Mechanical forces in lymphatic vascular development and disease

The lymphatic vasculature is essential for fluid homeostasis and transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a hierarchical network of blind-ended lymphatic capillaries and collecting lymphatic vessels, both lined by lymphatic endothelial cells (LECs). The low hydrostatic pressure in lymphatic capillaries, their loose intercellular junctions, and attachment to the surrounding extracellular matrix (ECM) permit passage of extravasated blood plasma from the interstitium into the lumen of the lymphatic capillaries. It is generally thought that interstitial fluid accumulation leads to a swelling of the ECM, to which the LECs of lymphatic capillaries adhere, for example via anchoring filaments. As a result, LECs are pulled away from the vascular lumen, the junctions open, and fluid enters the lymphatic vasculature. The collecting lymphatic vessels then gather the plasma fluid from the capillaries and carry it through the lymph nodes to the blood circulation. The collecting vessels contain intraluminal bicuspid valves that prevent fluid backflow, and are embraced by smooth muscle cells that contribute to fluid transport. Although the lymphatic vessels are regular subject to mechanical strain, our knowledge of its influence on lymphatic development and pathologies is scarce. Here, we discuss the mechanical forces and molecular mechanisms regulating lymphatic vascular growth and maturation in the developing mouse embryo. We also consider how the lymphatic vasculature might be affected by similar mechanomechanisms in two pathological processes, namely cancer cell dissemination and secondary lymphedema.     

Ultrastructural and seasonal aspects of the kidney lymphatic system of hibernating animals

The kidney lymphatic system of bat, dormouse and marmot consists of intraparenchymal (interlobar, arcuate, interlobular) and extraparenchymal (capsular) vessels sharing common ultrastructural aspects. We did not observe medullary lymphatics. The qualitative and quantitative seasonal changes in the ultrastructure of the lymphatic endothelium represent not only a species-linked feature but also (and mainly) an evident seasonal fluctuation in lymph formation. Furthermore, these ultrastructural changes emphasize the important role played by the different mechanisms involved in the translymphatic movement of proteins and interstitial fluid with particular regard to the 'vesicular route' and intraendothelial channels.     

Ultrastructural and seasonal aspects of the kidney lymphatic system of hibernating animals

Summary The kidney lymphatic system of bat, dormouse and marmot consists of intraparenchymal (interlobar, arcuate, interlobular) and extraparenchymal (capsular) vessels sharing common ultrastructural aspects. We did not observe medullary lymphatics. The qualitative and quantitative seasonal changes in the ultrastructure of the lymphatic endothelium represent not only a species-linked feature but also (and mainly) an evident seasonal fluctuation in lymph formation. Furthermore, these ultrastructural changes emphasize the important role played by the different mechanisms involved in the translymphatic movement of proteins and interstitial fluid with particular regard to the vesicular route and intraendothelial channels.     

Regulatory circuits controlling vascular cell calcification

Vascular calcification is a common feature of chronic kidney disease, cardiovascular disease, and aging. Such abnormal calcium deposition occurs in medial and/or intimal layers of blood vessels as well as in cardiac valves. Once considered a passive and inconsequential finding, the presence of calcium deposits in the vasculature is widely accepted as a predictor of increased morbidity and mortality. Recognition of the importance of vascular calcification in health is driving research into mechanisms that govern its development, progression, and regression. Diverse, but highly interconnected factors, have been implicated, including disturbances in lipid metabolism, oxidative stress, inflammatory cytokines, and mineral and hormonal balances, which can lead to formation of osteoblast-like cells in the artery wall. A tight balance of procalcific and anticalcific regulators dictates the extent of disease. In this review, we focus on the main regulatory circuits modulating vascular cell calcification.     

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

Congenital heart defects affect approximately 1–5 % of human newborns each year, and of these cardiac defects 20–30 % are due to heart valve abnormalities. Recent literature indicates that the key factors and pathways that regulate valve development are also implicated in congenital heart defects and valve disease. Currently, there are limited options for treatment of valve disease, and therefore having a better understanding of valve development can contribute critical insight into congenital valve defects and disease. There are three major signaling pathways required for early specification and initiation of endothelial-to-mesenchymal transformation (EMT) in the cardiac cushions: BMP, TGF-β, and Notch signaling. BMPs secreted from the myocardium set up the environment for the overlying endocardium to become activated; Notch signaling initiates EMT; and both BMP and TGF-β signaling synergize with Notch to promote the transition of endothelia to mesenchyme and the mesenchymal cell invasiveness. Together, these three essential signaling pathways help form the cardiac cushions and populate them with mesenchyme and, consequently, set off the cascade of events required to develop mature heart valves. Furthermore, integration and cross-talk between these pathways generate highly stratified and delicate valve leaflets and septa of the heart. Here, we discuss BMP, TGF-β, and Notch signaling pathways during mouse cardiac cushion formation and how they together produce a coordinated EMT response in the developing mouse valves.     

