Three-dimensional reconstruction of the mouse nephron

Renal function is crucially dependent on renal microstructure which provides the basis for the regulatory mechanisms that control the transport of water and solutes between filtrate and plasma and the urinary concentration. This study provides new, detailed information on mouse renal architecture, including the spatial course of the tubules, lengths of different segments of nephrons, histotopography of tubules and vascular bundles, and epithelial ultrastructure at well-defined positions along Henle's loop and the distal convolution of nephrons. Three-dimensional reconstruction of 200 nephrons and collecting ducts was performed on aligned digital images, obtained from 2.5-mum-thick serial sections of mouse kidneys. Important new findings were highlighted: (1) A tortuous course of the descending thin limbs of long-looped nephrons and a winding course of the thick ascending limbs of short-looped nephrons contributed to a 27% average increase in the lengths of the corresponding segments, (2) the thick-walled tubules incorporated in the central part of the vascular bundles in the inner stripe of the outer medulla were identified as thick ascending limbs of long-looped nephrons, and (3) three types of short-looped nephron bends were identified to relate to the length and the position of the nephron and its corresponding glomerulus. The ultrastructure of the tubule segments was identified and suggests important implications for renal transport mechanisms that should be considered when evaluating the segmental distribution of water and solute transporters within the normal and diseased kidney.     

The vulnerability of the thin descending limbs of Henle's loop in the isolated perfused rat kidney

In the isolated rat kidney perfused without erythrocytes, the medullary thick ascending limb shows extensive injury. Damage to the thin limbs of Henle's loop has been mentioned only briefly. Thin limbs were examined in the isolated perfused kidney under a variety of conditions that alter oxygenation and active transport in the medulla and are known to affect injury to the medullary thick ascending limb. The thin descending limbs of short loops were preserved in all experimental groups, but those of the long loop showed necrosis that was restricted to the proximal portion, where the epithelium is more complex. In oxygenated kidneys, necrosis involved 41% +/- 5% (mean +/- SE) of the medullary thick ascending limbs and 10% +/- 3% of the proximal portion of long loops of thin descending limbs. Under hypoxic conditions, necrosis involved 90% +/- 3% of the medullary thick ascending limbs and 70% +/- 5% of the proximal portion of long loops of thin descending limbs (P less than 0.0001 compared with oxygenated kidneys). Ouabain and absence of filtration completely prevented necrosis of both nephron segments. Thus, the proximal portions of long loops of thin descending limbs, in resemblance to medullary thick ascending limbs, are especially susceptible to transport-dependent hypoxic injury.     

Mechanisms to concentrate the urine: an opinion

PURPOSE OF REVIEW: Our goal is to suggest how the renal concentrating mechanism is regulated in vivo. RECENT FINDINGS: The majority of descending thin limbs of the loop of Henle lack aquaporin-1 water channels, and loops of Henle in the inner medulla lack urea transporters. SUMMARY: Lack of water permeability in the descending thin limbs of the loop of Henle offers several advantages. First, since much less water is added to the outer medullary interstitial compartment, inhibitory control mechanisms on sodium and chloride reabsorption from the medullary thick ascending of loop of Henle initiated by water addition from the medullary collecting duct can be effective. Second, recycling of urea is efficient, as little urea will be washed out of the medulla. Third, delivery of a larger volume of filtrate to the medullary thick ascending limb of the loop of Henle permits both an appreciable reabsorption of sodium along with only a small fall in the luminal concentration of sodium in each of these liters. Hence there need be only a small lumen positive voltage in the medullary thick ascending limb of the loop of Henle. The absence of urea transporters in the loop of Henle in the inner medulla is required for a passive mechanism of sodium and chloride reabsorption in the inner medulla. Control of urea reabsorption from the medullary collecting duct is needed to prevent excessive oliguria in electrolyte-poor urine.     

Cell proliferation in the loop of henle in the developing rat kidney.

