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AbstractThe skin manifestations in hypothyroidism are common but their advanced forms do not have adequate therapy. Along with achievement of euthyroidism, thyroid-associated dermatopathy (TAD) needs an inhibition of the proliferation and activation of fibroblasts, and of the glycosaminoglycans synthesis. Pentoxifylline (PTX) exerts multifaceted effects and has shown properties to inhibit certain constitutive activities of dermal fibroblasts. The aim of this study was to determine the histopathological changes and to establish the amount and distribution of mucin in hypothyroid skin, with or without treatment with PTX. Hypothyroid skin showed abundant diffuse mucin deposition in all dermal compartments in comparison with euthyroid skin. After administration of PTX in dose 100 mg/kg daily, there was a significant decreased amount of mucin in the dermis. The improvement of the dermal histological manifestations, in general demonstrated by narrowing between the dermal collagen fibers and by a decrease in mucin deposition, might be explained with dose-dependent PTX inhibition of the synthesis and accumulation of glycosaminoglycans. The histological results of this study confirmed several clinical observations about the beneficial effect of PTX on clinical skin manifestations in hypothyroid state. Therefore, we may propose PTX as a new potential therapeutic agent for TAD treatment. Further studies are needed to fully establish the effect of PTX on dermal fibroblasts after continuous administration.

Basal cell carcinoma (BCC) is the most common keratinocyte carcinoma and the most prevalent skin cancer in humans, worldwide. BCC is histologically characterized by the proliferation of basaloid cells, arranged in globular masses of varying size, often separated from the surrounding stroma by optically empty spaces. Although attributed to tumor retraction during tissue processing for the preparation of pathology slides, these spaces are also seen on cryostat sections. The aim of this study is to in vivo characterize amyloid and mucin deposits in primary BCC lesions through RCM, followed by histopathological correlation. We included twenty-two consecutive subjects totaling thirty-one primary BCCs. Each lesion underwent the same evaluation protocol which included: clinical and dermoscopic images, RCM imaging, excisional biopsy under local anesthesia, and histopathological examination (colloidal iron and cytokeratin 34betaE12 stains). Hypo-reflective peritumoral clefts and hyper-reflective globules were measured on RCM images and compared to mucin and amyloid deposits seen on histology slides. The mean differences between RCM and histology measurements in both mucin and amyloid were not statistically significant. There were medium and strong correlations between RCM and histology regarding mucin and amyloid deposits, respectively. [ABSTRACT FROM AUTHOR]

• Ferrous citrate significantly enhances the oxidation of Mn(II) and As(III) by AGS. • Bacteria rapidly degrade Fe(II)-citrate complex, producing abundant colloidal iron. • Colloidal iron (both II and III) stimulates extracellular proteins to generate ·O 2 −. • Extracellular ·O 2 − oxidizes Mn(II) to Mn(III/IV), which efficiently oxidizes As(III). Microbial manganese (Mn) oxidation plays a crucial role in shaping the fate of various elements, including arsenic (As). However, this process faces challenges in wastewater environments due to its inherent inefficiency and instability. In our initial research, a serendipitous discovery occurred: the addition of citrate to Fe(II)-containing wastewater stimulated the oxidation of Mn(II) by aerobic granular sludge (AGS). Subsequent experiments in four sequencing batch reactors (SBRs) over a 67-day period confirmed this stimulatory effect. The presence of Fe(II)-citrate led to a remarkable twofold increase in the oxidation of Mn(II) and As(III). The removal efficiency improved from 21±4 % to 87±7 % for Mn(II) and from 77.1 ± 1.8 % to 93.6 ± 0.2 % for As(III). The verification experiments demonstrated that the simultaneous addition of manganese-oxidizing bacteria (MnOB) and Fe(II)-citrate is an effective strategy for enhancing the oxidation and removal of Mn(II) and As(III) by AGS. Through a combination of genomic analysis, cell-free filtrate incubation, and bacterial batch cultivations (including monitoring the time-course changes of 17 substances and 2 free radicals), we elucidated a novel Mn(II) oxidation pathway in Pseudomonas , along with its stimulation method and mechanism. First, bacteria rapidly degrade citrate possibly via the citrate-Mg2+:H+ symporter (CitMHS) and the tricarboxylic acid (TCA) cycle, resulting in the formation of colloidal Fe(II), colloidal Fe(III), and biogenic iron (hydr)oxides (FeOx). Then, colloidal Fe(II) and colloidal Fe(III) stimulated extracellular proteins to produce superoxide radicals (·O 2 −). These radicals were responsible for oxidizing Mn(II) into Mn(III), ultimately forming biogenic manganese oxides (MnOx). Finally, MnOx effectively oxidized As(III) to the less toxic As(V). This innovative approach for bacterial Mn(II) oxidation holds promise for treating Mn(II) and As(III) in water and wastewater. Furthermore, the mechanism by which colloidal iron stimulates extracellular proteins to produce ·O 2 −, thereby facilitating Mn(II) oxidation, may widely occur across various engineering and natural ecosystems. [Display omitted] [ABSTRACT FROM AUTHOR]