The functioning and interrelationships of blood capillaries and lymphatics

Summary the structure and function of blood capillaries, as related to permeability, depends on tight, close and (in injured vessels) open junctional regions, small vesicles, vacuoles (in injured vessels) and fenestrae. The basement membrane presents a hindrance to the larger macromolecules, at high flow rates, but not to small molecules. The connective tissue channels are probably the paths by which macromolecules, and most of the small ones, pass from the arterial-limbs to the venous ones, and to the lymphatics. In some regions these channels are grouped in special systems: the prelymphatics. The initial lymphatics take up material via open junctions, which close during tissue-compression. The collecting lymphatics retain the lymph because they do not have open junctions.In the close junctional regions the motive force for water flow is the result of Starling's forces; diffusion is very important for other small molecules. The small vesicles transport macromolecules slowly by Brownian motion, as may the vacuoles, but possibly these latter are moved actively.There is much evidence that colloids can develop high effective osmotic pressures even across pores much larger than their molecules, and that proteins can be dragged up a concentration gradient by the resultant fluid flow. On the basis of this, hypotheses have been developed about the functioning of venous-limb fenestrae and the initial lymphatics, for which there is much theoretical, in vitro, and in vivo evidence. Thus, in fenestrae and regions there is held to be a large local circulation through the tissues, of which a quantitatively small, but qualitatively vital, part goes to the lymphatics. Material is considered usually to enter these latter because of the relative concentration of the lymph.It is becoming increasingly evident that in the study of the microvasculature, as with other systems, there is much to be gained by quantifying fine structural observations and by combining and contrasting this data, via physical laws, with that obtained by other methods where the characteristics of whole organs and regions are studied. Thus one can obtain interrelated information, which is not possible by either method alone, and which gives us a vital, comprehensive, perspective of the ways in which whole systems function, and how different systems interact. In this paper I shall show how this approach has yielded much that is new about the functioning of different kinds of blood capillaries, of the tissue channels, of the whole lymphatic system, and of the ways they affect each other.Most of the author's work reported here was financed by the Australian Research Grants Committee.     

Decrease of mast cells in regional lymph nodes in response to allogeneic antigens and syngeneic tumor antigens

Summary The absolute number of mast cells in regional lymph nodes decreases on the 5th day after stimulation by allogeneic lymphocytes and semisyngeneic leukaemic cells, despite the enlargement of stimulated lymph nodes. It is postulated that the reaction of lymphatic mast cells could be a sensitive test for tissue incompatibility, and probably also for the presence of tumor associated antigens.     

On the lymphatic spread of cancer

Summary Experimental demonstration that agents promoting cellular motility and/or vascular permeability enhance the spread of tumour to regional or distant lymph nodes. Tumour emboli appear to reach the regional glands via the lymphatic channels and the distant nodes by haematogenous route.     

On the lymphatic spread of cancer

Experimental demonstration that agents promoting cellular motility and/or vascular permeability enhance the spread of tumour to regional or distant lymph nodes. Tumour emboli appear to reach the regional glands via the lymphatic channels and the distant nodes by haematogenous route.     

Alzheimer’s disease and CADASIL are heritable,adult-onset dementias that both involve damaged small blood vessels

This essay explores an alternative pathway to Alzheimer’s dementia that focuses on damage to small blood vessels rather than late-stage toxic amyloid deposits as the primary pathogenic mechanism that leads to irreversible dementia. While the end-stage pathology of AD is well known, the pathogenic processes that lead to disease are often assumed to be due to toxic amyloid peptides that act on neurons, leading to neuronal dysfunction and eventually neuronal cell death. Speculations as to what initiates the pathogenic cascade have included toxic abeta peptide aggregates, oxidative damage, and inflammation, but none explain why neurons die. Recent high-resolution NMR studies of living patients show that lesions in white matter regions of the brain precede the appearance of amyloid deposits and are correlated with damaged small blood vessels. To appreciate the pathogenic potential of damaged small blood vessels in the brain, it is useful to consider the clinical course and the pathogenesis of CADASIL, a heritable arteriopathy that leads to damaged small blood vessels and irreversible dementia. CADASIL is strikingly similar to early onset AD in that it is caused by germ line mutations in NOTCH 3 that generate toxic protein aggregates similar to those attributed to mutant forms of the amyloid precursor protein and presenilin genes. Since NOTCH 3 mutants clearly damage small blood vessels of white matter regions of the brain that lead to dementia, we speculate that both forms of dementia may have a similar pathogenesis, which is to cause ischemic damage by blocking blood flow or by impeding the removal of toxic protein aggregates by retrograde vascular clearance mechanisms.     

Neurovascular development and links to disease

The developing central nervous system (CNS) is vascularized via ingression of blood vessels from the outside as the neural tissue expands. This angiogenic process occurs without perturbing CNS architecture due to exquisite cross-talk between the neural compartment and invading blood vessels. Subsequently, this intimate relationship also promotes the formation of the neurovascular unit that underlies the blood–brain barrier and regulates blood flow to match brain activity. This review provides a historical perspective on research into CNS blood vessel growth and patterning, discusses current models used to study CNS angiogenesis, and provides an overview of the cellular and molecular mechanisms that promote blood vessel growth and maturation. Finally, we highlight the significance of these mechanisms for two different types of neurovascular CNS disease.     