In the developing rat kidney, there is no separation of the medulla into an outer and inner zone. At the time of birth, ascending limbs with immature distal tubule epithelium are present throughout the renal medulla, all loops of Henle resemble the short loop of adult animals, and there are no ascending thin limbs. It was demonstrated previously that immature thick ascending limbs in the renal papilla are transformed into ascending thin limbs by apoptotic deletion of cells and transformation of the remaining cells into a thin squamous epithelium. However, it is not known whether this is the only source of ascending thin limb cells or whether cell proliferation occurs in the segment undergoing transformation. This study was designed to address these questions and to identify sites of cell proliferation in the loop of Henle. Rat pups, 1, 3, 5, 7, and 14 d old, received a single injection of 5-bromo-2'-deoxyuridine (BrdU) 18 h before preservation of kidneys for immunohistochemistry. Thick ascending and descending limbs were identified by labeling with antibodies against the serotonin receptor, 5-HT(1A), and aquaporin-1, respectively. Proliferating cells were identified with an antibody against BrdU. BrdU-positive cells in descending and ascending limbs of the loop of Henle were counted and expressed as percentages of the total number of aquaporin-1-positive and 5-HT(1A)-positive cells in the different segments. In the developing kidney, numerous BrdU-positive nuclei were observed in the nephrogenic zone. Outside of this location, BrdU-positive tubule cells were most prevalent in medullary rays in the inner cortex and in the outer medulla. BrdU-labeled cells were rare in the papillary portion of the loop of Henle and were not observed in the lower half of the papilla after 3 d of age. BrdU-labeled nuclei were not observed in segments undergoing transformation or in newly formed ascending thin limb epithelium. It was concluded that the growth zone for the loop of Henle is located around the corticomedullary junction, and the ascending thin limb is mainly, if not exclusively, derived from cells of the thick ascending limb.     

Altered expression of aquaporin-2 in human explants with chronic renal allograft dysfunction

OBJECTIVE: To investigate the distribution of aquaporins, a recently discovered family of transmembrane water channels, in human renal explants, with specific reference to chronic renal allograft dysfunction (CRAD). MATERIALS AND METHODS: Immunohistochemistry for aquaporin-1 and -2 was used in 11 explants, of which five had clinically and histologically confirmed CRAD. Controls were taken from the six explants unaffected by CRAD and from histologically normal areas of six kidneys excised for renal tumours. RESULTS: In the renal tumour control group, aquaporin-1 immunoreactivity was detected in the glomerular endothelium, Bowman's capsule, the proximal convoluted tubules and the thin limb of the loop of Henle, whereas immunoreactivity for aquaporin-2 was detected in the collecting ducts only. Of the explants without CRAD, where architecture was preserved, immunoreactivity for aquaporin-1 and -2 was the same as in the renal tumour controls. In the two explants with no CRAD and loss of collecting ducts, there was no aquaporin-2 immunoreactivity. In five explants with CRAD, immunoreactivity for aquaporin-2 was decreased or absent from the medulla to the cortex. The apparent decreased immunoreactivity of aquaporin-1 in this group was secondary to a decrease in the number of viable proximal tubules. CONCLUSION: There was less aquaporin-2 immunoreactivity in human renal explants diagnosed with CRAD, starting from the medullary region. In explants with no CRAD and viable collecting ducts, or in normal controls, aquaporin-2 immunoreactivity remained unchanged. Aquaporins might be useful as markers for CRAD.     

Historadioautographic localization, pharmacology and ontogeny of V(1a) vasopressin binding sites in the rat kidney.

The localization and pharmacological characteristics of vasopressin (VP) binding sites of the V(1a) subtype in developing and adult rat kidney were investigated by radioautography on kidney sections incubated in the presence of a radioiodinated selective V(1a) antagonist. Their localization after in vivo systemic infusion of the radioligand was also investigated. V(1a) binding sites first appear at embryonic day 16 on vascular elements. In the adult, they were localized in the cortex (vascular and tubular structures, juxtaglomerular apparatus), the outer medulla outer stripe (vasa recta) and inner stripe (thin descending limbs of short looped nephrons) and the inner medulla (collecting ducts). Data obtained in vitro were confirmed by in vivo binding at postnatal day 30 (PN30). Whatever their localizations, the V(1a) binding sites exhibited full V(1a) pharmacological profile in postnatal stages rats and in adult rats: a high affinity (nM range) for VP and for the V(1a) agonist, a lower affinity (microM range) for oxytocin and no affinity for the oxytocin agonist. The presence of V(1a) binding sites in these different structures raises the question of the putative roles of VP in modulating renal functions. A striking finding is the presence of V(1a) binding sites in the outer medullary thin descending limbs of short looped nephrons suggesting their colocalization with urea transporters.     

