties of the lymphatic vasculature, such as mechanisms of lymphatic contraction (which immunologists traditionally do not take into account), are nonetheless integral in the immune system. Much is yet unknown, and this nascent subject is ripe for exploration. We argue that to consider the impact of lymphatic biology in any given immunological interaction is a key step toward integrating immunology with organ physiology and ultimately many complex pathologies.