The skin manifestations in hypothyroidism are common but their advanced forms do not have adequate therapy. Along with achievement of euthyroidism, thyroid-associated dermatopathy (TAD) needs an inhibition of the proliferation and activation of fibroblasts, and of the glycosaminoglycans synthesis. Pentoxifylline (PTX) exerts multifaceted effects and has shown properties to inhibit certain constitutive activities of dermal fibroblasts. The aim of this study was to determine the histopathological changes and to establish the amount and distribution of mucin in hypothyroid skin, with or without treatment with PTX. Hypothyroid skin showed abundant diffuse mucin deposition in all dermal compartments in comparison with euthyroid skin. After administration of PTX in dose 100 mg/kg daily, there was a significant decreased amount of mucin in the dermis. The improvement of the dermal histological manifestations, in general demonstrated by narrowing between the dermal collagen fibers and by a decrease in mucin deposition, might be explained with dose-dependent PTX inhibition of the synthesis and accumulation of glycosaminoglycans. The histological results of this study confirmed several clinical observations about the beneficial effect of PTX on clinical skin manifestations in hypothyroid state. Therefore, we may propose PTX as a new potential therapeutic agent for TAD treatment. Further studies are needed to fully establish the effect of PTX on dermal fibroblasts after continuous administration. [ABSTRACT FROM AUTHOR]

Basal cell carcinoma (BCC) is the most common keratinocyte carcinoma and the most prevalent skin cancer in humans, worldwide. BCC is histologically characterized by the proliferation of basaloid cells, arranged in globular masses of varying size, often separated from the surrounding stroma by optically empty spaces. Although attributed to tumor retraction during tissue processing for the preparation of pathology slides, these spaces are also seen on cryostat sections. The aim of this study is to in vivo characterize amyloid and mucin deposits in primary BCC lesions through RCM, followed by histopathological correlation. We included twenty-two consecutive subjects totaling thirty-one primary BCCs. Each lesion underwent the same evaluation protocol which included: clinical and dermoscopic images, RCM imaging, excisional biopsy under local anesthesia, and histopathological examination (colloidal iron and cytokeratin 34betaE12 stains). Hypo-reflective peritumoral clefts and hyper-reflective globules were measured on RCM images and compared to mucin and amyloid deposits seen on histology slides. The mean differences between RCM and histology measurements in both mucin and amyloid were not statistically significant. There were medium and strong correlations between RCM and histology regarding mucin and amyloid deposits, respectively.

This study used an ocean general circulation model to simulate the marine iron cycle in an investigation of how simulated distributions of weak iron-binding ligands would be expected to control dissolved iron concentrations in the ocean, with a particular focus on deep ocean waters. The distribution of apparent oxygen utilization was used as a proxy for humic substances that have recently been hypothesized to account for the bulk of weak iron-binding ligands in seawater. Compared to simulations using a conventional approach with homogeneous ligand distributions, the simulations that incorporated spatially variable ligand concentrations exhibited substantial improvement in the simulation of global dissolved iron distributions as revealed by comparisons with available field data. The improved skill of the simulations resulted largely because the spatially variable ligand distributions led to a more reasonable basin-scale variation of the residence time of iron when present at high concentrations. The model results, in conjunction with evidence from recent field studies, suggest that humic substances play an important role in the iron cycle in the ocean.