Intracellular distribution of carbonic anhydrase in the rat kidney

The rat kidney was studied by light and electron microscope after it was histochemically stained for carbonic anhydrase activity. Glomeruli and Bowman's capsule were inactive. Convoluted proximal tubules showed intense activity at the brush border and the basolateral membranes. Cytoplasmic activity also was found. Straight proximal tubules had considerable enzyme activity at basolateral membranes but only low activity at the brush border and in the cytoplasm. In nephrons with long loops, the descending thin limb contained cytoplasmic enzyme activity, whereas the ascending thin limb was inactive. Thin limbs of short loops showed a varying enzyme pattern. In the thick limb of Henle's loop, most enzyme activity was found at the luminal cell border. Distal convoluted tubules showed enzyme activity only at basal infolded membranes. In the late distal tubule, intercalated cells appear among the "ordinary" distal cells, and they contained abundant cytoplasmic enzyme. Many highly active intercalated cells were found also in the cortical and outer medullary segments of the collecting duct. The chief cells in these segments also showed some cytoplasmic enzyme activity. In the inner medullary segment of the collecting duct, enzyme activity disappeared gradually, and the tip of the papilla lacked activity. Acetazolamide (10 microM) completely abolished visible staining, whereas Cl 13850 (10 microM), an inactive acetazolamide analogue, did not interfere with the staining.     

Magnetic resonance imaging of urea transporter knockout mice shows renal pelvic abnormalities

Many transgenic and knockout mice with increased urine flow have structural abnormalities of the renal pelvis and inner medulla. Here, we used high resolution contrast enhanced T1-weighted magnetic resonance imaging of mice whose urea transporters UT-A1 and UT-A3 were deleted (UT-A1/3(-/-) mice) as a model for the in vivo study of such abnormalities. Three distinct variations in the appearance of the renal pelvis were found. These included normal kidneys with no accumulation of contrast agent in the renal pelvis; infrequent frank right-sided unilateral hydronephrosis with marked atrophy of the renal medulla; and a renal pelvic reflux pattern characterized by the presence of contrast agent in the renal pelvis surrounding the renal inner medulla but no substantial atrophy of the medulla. This last pattern was found in most of the advanced age UT-A1/3(-/-) mice and in aquaporin-1 knockout mice. The UT-A1/3(-/-) mice also had increased mean arterial blood pressures. Feeding the mice a low protein diet did not prevent development of their renal pelvic abnormalities. Our studies show that real time imaging of renal pelvic structure in genetically manipulated mice provides a tool for the non-destructive, temporal evaluation of kidney structure.     

The consequences for renal function of widening of the interstitium and changes in the tubular epithelium of the renal cortex and outer medulla in various renal diseases.

The following parameters were measured in 63 renal biopsy specimens, most of which exhibited mesangioproliferative glomerulonephritis: the relative area of the interstitium and the area of the capillaries in the cortex and outer stripe of the outer medulla, and the area of the epithelium of the proximal tubules and of the ascending limbs of Henle's loop. The percentage of hyalinized glomeruli was also determined. Investigation of correlations between these values and parameters of renal function revealed the following: 1) The serum creatinine concentration increases and the endogenous creatinine clearance decreases significantly as the interstitium of both the cortex and the outer stripe of the outer medulla increases in width. 2) The urine osmolality decreases significantly as the epithelial areas of the proximal tubules and ascending limbs of Henle's loop decrease, and as the serum creatinine concentration rises and the areas of the interstitium increase. 3) No significant correlation exists between the percentage of hyalinized glomeruli and the urine osmolality. 4) The total area of the intertubular capillaries in the cortex decreases significantly as the interstitium in this area increases in width and as the serum creatinine concentration increases. 5) Compensatory hypertrophy of individual nephrons, as proposed by Bricker's hypothesis, was found only where more than 90% of the glomeruli were hyalinized.(ABSTRACT TRUNCATED AT 250 WORDS)     

Renal microradiography of experimental ureteral obstruction in the rabbit

Microradiography of nephrons in kidneys perfusion-fixed with glutaraldehyde permits examination of large numbers of nephrons. This technique was applied to rabbits between one and 14 days following unilateral ureteral ligation. Kidneys without ureteral occlusion served as controls. By two days after ureteral obstruction there was dilatation of the ducts of Bellini and papillary collecting ducts. At three to four days there was splaying and tortuosity of the loops of Henle. By eight to 10 days the proximal straight tubules were noted to be dilated and helically twisted. After two weeks of ureteral obstruction there was dilatation of Bowman's space with encroachment on the glomerular capillary tuft. At this time many proximal convoluted tubules began to show atrophic changes. These morphologic alterations may due in part to back pressure on the nephrons, with retrograde progression as the duration of urinary tract obstruction is increased. The distal convoluted tubule and the descending limb of the loop of Henle were not noted to be abnormal during the study.     

