Renal lymphatics

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

Renal Lymphatics: Anatomy, Physiology, and Clinical Implications

Renal lymphatics are abundant in the cortex of the normal kidney but have been largely neglected in discussions around renal diseases. They originate in the substance of the renal lobule as blind-ended initial capillaries, and can either follow the main arteries and veins toward the hilum, or penetrate the capsule to join capsular lymphatics. There are no valves present in interlobular lymphatics, which allows lymph formed in the cortex to exit the kidney in either direction. There are very few lymphatics present in the medulla. Lymph is formed from interstitial fluid in the cortex, and is largely composed of capillary filtrate, but also contains fluid reabsorbed from the tubules. The two main factors that contribute to renal lymph formation are interstitial fluid volume and intra-renal venous pressure. Renal lymphatic dysfunction, defined as a failure of renal lymphatics to adequately drain interstitial fluid, can occur by several mechanisms. Renal lymphatic inflow may be overwhelmed in the setting of raised venous pressure (e.g., cardiac failure) or increased capillary permeability (e.g., systemic inflammatory response syndrome). Similarly, renal lymphatic outflow, at the level of the terminal thoracic duct, may be impaired by raised central venous pressures. Renal lymphatic dysfunction, from any cause, results in renal interstitial edema. Beyond a certain point of edema, intra-renal collecting lymphatics may collapse, further impairing lymphatic drainage. Additionally, in an edematous, tense kidney, lymphatic vessels exiting the kidney via the capsule may become blocked at the exit point. The reciprocal negative influences between renal lymphatic dysfunction and renal interstitial edema are expected to decrease renal function due to pressure changes within the encapsulated kidney, and this mechanism may be important in several common renal conditions.

Lymphangiogenesis in renal diseases: passive bystander or active participant?

Lymphatic vessels (LVs) are involved in a number of physiological and pathophysiological processes such as fluid homoeostasis, immune surveillance, and resolution of inflammation and wound healing. Lymphangiogenesis, the outgrowth of existing LVs and the formation of new ones, has received increasing attention over the past decade on account of its prominence in organ physiology and pathology, which has been enabled by the development of specific tools to study lymph vessel functions. Several studies have been devoted to renal lymphatic vasculature and lymphangiogenesis in kidney diseases, such as chronic renal transplant dysfunction, primary renal fibrotic disorders, proteinuria, diabetic nephropathy and renal inflammation. This review describes the most recent findings on lymphangiogenesis, with a specific focus on renal lymphangiogenesis and its impact on renal diseases. We suggest renal lymphatics as a possible target for therapeutic interventions in renal medicine to dampen tubulointerstitial tissue remodelling and improve renal functioning.

The role of lymphatics in renal inflammation

Progressive renal diseases are characterized by tubulointerstitial inflammatory cell recruitment, tubular atrophy and fibrosis. Various aspects of the recruitment of leukocytes have been extensively studied, but the exit routes (i.e. the lymphatic vessels and their biology) have only recently found attention. Similar to the recruitment of inflammatory cells, the exit is coordinated by an orchestrated interaction of chemotactic cytokines and adhesion molecules. During inflammatory injury, new routes are created by the de novo formation of lymphatic vessels, i.e. neolymphangiogenesis. These newly formed lymphatic vessels help to cope with the increase in interstitial fluid related to inflammation. Here, we review some aspects of lymphatic biology and the current knowledge about lymphatic vessels in renal inflammation.

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.

Extracapillary tumourous metastatic crescents in glomeruli of the kidney

This report describes intraglomerular tumor cell metastases, as a rare pathologic observation. Specimens of one hundred autopsy cases (each having extrarenal carcinoma) were examined by light microscopy, and in seven of them metastatic crescents were found in the urinary space of the Bowman's capsule. The capsule itself and the layer of the parietal epithelium seemed to be intact. Glomeruli with metastases in five cases were near or inside the extraglomerular metastatic tumor mass. This phenomenon must be distinguished from metaplastic transformation of the glomerular capsular epithelium.