Key Points: Size‐fractionation reveals that colloidal iron is more important than soluble iron for distribution of dissolved iron in North AtlanticColloidal iron is the predominantly active species at key supply and removal processes of the dissolved iron cycleGlobal comparison suggests that the colloidal iron fraction in mesopelagic zone is determined by the local lithogenic contribution The low availability of the essential micronutrient iron (Fe) in the ocean impacts the efficiency of the biological carbon pump, and hence, it is vital to elucidate its sources, sinks, and internal cycling. We present size‐fractionated dissolved Fe (dFe, <0.2 μm) measurements from 130 surface samples and 7 full‐depth profiles from the subtropical North Atlantic during summer 2017 and demonstrate the pivotal role of colloidal (cFe, 0.02 to 0.2 μm) over soluble (sFe, <0.02 μm) Fe in controlling the dFe distribution. In the surface (<5 m), a strong west‐to‐east decrease in dFe (1.53 to 0.26 nM) was driven by a dust gradient, which retained dFe predominantly as cFe (61% to 85% of dFe), while sFe remained largely constant at 0.19 ± 0.05 nM. In the euphotic zone, the attenuation of dFe resulted from the depletion of cFe (0% to 30% of dFe), with scavenging as an important driver. In the mesopelagic, cFe was released from sinking biogenic and lithogenic particles, creating a zone of elevated dFe (0.7 to 1.0 nM) between 400 to 1100 m depth. While the ocean interior, below the mesopelagic and above the seafloor boundary, exhibited a narrow range of cFe (40% to 60% of dFe), the abyssal cFe fraction varied in range from 26% to 76% due to interactions with seafloor sediments and a hydrothermal source with almost 100% cFe. Overall, our results produced an hourglass shape for the vertical cFe‐to‐dFe fraction and highlight the primary control of cFe on the dFe distribution. Plain Language Summary: Phytoplankton require nutrients such as phosphorus, nitrogen, and iron. Of these, iron is particularly interesting due to the paradox of its requirement for life‐supporting mechanisms on the one hand and its low oceanic concentrations on the other. Iron is >1000‐fold lower than the "traditional" nutrients. Hence, it is important to know how much iron is introduced to the ocean (sources), how much is removed (sinks), and how it is processed during its residence in the water. This study addressed these questions by measuring the iron concentrations in the subtropical North Atlantic. Our samples were filtered through two filter sizes to investigate the distributions of iron's different size fractions, a popular tool to gain a detailed understanding of the overall iron cycle. We found that the smallest size fraction, "soluble iron," does not vary much throughout the water column, but the slightly larger "colloidal iron" varies a lot, especially in the upper ocean and close to the seafloor, where dynamic supply and removal processes occur. The unequal behaviour of these fractions is an important finding that will improve the accuracy of biogeochemical models for iron, which in turn can improve the prediction of phytoplankton growth in the present and future ocean. [ABSTRACT FROM AUTHOR]

Our study followed the seasonal cycling of soluble (SFe), colloidal (CFe), dissolved (DFe), total dissolvable (TDFe), labile particulate (LPFe), and total particulate (TPFe) iron in the Celtic Sea (NE Atlantic Ocean). Preferential uptake of SFe occurred during the spring bloom, preceding the removal of CFe. Uptake and export of Fe during the spring bloom, coupled with a reduction in vertical exchange, led to Fe deplete surface waters (<0.2 nM DFe; 0.11 nM LPFe, 0.45 nM TDFe, and 1.84 nM TPFe) during summer stratification. Below the seasonal thermocline, DFe concentrations increased from spring to autumn, mirroring NO3− and consistent with supply from remineralized sinking organic material, and cycled independently of particulate Fe over seasonal timescales. These results demonstrate that summer Fe availability is comparable to the seasonally Fe limited Ross Sea shelf and therefore is likely low enough to affect phytoplankton growth and species composition. [ABSTRACT FROM AUTHOR]

In many parts of the global ocean, iron bioavailability is a critical factor controlling primary productivity. Because colloidal Fe (cFe) makes up a substantial proportion of the surface water Fe inventory, it is critical to evaluate the biogeochemical behaviour and availability of this quantitatively important Fe pool. Here we present and review a compilation of over 1300 soluble and corresponding dissolved Fe concentration measurements from which operationally-defined colloidal Fe data have been determined. Partitioning between cFe and the smaller soluble Fe (sFe) size fraction shows a high degree of variability in the surface ocean caused by the range of factors affecting size fractionation (e.g., Fe sources, ligand controls, thermodynamic controls, biological utilisation). Evaluation of the seasonality of sFe and cFe partitioning indicates that a strong decoupling between their respective biogeochemical cycling occurs in the spring time. This suggests that a seasonal component (e.g., seasonal biological growth, seasonal changes in Fe supply) is responsible for a proportion of the observed surface ocean variability. Below depths of 200 m, the partitioning becomes more constant (49 ± 17%). To better understand the bioavailability of the cFe pool, future work should complement traditional size-fractionated data with molecular level investigation into the functional group chemistry of organic Fe colloids and the mineralogy and surface chemistry of inorganic Fe colloids. [ABSTRACT FROM AUTHOR]

Primary mucinous carcinoma (PMC) of the skin is a rare adnexal tumor of sweat gland origin. A case report is presented of a 50-year-old female who presented with a gluteal mass, which was diagnosed as an injection abscess. Following incision and drainage, the incision site persisted as a non-healing ulcer. An edge biopsy of the lesion revealed mucinous carcinoma of the skin. Investigations excluded the possibility of a metastatic mucinous carcinoma. Thus, the lesion in the gluteal region was diagnosed as PMC of the skin, a rare site of occurrence.