Creation of a functioning chimeric mammalian kidney

The possibility of adding new nephrons to the mammalian kidney was studied. Embryonic metanephric tissue was implanted into the renal cortex of neonatal mice less than 24 hours old, and the development of the chimeric kidney was followed over the following two to four weeks. Donor tissue was obtained from the homozygous beige mouse and a mouse line transgenic for the beta-globin gene, which provided distinct cellular and nuclear markers which were used to distinguish donor from recipient nephrons. Differentiation and growth of donor nephrons occurred in the host kidney and included vascularized glomeruli, mature proximal tubules, and tubular extensions into the renal medulla. Glomerular filtration was demonstrable in donor nephrons using FITC-dextran as a marker of filtration into the proximal tubules. Transplantation of metanephric tissue into adult mouse kidneys did not lead to glomerular or tubular differentiation. This study demonstrates the feasibility of adding functioning nephrons to mammalian kidneys in species in which there is ongoing nephrogenesis post-natally.     

"Avian-type" renal medullary tubule organization causes immaturity of urine-concentrating ability in neonates

BACKGROUND: While neonatal kidneys are not powerful in concentrating urine, they already dilute urine as efficiently as adult kidneys. To elucidate the basis for this paradoxical immaturity in urine-concentrating ability, we investigated the function of Henle's loop and collecting ducts (IMCDs) in the inner medulla of neonatal rat kidneys. METHODS: Analyses of individual renal tubules in the inner medulla of neonatal and adult rat kidneys were performed by measuring mRNA expression of membrane transporters, transepithelial voltages, and isotopic water and ion fluxes. Immunofluorescent identification of the rCCC2 and rCLC-K1 using polyclonal antibodies was also performed in neonatal and adult kidney slices. RESULTS: On day 1, the transepithelial voltages (V(Ts)) in the thin ascending limbs (tALs) and IMCDs were 14.6 +/- 1.1 mV (N = 27) and -42.7 +/- 6.1 mV (N = 14), respectively. The V(Ts) in the thin descending limbs (tDLs) were zero on day 1. The V(Ts) in the tALs were strongly inhibited by luminal bumetanide or basolateral ouabain, suggesting the presence of a NaCl reabsorption mechanism similar to that in the thick ascending limb (TAL). The diffusional voltage (V(D)) of the tAL due to transepithelial NaCl gradient was almost insensitive to a chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB). The V(Ts) in the IMCDs were strongly inhibited by luminal amiloride. On day 1, both the tDL and tAL were impermeable to water, indicating the water impermeability of the entire loop. Diffusional water permeability (P(dw)) and urea permeabilities (P(urea)) in the IMCDs indicated virtual impermeability to water and urea on day 1. Stimulation by vasopressin (1 nmol/L) revealed that only P(dw) was sensitive to vasopressin by day 14. A partial isoosmolar replacement of luminal urea by NaCl evoked negligible water flux across the neonatal IMCDs, indicating the absence of urea-dependent volume flux in the neonatal IMCD. These transport characteristics in each neonatal tubule are similar to those in quail kidneys. Identification of mRNAs and immunofluorescent studies for specific transporters, including rAQP-1, rCCC2, rCLC-K1, rENaC beta subunit, rAQP-2, and rUT-A1, supported these findings. CONCLUSION: We hypothesize that the renal medullary tubule organization of neonatal rats shares a tremendous similarity with avian renal medulla. The qualitative changes in the organization of medullary tubules may be primarily responsible for the immature urine-concentrating ability in mammalian neonates.     