The renal cortical lymphatic system in the rat, hamster, and rabbit

Rat, hamster, and rabbit renal cortical lymphatics were examined by light and electron microscopy. Rat and hamster kidneys possessed both intra- and interlobular lymphatics that were structurally similar at the light microscopic level. Ultrastructural examination of the hamster lymphatic endothelium, however, revealed an unusual arrangement of cytoplasmic extensions not seen in the other two species. The intralobular lymphatics were related primarily to tubules, afferent arterioles, and renal corpuscles and were consistent with lymph formation from both plasma filtrate and tubular reabsorbate. Interlobular lymphatics were seen in connective tissue associated with the interlobular blood vessels. Rabbit cortex contained only interlobular lymphatics. Cross-sectional area, maximum diameter, volume density, and profile density were determined by stereological measurements using a computer-based image analyzer. The morphological data from the rat were used, in combination with published values for lymph flow, to calculate the rate of lymph formation per unit area of endothelium in lymphatics of the renal cortex. Among kidneys fixed by retrograde perfusion, the cortical lymphatic system was most extensive in maximum diameter, volume density, and profile density. It was smallest in the rabbit and intermediate in the rat. Lower volume and profile density were found for rat kidneys fixed by the dripping technique. It was concluded that: tubular reabsorbate probably contributes to renal lymph in the rat and hamster, but not in the rabbit; significant differences exist in the extent of the renal lymphatic systems among the three species, with the hamster kidney having the richest network and the rabbit the poorest; the method of fixation influences the measured size and density of renal cortical lymphatics; and the estimated rate of lymph formation in the kidney of the rat is roughly comparable to that in the dog.

Intraglomerular metastases. Report of two cases

Glomerular metastases are rarely observed. Two cases of such metastases are reported. In one patient, proteinuria was detected and the diagnosis was made by kidney needle biopsy; light microscopy, immunofluorescence, and electron microscopy studies were performed. The second case was an autopsy finding. The histologic patterns were, respectively, an intracapillary metastasis and associated intra- and extracapillary metastases. These findings raise a number of questions concerning the early detection of such neoplastic diseases, the nature of primary tumors, and the mechanisms of malignant dissemination and glomerular localization.

The renal cortical lymphatic system in dogs with unimpeded lymph and urine flow

The distribution and extent of the lymphatic circulation in the renal cortex was analyzed in three dogs under conditions of unimpeded lymph and urine flow. The kidneys were drip fixed with acrolein in vivo, and cortical tissue strips were prepared for light and electron microscopic examination. Analysis of 90 tissue strips revealed 38 cortical lymphatics, one third of which were intralobular in position. The intralobular lymphatic capillaries were related primarily to tubules, afferent arterioles, or renal corpuscles. The remainder of the lymphatics were located in interlobular connective tissue areas in association with the interlobular blood vessels. Interlobular lymphatics had a surface area twice that of intralobular vessels. Stereological analysis was used to estimate the volume density of the components of the renal cortex. The volume density of lymphatics was found to be 0.0014, but because of the relative infrequency of lymphatics, this value was considered to be approximate. The volume density data for non-lymphatic renal components were found to be in close agreement with published data. From these volume density values it was concluded that the volume of cortical lymph in a functioning dog kidney is equivalent to about 1% of the volume of blood in the cortical peritubular capillaries.

Distribution and density of the canine renal cortical lymphatic system

The pattern, distribution, and extent of the lymphatic circulation in the canine renal cortex was studied with light and electron microscopy, in two groups of animals, one with and one without ipsilateral ureteric obstruction for 3 days. Recognition of lymphatics in tissue sections was facilitated by mild dilatation, induced in both groups by ligation of the renal collecting vessels for 4 to 6 hours, and by retrograde injection of tracer in a third group. Of 77 lymphatics present in 180 blocks from six kidneys, approximately one third were intralobular, the remainder being primarily associated with interlobular blood vessels. The cross-sectional area of interlobular lymphatics was almost twice that of intralobular lymphatics. The relationships of these lymphatics were analyzed quantitatively. Intralobular lymphatics had primary relationships with terminal arteries, arterioles, renal corpuscles, and tubular elements. Both inter- and intralobular lymphatics had secondary relationships with a small proportion of all components of the cortical parenchyma including juxtaglomerular complexes. The most common association was between lymphatics and elements of the vascular tree. Morphometric analysis was used to obtain volume density data on the composition of the renal cortex. The volume density of lymphatics was 0.0026 in ureter-obstructed kidneys and 0.0017 in nonobstructed kidneys. The cross-sectional surface area of lymphatics in ureter-obstructed kidneys was significantly larger than those in nonobstructed kidneys. The volume density of other cortical components was found to be in good agreement with published data. From the volume density data, it was concluded that the volume of lymph in the renal cortex, under conditions of mild lymphatic dilatation, was about 1% that of the volume of blood in the cortical peritubular capillaries.