The role of platelets in the pathogenesis of cerebral malaria

Malaria is a major cause of morbidity and mortality in the developing world and cerebral malaria is responsible for the majority of malaria-associated deaths. There is a strong association between thrombocytopenia and outcome in malaria, suggesting a role for platelets in the pathogenesis of malaria. This thrombocytopenia is likely due to platelet activation possibly through an interaction between PfEMP1 on plasmodium and CD36 on platelets. Platelet activation by plasmodium has two potential consequences. It can lead to the formation of micro-aggregates of infected red blood cells and platelets which can occlude blood vessels and it also leads to binding to and activation of the endothelium.     

Amyloid fibrils from the viewpoint of protein folding

In amyloid related diseases, proteins form fibrillar aggregates with highly ordered -sheet structure regardless of their native conformations. Formation of such amyloid fibrils can be reproducible in vitro using isolated proteins/peptides, suggesting that amyloid fibril formation takes place as a result of protein conformational change. In vitro studies revealed that perturbation of the native structure is important for the fibril formation, and it is suggested that the mechanisms of amyloid fibril formation share the mechanisms of protein folding. In particular, amyloid fibril formation is similar to one of the common features of proteins, i.e. amorphous aggregation upon partial unfolding, which is likely driven by hydrophobic interactions through exposed protein interior. However, these molecular associations are distinct phenomena, and identifying factors that lead to amyloid fibril formation would precede our understanding of the mechanisms of amyloid fibrillization. The necessity of understanding the nature of protein denatured states is also suggested.Received 6 July 2003; accepted 19 August 2003     

Lysophosphatidic acid up-regulates vascular endothelial growth factor-C and lymphatic marker expressions in human endothelial cells

Lysophosphatidic acid (LPA) is a low-molecular-weight lipid growth factor, which binds to G-protein-coupled receptors. Previous studies have shown that LPA enhances vascular endothelial growth factor-A (VEGF-A) expression in cancer cells and promotes angiogenesis process. However, the roles of LPA in lymphatic vessel formation and lymphangiogenesis have not been investigated. Here, we demonstrated that LPA up-regulated VEGF-C mRNA and protein expressions in human umbilical vein endothelial cells (HUVECs). Furthermore, the expression levels of lymphatic markers, including Prox-1, LYVE-1 and podoplanin, were enhanced in LPA-stimulated tube forming endothelial cells in vitro and in vivo. Moreover, we showed that pretreatment with MAZ51, a VEGFR-3 kinase inhibitor, and introduction of VEGFR-3 siRNA suppressed LPA-induced HUVEC tube formation and lymphatic marker expressions. These results demonstrated that LPA enhances expression of lymphatic markers through activating VEGF-C receptors in endothelial cells. This study provides basic information that LPA might be a target for therapeutics against lymphangiogenesis and tumor metastasis.     

Access of reabsorbed glucose to renal lymph

Summary Measurements of the glucose to mannitol tracer concentration ratios in renal venous blood and renal lymph of rats supported the hypothesis that reabsorbed glucose may have direct access to renal lymph by passage through the interstitium.Acknowledgment. This research was supported by grant No. AM-17093 of the USPHS.     

Contrôle direct,continu, simultané et in situ du taux d'oxyhémoglobine en milieux artériel et veineux profonds chez le Mammifère

Summary An original instrument, the probe-photometer, permits us to evaluate instantaneously, continuously, in situ and simultaneously into arterial and venous vessels the relative amount of Hb-Hb O₂. The CO₂ reflex described previously can also be fully recorded in terms of hemoglobin.     

Molecular mechanisms of lymphatic vascular development

Lymphatic vasculature has recently emerged as a prominent area in biomedical research because of its essential role in the maintenance of normal fluid homeostasis and the involvement in pathogenesis of several human diseases, such as solid tumor metastasis, inflammation and lymphedema. Identification of lymphatic endothelial specific markers and regulators, such as VEGFR-3, VEGF-C/D, PROX1, podoplanin, LYVE-1, ephrinB2 and FOXC2, and the development of mouse models have laid a foundation for our understanding of the major steps controlling growth and remodeling of lymphatic vessels. In this review we summarize recent advances in the field and discuss how this knowledge as well as use of model organisms, such as zebrafish and Xenopus, should allow further in depth analysis of the lymphatic vascular system. Received 26 January 2007; received after revision 5 March 2007; accepted 29 March 2007     

Lamellated sensory nerve endings in guinea-pig adrenals

Summary Lamellated sensory nerve endings were observed in the adrenal gland of guinea-pigs located at the corticomedullary junction close to venous blood vessels. We suggest that they form the afferents of a system contributing to a local regulation of adrenal blood flow.