Urea transporters and renal function: lessons from knockout mice

PURPOSE OF REVIEW: Gene knockout mice have been created for the collecting duct urea transporters UT-A1 and UT-A3, the descending thin-limb urea transporter UT-A2 and the descending vasa recta isoform, UT-B. In this brief review, the new insights in our understanding of the role of urea in the urinary concentrating mechanism and kidney function resulting from studies in these mice are discussed. RECENT FINDINGS: The major findings in studies on urea transporter knockout mice are as follows: rapid transport of urea from the inner medulla collecting duct lumen via UT-A1 or UT-A3 is essential for urea accumulation in the inner medullary interstitium; inner medulla collecting duct urea transporters are essential in water conservation by preventing urea-induced osmotic diuresis; an absence of inner medulla collecting duct urea transport does not prevent the concentration of sodium chloride in the inner medulla interstitium; deletion of the vasa recta isoform UT-B has a much greater effect on urinary concentration than deleting the descending limb isoform UT-A2. SUMMARY: Multiple urea transport mechanisms within the kidney are essential for producing maximally concentrated urine.     

Urea and renal function in the 21st century: insights from knockout mice

Since the turn of the 21st century, gene knockout mice have been created for all major urea transporters that are expressed in the kidney: the collecting duct urea transporters UT-A1 and UT-A3, the descending thin limb isoform UT-A2, and the descending vasa recta isoform UT-B. This article discusses the new insights that the results from studies in these mice have produced in the understanding of the role of urea in the urinary concentrating mechanism and kidney function. Following is a summary of the major findings: (1) Urea accumulation in the inner medullary interstitium depends on rapid transport of urea from the inner medullary collecting duct (IMCD) lumen via UT-A1 and/or UT-A3; (2) as proposed by Robert Berliner and colleagues in the 1950s, the role of IMCD urea transporters in water conservation is to prevent a urea-induced osmotic diuresis; (3) the absence of IMCD urea transport does not prevent the concentration of NaCl in the inner medulla, contrary to what would be predicted from the passive countercurrent multiplier mechanism in the form proposed by Kokko and Rector and Stephenson; (4) deletion of UT-B (vasa recta isoform) has a much greater effect on urinary concentration than deletion of UT-A2 (descending limb isoform), suggesting that the recycling of urea between the vasa recta and the renal tubules quantitatively is less important than classic countercurrent exchange; and (5) urea reabsorption from the IMCD and the process of urea recycling are not important elements of the mechanism of protein-induced increases in GFR. In addition, the clinical relevance of these studies is discussed, and it is suggested that inhibitors that specifically target collecting duct urea transporters have the potential for clinical use as potassium-sparing diuretics that function by creation of urea-dependent osmotic diuresis.     

Distribution of the sodium/phosphate transporter during postnatal ontogeny of the rat kidney.

Renal phosphate reabsorption via the type II sodium/ phosphate cotransporter (NaPi-2) in the brush border membrane (BBM) of proximal tubules underlies alterations during aging. The ontogeny of NaPi-2 in kidneys from newborn to 6-wk-old rats was investigated. NaPi-2 protein distribution in the kidneys of neonatal, 13-d-old, 22-d-old, and 6-wk-old rats was immunohistochemically analyzed, and NaPi-2 mRNA distribution in neonatal and 6-wk-old rats was analyzed by in situ hybridization. In kidneys of newborn rats, the appearance of NaPi-2 protein and mRNA coincided with the development of the brush border (assessed by actin staining) on proximal tubular cells. NaPi-2 was not detectable in the nephrogenic zone or in the outgrowing straight sections of proximal tubules, which lack a brush border. In 13-d-old suckling rats, strong NaPi-2 staining was seen in the BBM of convoluted proximal tubules of all nephron generations. In contrast, in 22-d-old weaned rats, NaPi-2 staining in the BBM of superficial nephrons was weaker than that in the BBM of juxtamedullary nephrons. Western blotting demonstrated that the overall abundance of NaPi-2 protein in the BBM of 22-d-old rats was decreased to approximately 70% of that in 13-d-old rats. In kidneys of 6-wk-old rats, the internephron gradient for NaPi-2 abundance in the BBM corresponded to that in adult rats. The data suggest that the NaPi-2 system in the kidney is fully functional and possesses the capacity for regulation as soon as nephrogenesis is completed. The manifestation of NaPi-2 internephron heterogeneity immediately after weaning might be related to the change in dietary inorganic phosphate content.     