Objective: To compare the efficacy of ferrous ascorbate and colloidal iron in the treatment of iron deficiency anemia in children. Methods: Eighty one children, aged 6 mo to 12 y, were screened for iron deficiency anemia (IDA) and those diagnosed with IDA were randomized to receive ferrous ascorbate or colloidal iron for a period of 12 wk, such that each child received elemental iron 3 mg/kg body weight/d. Increase in hemoglobin (Hb) level was the primary outcome measure. Assessment was performed at baseline, wk 4, wk 8 and wk 12. Results: Of 81 children screened, 73 were included in the study. The mean rise in Hb at the end of the 12 wk was significantly higher in ferrous ascorbate group than the colloidal iron group [3.59 ± 1.67 g/dl vs. 2.43 ± 1.73 g/dl; P < 0.01]. Significantly higher proportion of children receiving ferrous ascorbate (64.86 % vs. 31.03 %; P < 0.01) became non-anemic in comparison to colloidal iron. Conclusions: Ferrous ascorbate provides a significantly higher rise in hemoglobin levels in comparison to colloidal iron. The study supports the use of ferrous ascorbate in the pediatric age group, providing evidence for its role as an efficient oral iron supplement in the treatment of iron deficiency anemia. [ABSTRACT FROM AUTHOR]

Re-staining of formalin fixed paraffin sections sometimes is required and this requires prior de-staining. Some simple and effective protocols for de-staining are described. Mucihematoxylin and mucicarmine can be removed with acid alcohol. Zirconyl hematoxylin can be removed with periodic acid or Sinha's fixative. Alcian blue can be removed with 5% trifluoroacetic acid in dichloromethane. Colloidal iron can be bleached in 1% household bleach in alcohol. PAS can be removed with hydrogen peroxide or ammonium hydroxide. With few exceptions, de-stained sections can be re-stained with mucihematoxylin, PAS or Gabe's trichrome. [ABSTRACT FROM AUTHOR]

Abstract: Experiments were conducted using a semi-continuous batch protocol to leach iron from aerosol samples. In this procedure, aerosol samples were leached in an all-Teflon flow-through reaction chamber by multiple (3–4) aliquots (~40 mL) of leaching solution per predetermined leaching period. Several types of aerosols and leaching solutions were tested. The percent iron solubility obtained from aerosols collected in two urban regions was lower (~1%) than the solubility from samples collected in the open ocean (6–13%). This regional variability was greater than the variability obtained when leaching the same aerosols using different leaching solutions. A review of field and laboratory studies from the literature also suggests that different types of aerosols produce more variability in aerosol iron solubility estimates than do different leaching protocols. Colloidal iron was the dominant fraction of aerosol derived dissolved iron in all the experiments carried out during this study, suggesting that the dissolution of aerosol iron in seawater is dominated by the colloidal iron fraction. [Copyright &y& Elsevier]

Abstract: Peat bogs have the ability to produce strong chelate ligands (humic and fulvic acids) which enhance the weathering rates of iron-silicate minerals and greatly increase the solubility of the essential trace metal iron in river water. Fluvial networks link peat bogs with the ocean, and thus terrestrial-derived fulvic-iron complexes fuel the ocean''s biological productivity and biological carbon pump, but understanding this role is constrained by inconsistent observations regarding the behaviour of riverine iron in the estuarine mixing zone, where precipitation reactions remove iron from the water column. We applied a characterization of the colloidal iron carriers in peatland-draining rivers in North Scotland, using field-flow fractionation (FFF), in combination with end-member mixing experiments of river water sampled near the river mouth and coastal seawater using a 59Fe radiotracer method. According to our results, the investigated river contributed “truly dissolved” Fe concentrations of about 3300nmolL−1 to the ocean which is nearly two orders of magnitude higher than the dissolved iron contribution of the “average world” river (∼40nmolL−1). Thus we conclude that peatland-draining rivers are important sources of dissolved iron to the ocean margins. We propose highly electrostatic and sterical stabilized iron-organic matter complexes in the size range of <2kDa to be responsible for iron transport across the estuarine mixing zone. [Copyright &y& Elsevier]

