Ultrastructure of the renal tubules in acute renal insufficiency

Extravasation of Blood and Blood Toxicity Drives Tubular Injury from RBC Trapping in Ischemic AKI

Red blood cell (RBC) trapping is common in ischemic acute kidney injury (AKI) and presents as densely packed RBCs that accumulate within and engorge the kidney medullary circulation. In this study, we tested the hypothesis that "RBC trapping directly promotes tubular injury independent of extending ischemia time." Studies were performed on rats. Red blood cell congestion and tubular injury were compared between renal arterial clamping, venous clamping, and venous clamping of blood-free kidneys. Vessels were occluded for either 15 or 45 min with and without reperfusion. We found that RBC trapping in the medullary capillaries occurred rapidly following reperfusion from renal arterial clamping and that this was associated with extravasation of blood from congested vessels, uptake of blood proteins by the tubules, and marked tubular injury. To determine if this injury was due to blood toxicity or an extension of ischemia time, we compared renal venous and arterial clamping without reperfusion. Venous clamping resulted in RBC trapping and marked tubular injury within 45 min of ischemia. Conversely, despite the same ischemia time, RBC trapping and tubular injury were minimal following arterial clamping without reperfusion. Confirming the role of blood toward tubular injury, injury was markedly reduced in blood-free kidneys with venous clamping. Our data demonstrate that RBC trapping results in the rapid extravasation and uptake of blood components by tubular cells, causing toxic tubular injury. Tubular toxicity from extravasation of blood following RBC trapping appears to be a major component of tubular injury in ischemic AKI, which has not previously been recognized.

Hidden in Plain Sight: Does Medullary Red Blood Cell Congestion Provide the Explanation for Ischemic Acute Kidney Injury?

Acute kidney injury (AKI) represents a sudden reduction in renal function and is a major clinical problem with a high mortality rate. Despite decades of research, there are currently no direct therapies for AKI. The failure of therapeutic approaches identified in rodents to translate to human beings has led to questions regarding the appropriateness of these models. Our recent data indicate that there are two distinct processes driving tubular injury in the commonly used rat model of warm bilateral renal ischemia reperfusion injury, which often is used to mimic ischemic AKI. One results from the period of warm ischemia, manifesting as sublethal injury and coagulative necrosis of the proximal tubules in the renal cortex. This is the predominate type of injury observed 24 hours after reperfusion and the most well studied. The other results from red blood cell congestion of the outer medullary vasculature. This type of injury manifests as cell sloughing, along with the later formation of heme casts that fill distal nephron segments. Cell sloughing from congestion is most prominent in the early hours after reperfusion and often is masked by regeneration of the tubular epithelium by 24 hours postischemia. In this review, we argue that injury from outer medullary red blood cell congestion reflects the pathology observed in human kidneys and likely is representative of injury in most cases of ischemic AKI after shock. Greater focus on this congestive injury is likely to lead to improved translation in AKI.

Transmission electron microscopy of urinary sediment in human acute renal failure

Urinary sediments from 31 patients with acute tubular necrosis (ATN) were studied using transmission electron microscopy (TEM). Variable quantities of renal tubule cells (RTCs), characterized by abundant cytoplasmic organelles, were a consistent finding in these sediments. TEM analyses of well-preserved RTCs permitted identification of their nephron segment(s) of origin. On the basis of the severity of ultrastructural changes observed, the sediments were divided into three types. Type I (N = 11) and II (N = 8) consisted of homogeneous populations of severely and mildly affected RTCs, respectively, while Type III (N = 12) sediments were composed of a heterogeneous population of variably affected cells. A correlation was found between the sediment type and the severity of the clinical illness. Of the 11 patients with Type I sediments, only two survived and they required chronic dialysis support. In contrast, seven of eight patients exhibiting Type II sediments survived, with only one requiring temporary dialysis therapy. Patients with Type III sediments had an intermediate course, with six of seven survivors regaining sufficient renal function to remain off dialysis over a 3-month period of observation. Our study suggests that TEM of urinary sediment can provide a useful non-invasive means of studying patients with ATN.