Morphology of the intrarenal lymphatic system. Capsular and hilar communications

Recent functional evidence has indicated that ureteric obstruction for three or more days in dogs causes a diversion of renal lymph from the hilar to the capsular system. The present study was concerned with the structural correlate of this functional evidence, having special reference to communications between the two systems. Within three days of ureteric occlusion the capsular lymphatic system became dilated. On histological examination two types of tributaries were found. One (termed a perforating lymphatic) served as a primary pathway for superficial cortical lymph from the subcapsular plexus, and penetrated the capsule either alone or in company with a small vein. The other (termed a communicating lymphatic) was closely associated with the occasional penetrating interlobular blood vessel which traversed the capsule to ramify in the perirenal tissue. Approximately 60% of the penetrating arteries in eight dog kidneys had associated communicating lymphatics. On the renal surface the perforating and communicating lymphatics formed the primary collecting vessels of the capsular system. Within the renal substance the communicating lymphatics were directly continuous with lymphatics which surrounded the interlobular blood vessels and which have been shown to drain into the hilar network. Thus the communicating lymphatics formed direct connections between the hilar and capsular systems. It was concluded that these communications, although dilated by hydronephrosis, existed under control conditions. Functionally they are probably of importance only under special circumstances when intrarenal lymph may be diverted from one to the other system.

Lymphatic network of kidney. I. Anatomic and physiologic considerations

A small number of investigators, including our team, has been responsible during the past two decades for many studies focusing on the kidney lymphatics. Interesting new information and concepts have been published, but unfortunately some of these data are scattered in obscure journals. The major purpose of this, and three subsequent reviews, is to present these new concepts in a more organized fashion. The reviews will also allow our group an opportunity to emphasize physiologic principles which have been ignored in the past. In a future review we will emphasize the role of the immunoblast, found only in kidney lymph and responsible for rejecting the renal allograft. Azathioprine (Imuran), if given early and in adequate dosage, will reduce the role of these transformed lymphocytes. Still another review article will emphasize the importance of renal tissue concentrations of antibacteria in eradicating acute pyelonephritis. Blood and urine levels of antibiotics are also important in eradicating pyelonephritis. Since renal lymph represents interstitial fluid, the concentration of appropriate bacteria-sensitive antibacterials is probably more important. We have now evaluated fourteen antibacterials individually in the mongrel dog, comparing plasma, urine, and renal lymph levels during similar time intervals. Finally, we will review the role of diuretics in altering the interstitial gradient of various solutes. Osmotic diuretics of more potent diuretics which affect renal tubular reabsorption manifest their action by different means when renal lymph fluid is evaluated.

Renal hilar lymph. Effects of diuresis on flow and composition in dogs

Renal lymph was collected from single hilar lymphatics in 58 anesthetized dogs (1) to study the mechanism by which lymph production is affected during diuresis and (2) to determine whether a medullary contribution to renal lymph could be defined by changes in the electrolyte concentration of hilar lymph with concomitant alterations in the concentration gradient of the renal medulla. When diuresis was induced by a solute load (mannitol), lymph flow increased by 25 to 300%. On the other hand, when diuresis was induced without such a solute load, lymph flow was either unaffected (mersalyl) or slightly reduced (furosemide). It was concluded that the effect of mannitol on renal lymph flow was mediated primarily through its general effect on extracellular fluid rather than through any specific intrarenal consequence of the diuresis itself.

Control hilar lymph-to-plasma concentration ratios for Na + (1.057 ± 0.040), Cl - (1.129 ± 0.040) and Ca 2+ (0.770 ± 0.046) but not K + (0.0986 ± 0.086) were found to be significantly different from 1.0. Failure of mannitol diuresis to alter significantly the lymph-plasma ratios of Na + and Cl - provided evidence that the high electrolyte concentrations of the inner medulla were not reflected in hilar lymph. The finding that furosemide abolished the lymph-plasma concentration difference for Na + and significantly reduced that for Cl - was taken as evidence that the outer medulla was a significant source of renal hilar lymph.