Ingestion rates of free-living marine nematodes were measured using colloidal iron as a tracer. Detritus labeled with colloidal iron was fed to the dominant nematode species and cultured. The iron ingested by the animals was quantified using a scanning electron microscope equipped with differential X-ray energy analyzer (EDX). Average ingestion rates (µgC ind.−1day−1) measured were 0.28 for Symplocostoma sp., 0.29 for Polygastrophora sp., 0.73 for Mesacanthion sp. and 0.15 for Metachromadora sp. These values were higher than the values determined using radioactive organic materials as tracers, though the present method is considered to provide conservative values. This result thus strongly suggests that the ingestion rates of nematodes measured so far were underestimated, and the use of colloidal iron has advantages over the use of radioactive organic matter as a tracer. [ABSTRACT FROM AUTHOR]

The finding of various forms of iron, and less commonly of manganese, in numerous reverse osmosis (RO) membrane autopsies we have performed, have led us to call attention to the diverse forms of iron and manganese species in natural waters. Correlation of elemental composition analyses of foulants with contaminants in RO feedwater and pretreatment steps led to successful solutions of fouling problems by the modification of pretreatments, such as substitution of oxidation filtration strategies for iron and manganese reduction with sequestration with appropriate antiscalants. Cases where iron and manganese fouling failed to respond to sequestration with antiscalants, the existence in the RO feedwater of pre-existing colloidal iron and manganese particles are suspected. To fully control such fouling, speciation of the different forms of iron and manganese that exist in natural, industrial and wastewaters are central to process design, troubleshooting and RO system operation and maintenance. We have undertaken a literature review of iron and manganese colloidal chemistry as it pertains to the RO membrane process. This paper reviews the background for the association of iron with manganese in water treatment, natural sources of colloidal iron and manganese, early works on methods for the reduction of iron and manganese in traditional municipal waterworks where colloidal particles may be formed and not removed, speciation of forms of iron and manganese in water treatment, and the effects of colloidal iron and manganese on RO operation. The overall conclusion is that methods for the speciation and quantitation of colloidal forms of iron and manganese need to be fully developed and employed for the validation of methods useful for the control and/or removal of such foulants in RO feedwaters. [ABSTRACT FROM AUTHOR]

Abstract: During the Sub-arctic Ecosystem Response to Iron Enrichment Study (SERIES), the addition of ferrous iron to high-nitrate low-chlorophyll (HNLC) waters near Ocean Station PAPA (OSP: 50°N, 145°W) produced a phytoplankton bloom and CO2 drawdown, as evidenced by decreasing CO2 fugacity (fCO2). We analyzed five fractions or phases of iron: soluble (<0.03μm), dissolved (<0.22μm), total dissolved (acidified dissolved, <0.22μm), labile (unfiltered), and total (acidified, unfiltered). From these, we also calculated non-labile iron, colloidal iron (0.03–0.22μm), and both labile and non-labile particulate iron (>0.22μm). Here, we describe iron distributions and the evolution of iron phases in the upper ocean during the experiment. We also present an iron budget accounting for horizontal and vertical dilution. At the time of our first sampling eight hours after fertilization was completed, total iron reached 8.6nmolL−1 and dissolved iron was approximately 3nmolL−1. Early in the experiment the dissolved iron phase decreased the most rapidly and by late day 6 the integrated dissolved iron (8.6μmolm−2) represented less than 10% of the initial addition (90–95μmolm−2). However at this same time the total integrated iron at the centre of the patch was still 52μmolm−2 or almost 60% of the calculated initial addition. By day 12,45% of the added iron (from both injections) could be accounted for in the patch. The half-life of total iron in the patch for the first injection was estimated to be less than 5 days if dilution is not considered, but more than 13 days if dilution is taken into account. The most notable change in iron percentages from one form to another occurred early in the first week of the experiment where the predominant phase shift was from the colloidal portion of dissolved iron to labile particulate iron that could have been biologically induced or simply aggregation of oxyhydroxides. This was immediately followed by a physical event resulting in a reduction in the non-labile particulate iron due to sinking out of the patch. The second infusion did not change the relative concentration of the various pools of iron as might be expected, but this was likely due to the fact that it was a much smaller injection than the first. The most pronounced change after the second infusion was the reduction in the labile particulate pool which coincided with one of the largest decreases in silicate observed during the entire experiment. In general the gradual decrease in the fraction of the 10m colloidal iron as well as episodic losses of, or shifts in, integrated colloidal iron are thought to be the result of adsorption of colloidal iron to the plankton cell surfaces as well as aggregation of oxyhydroxides but could also be the result of utilization of colloidal iron by mixotrophic phytoplankton. [Copyright &y& Elsevier]