Tubular ultrastructure in acute renal failure in man: epithelial necrosis and regeneration

It is not clear whether tubular cell necrosis is present or not in acute renal failure (ARF) of ischaemic type ("acute tubular necrosis"). In order to get quantitative data, using precisely defined criteria for tubular cell necrosis, 25 renal biopsies from 24 patients with ARF (11 obtained in the active phase, 14 in the early recovery period) were compared with 12 control biopsies. In all 1959 proximal cells and 1603 distal cells were analysed by electron microscopy. Cellular disintegration was very rare in all groups. Shrinkage necrosis (apoptosis) was not present in the proximal tubules of the controls and was rare in ARF (1.6-2.1%). In the distal tubules of controls 2.7% of all cells showed shrinkage necrosis. The incidence in ARF was not significantly increased. "Non-replacement sites" in distal tubules (probably loci where cells have recently been desquamated) were significantly increased in number (5.2%) in the active phase in ARF compared to controls and recovery. The relative number of regenerating cells was not increased. These data show that there is no widespread necrosis of tubular cells in ARF. The increased incidence in distal tubules of focal, denuded areas of the basement membrane in the active phase of ARF indicates a slightly increased desquamation of cells and/or a failure to cover such sites by adjacent cells. This process is not restricted to the brief induction phase of ARF but continues during the whole active phase.

Tubular ultrastructure in acute renal failure: alterations of cellular surfaces (brush-border and basolateral infoldings)

Using a blind, semiquantitative technique, the degree of reduction of proximal tubular brush border (BB) and proximal and distal basolateral infoldings (BI) were measured in 25 renal biopsies from patients with acute renal failure (ARF) of ischaemic type. For comparison 12 biopsies from patients without ARF were studied, 6 were normal controls, six were from patients with minor change disease and slight glomerulonephritis. The mean scores for reduction of BB as well as proximal and distal BI were strongly increased in ARF compared to controls and the differences were highly significant. Some of the biopsies were taken during recovery and there was a significant negative correlation between the individual scores for reduction of BB and BI and simultaneous renal function. The disappearance of BB microvilli was correlated to tubular dilatation, but it could not be explained exclusively by "stretching" of the luminal surface due to dilatation. There was no correlation between reduction of BI and tubular dilatation. The data indicate a disturbance of cell membrane turnover in the active phase of ARF, possibly due to decreased synthesis, and they are consistent with a pathogenetic hypothesis implicating a decreased proximal Na+ resorption and consequently a pre-glomerular vasoconstriction.

Hemodynamic basis for human acute renal failure (vasomotor nephropathy)

Oliguric acute renal failure in man is characterized by intense outer cortical vasoconstriction and a marked increase in preglomerular resistance. The degree of preglomerlar resistance change needed to cause the expected 50 to 80 percent fall in blood flow far exceeds the level that would totally abolish filtration. By contrast, equal 3.0-fold increases in both pre- and postglomerular resistance provide this same degree of ischemia but leave filtration very well maintained. Such a scenario seems unlikely, however, since it would entail a mere 15 to 25 percent decrease in preglomerular resistance vessel caliber rather than the extreme attenuation observed. By contrast, there are reasons to believe that preglomerular constriction may be accompanied by postglomerular vascular relaxation. In sum, unless cortical ischemia reflects precisely matched increases in pre- and postglomerular resistances, filtration failure is inevitable in human vasomotor nephropathy.

Renal ultrastructure in acute tubular necrosis following Russell's viper envenomation

Electron microscopic examination of renal biopsies from nine patients who developed renal failure after Russell's viper bite showed glomerular mesangial hypercellularity, vascular endothelial swelling, tubular epithelial necrosis and shedding, acellular segments of tubular basement membrane and an interstitial infiltrate composed of mast cells, eosinophils, lymphocytes and plasma cells. These cells showed many close contacts with each other and with fibroblasts. Severe vascular and tubular lesions were also present in the medulla. These abnormalities and other reported clinical and laboratory investigations in this condition, suggest that the primary lesion is a reversible intravascular coagulation with secondary ischaemic tubular damage. The inflammatory response in the interstitium is probably initiated by a release of antigens from damaged tubules.