Expression of RhCG,a new putative NH(3)/NH(4)(+) transporter,along the rat nephron

Two non-erythroid members of the erythrocyte Rhesus (Rh) protein family, RhBG and RhCG, have been recently cloned in the kidney. These proteins share homologies with specific NH(3)/NH(4)(+) transporters (Mep/Amt) in primitive organisms and plants. When expressed in a Mep-deficient yeast, RhCG can function as a bidirectional NH(3)/NH(4)(+) transporter. The aim of this study was to determine the intrarenal and intracellular location of RhCG in rat kidney. RT-PCR on microdissected rat nephron segments demonstrated expression of mRNAs encoding RhCG in distal convoluted tubules, connecting ducts, and cortical and outer medullary collecting ducts but not in proximal tubules and thick ascending limbs of Henle's loop. Immunolocalization studies performed on rat kidney sections with rabbit anti-human RhCG 31 to 48 antibody showed labeling of the apical pole of tubular cells within the cortex, the outer medulla, and the upper portion of the inner medulla. All cells within connecting tubules had identical apical staining. In cortical collecting ducts, a subpopulation of cells that has either apical staining (alpha-intercalated cells) or diffuse staining (beta-intercalated cells) for the beta1 subunit of the H(+)-ATPase, was heavily stained at their apical pole with the RhCG antibody while principal cells identified as H(+)-ATPase negative cells showed a faint apical staining for RhCG that was much less intense than in adjacent intercalated cells. In the outer medulla and the upper portion of the inner medulla, RhCG labeling was restricted to a subpopulation of cells within the collecting duct that apically express the beta1 subunit of the H(+)-ATPase, indicating that RhCG expression in medullary collecting ducts is restricted to intercalated cells. No labeling was seen in glomeruli, proximal tubules, and limbs of Henle's loop. Immunoblotting of apical membrane fractions from rat kidney cortex with the rabbit anti-human RhCG 31 to 48 antibody revealed a doublet band at approximatively 65 kD.     

Molecular mechanisms of impaired urinary concentrating ability in glucocorticoid-deficient rats

The purpose of this study was to examine urinary concentrating ability and protein expression of renal aquaporins and ion transporters in glucocorticoid-deficient (GD) rats in response to water deprivation as compared with control rats. Rats underwent bilateral adrenalectomies, followed only by aldosterone replacement (GD) or both aldosterone and dexamethasone replacement (control). As compared with control rats, the GD rats demonstrated a decrease in cardiac output and mean arterial pressure. In response to 36-h water deprivation, GD rats demonstrated significantly greater urine flow rate and decreased urine osmolality as compared with control rats at comparable serum osmolality and plasma vasopressin concentrations. The initiator of the countercurrent concentrating mechanism, the sodium-potassium-2 chloride co-transporter, was significantly decreased, as was the medullary osmolality in the GD rats versus control rats. There was also a decrease in inner medulla aquaporin-2 (AQP2) and urea transporter A1 (UT-A1) in GD rats as compared with control rats. There was a decrease in outer medulla Gsalpha protein, an important factor in vasopressin-mediated regulation of AQP2. Immunohistochemistry studies confirmed the decreased expression of AQP2 and UT-A1 in kidneys of GD rats as compared with control. In summary, impairment in the urinary concentrating mechanism was documented in GD rats in association with impaired countercurrent multiplication, diminished osmotic equilibration via AQP2, and diminished urea equilibration via UT-A1. These events occurred primarily in the relatively oxygen-deficient medulla and may have been initiated, at least in part, by the decrease in mean arterial pressure and thus renal perfusion pressure in this area of the kidney.     

Localization of urinary procoagulant in the human kidney.

By the indirect immunofluorescent and immunoenzymatic techniques with monospecific antiserum against urinary procoagulant (a tissue factor which accelerates blood coagulation), we found the urinary procoagulant in the kidney distributed to the loop of Henle and distal convoluted tubules. In these areas urinary procoagulant was found in association with the luminal and intercellular borders as well as in the cytoplasm of epithelial cells. Both the descending and ascending limbs of Henle were equally stained. The cytoplasmic staining was patchy in distribution among cells of distal tubules and was predominantly localized in the supranuclear areas. Glomeruli, the proximal tubular cells, the vascular wall, and the interstitium were not stained. There was, however, fluorescent staining along the epithelial layers of the Bowman's capsule, which was observed only in the frozen sections. Casts in the distal tubules were also positively stained. These findings suggest that urinary procoagulant is synthesized in the epithelial cells of these particular parts of nephron and is secreted into urine, although its physiologic roles and pathologic significance are not entirely known.