To observe changes in the concentrations of size- fractionated iron and related environmental factors, experiments were conducted in the northeastern part of the shallow eutrophic lake Dianchi (China) from March 2003 to February 2004. lion concentrations were measured for three size fractions: particulate iron (φ >0.22 μm), colloidal iron (φ = 0.025-0.22 μm) and soluble iron (φ < 0.025 μm), and environmental factors (physicochemical and biological factors) were synchronously analyzed. Results showed that size-fractionated iron and the related environmental factors all varied with season. Colloidal iron accounted for only 5-9% of total iron, while particulate and soluble iron each accounted for 40-50% of total iron. The results suggested that size-fractionated iron can transform into each other, especially the highly reactive colloidal iron. Significant linear correlations were found between iron in different size fractions, and significant correlations were also obtained between chlorophyll a and environmental factors, such as TN, TP and secchi depth. No significant correlation between iron and chlorophyll a was found in this study. [ABSTRACT FROM AUTHOR]

Fouling of 50 kDa MWCO OF membranes by iron colloid solutions was studied under constant pressure conditions and constant flux conditions at pH 5 and 7 and at various iron concentrations. By using measurements of both deposit density and changes in the transmembrane zeta potential, threshold fluxes were found that were significantly lower than the critical flux based on the increase in membrane resistance. Results are consistent with the initial phase of fouling involving pore blockage followed by cake filtration-like behavior. The dependence of fouling rate on iron colloid concentration can best be understood by realizing that only a small fraction of the iron colloid is of a size that contributes to fouling. [Copyright &y& Elsevier]

Pore water from a wetland receiving acid mine drainage was studied for its iron and natural organic matter (NOM) geochemistry on three different sampling dates during summer 1994. Samples were obtained using a new sampling technique that is based on screened pipes of varying length (several centimeters), into which dialysis vessels can be placed and that can be screwed together to allow for vertical pore-water sampling. The iron concentration increased with time (through the summer) and had distinct peaks in the subsurface. Iron was mainly in the ferrous form; however, close to the surface, significant amounts of ferric iron (up to 40% of 2 mmol L-1 total iron concentration) were observed. In all samples studied, iron was strongly associated with NOM. Results from laboratory experiments indicate that the NOM stabilizes the ferric iron as small iron oxide colloids (able to pass a 0.45μm dialysis membrane). We hypothesize that, in the pore water of the wetland, the high NOM concentrations (>100 mg C L-1) allow formation of such colloids at the redoxcline close to the surface and at the contact zone to the adjacent oxic aquifer. Therefore, particle transport along flow paths and resultant export of ferric iron from the wetland into ground water might be possible. [ABSTRACT FROM AUTHOR]

The action of trypsin (E.C. 3.4.4.4) and papain (E.C. 3.4.4.10) on the surface structure of the microvillus membrane was investigated using isolated brushborder fragments of rat kidney cortex. Negative staining was employed to demonstrate the surface structure. The composition of the membrane was determined by biochemical analysis. Untreated membranes show a smooth surface without any structural arrangement. These membranes contain alkaline phosphatase (E.C. 3.1.3.1), aminopeptidase (E.C. 3.4.1.2), protein and 12 μg hexosamine per mg protein. Treatment with trypsin lowers the equilibrium density of the brushborder fragments from 1.17 to 1.15 and releases glycoproteins from the membranes which contain 54% of the hexosamine present in the starting material. There is no change in the content of alkaline phosphatase and aminopeptidase following trypsin treatment. Negative staining now reveals the presence of particles with a diameter of 40 Å on the surface of the membrane. When brushborder fragments are treated with papain the surface particles as well as glycoproteins are removed. Aminopeptidase activity is no longer present in the brushborder membrane and the equilibrium density is decreased to 1.12. It is proposed that the composition of the surface of kidney microvilli is as follows: particles with aminopeptidase activity are attached to a membrane matrix which contains alkaline phosphatase, other proteins and hexosamine. Both structure are covered by a glycoprotein layer which gives the microvillus surface its smooth appearance. [ABSTRACT FROM AUTHOR]