The prevalence of focal tubulo-interstitial lesions in various renal diseases

The focal tubulo-interstitial lesions (FTIL) described in the present paper appear as short segments of tubules showing epithelial degeneration, necrosis and desquamation. The foci are surrounded by mononuclear cells, among which lymphocytes are most numerous. They infiltrate through the damaged tubular basement membranes and may thus be located between epithelial cells and in the tubular lumen. Older lesions show focal tubular atrophy with thickened basement membranes together with focal interstitial fibrosis and lymphocytic infiltrates. The overall prevalence of this type of lesion in a consecutive renal biopsy series was 36%. It was not detected in 24 biopsies from patients without renal disease. The frequency of FTIL was high in acute renal failure due to hemolysis, shock etc. or to the action of drugs, as well as in some types of glomerulonephritis (membranous, membrano-proliferative, SLE). Tubular immunoglobulins were not demonstrated in 38 biopsies with FTIL. The pathogenesis and possible consequence of the lesions are still unclear, but it is suggested that they may in some situations contribute to the nephron atrophy in chronic renal disease.

Nephron injury in the haemolytic-uraemic syndrome complicating bacillary dysentery

The electron microscopic appearances of renal biopsy material from four children who developed the haemolytic-uraemic syndrome as a complication of bacillary dysentery are described. Glomerular lesions present included deposits of fibrin and platelets, subendothelial rarefaction and lysis of endocapillary cells with or without necrosis of glomerular epithelial cells and periglomerular tissue. Tubular epithelial necrosis and cytoplasmic degeneration with an infiltrate of lymphocytes and plasma cells were also seen. It is likely that the different glomerular lesions result from thrombosis and obstruction of blood vessels at various levels of the intrarenal circulation. The tubulo-interstitial changes appear to be secondary to glomerular lesions.

Renal tubular necrosis due to shock: light and electron-microscope observations

The renal biopsy findings in a 76 yr-old woman suffering grom anuria due to acute tubular necrosis are described. The glomeruli were normal on light- and electron microscopy. Immunofluorescent studies failed to reveal any fibrin or immunoglobulins in the glomerular capillaries. Extensive focal areas of necrosis were seen in the tubular epithelium often exposing the lumen of the tubule directly to the tubular basement-membrane. In some areas necrotic cells lay adjacent to normal or near normal cells. The proximity of the necrotic tubular epithelium to the oedematous interstitial tissue and the peritubular capillaries, together with the finding of normal glomeruli is compatible with the theory of back diffusion as a mechanism for the oliguria.

Kidney tubular cell proliferation in burned mice

Kidney tubular cell proliferation after a 17%, sublethal, third-degree burn injury in mice was studied in vivo using tritiated thymidine. Changes in cell morphology were also followed. Total incorporation of 3H-TdR in renal cells was measured and autoradiographic analyses of the tubular cells in the renal cortex and in the stratum subcorticale were made 2 hours to 30 days after the injury. After the second day postburn there was a 3.5-fold stimulation in the total incorporation of 3H-TdR in DNA, and an 18-fold increase in the labelling index of tubular cells. The majority of the labelled cells were proximal tubular cells. After the third day postburn a moderate increase in the labelling index (7.5-fold) was observed; 40% of the labelled cells were distal tubular cells. No morphological signs of frank necrosis of the tubular cells could be observed in the burned mice within 3 days postburn, which indicates that the stimulated proliferation of the kidney tubular cells was induced by minor reversible tubular cell damage during the first few hours postburn. It is suggested that the tubular cells loose their normal growth-inhibiting factors through an altered cell wall which results in an abnormal stimulation of cell growth 2 days later. These findings support the idea that there are abnormalities in the regulation of cell growth in several organs after burn injury; in some tissues--as in the kidney--this results in a temporary insufficiency of organ function.