[Imaging the tear film and in vivo cornea. Initial results with a modified confocal laser scanning ophthalmoscope]

BACKGROUND

As the slit-lamp technique is an restrictive method for the in-vivo examination and documentation of the multi-layer structure and the dynamics of the tear film and the cornea, e.g. during wound-healing processes, our aim was to find out whether it is possible to change the confocal laser scanning ophthalmoscope (CLSO; Zeiss) into a high-resolution confocal laser scanning microscope (CLSM) for the visualization of the anterior segments of the eye and the tear film by adapting a special objective system. In order to gain information concerning the function of such a cornea microscope we examined selected patients with erosion or pterygium before and after operation and subsequent phototherapeutic keratectomy (PTK), as well as some in-vitro corneal surfaces after photorefractive keratectomy (PRK).

MATERIAL AND METHODS

We developed an objective adapter for the CLSO in order to focus the laser beam onto anterior segments of the eye to visualize the tear film and the layer structure of the cornea. By combining the long-distance objective or a contract objective with different zoom-optic lenses it was possible to alter the scan field size and so the magnification of the CLSM by a factor of up to 1000.

RESULTS

The CLSM provides a new method for the in-vivo examination of the tear film and its dynamics as well as the thin layers of the cornea in real-time imaging technique with high contrast and resolution in non-contact or contact procedures.

CONCLUSION

This system is a unique took for evaluating and monitoring the effect of excimer laser ablation after PTK and PRK on the corneal surface and the dynamics of the tear film. The application of this method causes no pain for the patient.

Polymorphonuclear leukocyte cells and elastase in tears

PURPOSE

To characterize the effects that mode of sampling and overnight eye closure have on the nature of caseinolytic activity recovered in tear fluid.

METHODS

Reflex, open and closed (R, O and C) eye tear fluids were collected by microcapillary tubes or from the inferior formix by Schirmer strip. Microcapillary collected samples were centrifuged and recovered cells cytochemically characterized and probed by immunofluorescence microscopy, or alternatively extracted in acidic PBS. Tear supernatants, pellets and Schirmer strip extracts were subjected to casein zymography or SDS-PAGE and immunoprobed for plasmin/plasminogen. To identify caseinolytic activity, samples were immunoprecipitated with antibodies to plasmin/plasminogen or to elastase, and the immunoprecipitated materials were subjected to zymographic analysis.

RESULTS

Immunoblot assays revealed R and O samples contained low levels of plasminogen (approximately 1.1 micrograms/ml) and only trace levels of plasmin (< 0.1 ng/ml). Insufficient levels of caseinolytic activity were present to allow zymographic detection. Cytochemical analysis revealed that R and O pellets consisted almost exclusively of desquamated epithelium. Immunoblot analysis revealed that C fluid was associated with an increase in plasminogen and its partial conversion to plasmin (approximately 3.2 ng/ microliter), high molecular weight covalent complexes and degradative products. Zymographic analysis disclosed much greater caseinolytic activity than could be attributed to plasmin or its cleavage products. This consisted primarily of three bands (30-26 kDa) which were identified as polymorphonuclear leukocyte (PMN) cell elastase based on size and antigenicity. This is derived from PMNs recovered from the C pellet. Elastase could also be recovered from Schirmer strips from 90% of donors, provided that the strips were extracted in sample loading buffer. The activity was restricted to the portion of the strip that had been in contact with the ocular tissue.

CONCLUSIONS

The main source of caseinolytic activity in C fluid is elastase. This arises from PMNs that undergo recruitment, activation and degranulation in the C environment. In contrast, the elastase recovered in Schirmer strip extracts is derived from intact PMNs that adhere to the strip during sample collection. This would suggest that PMN cells undergo a low level of recruitment into the open eye environment.

Efficacy of lodoxamide eye drops on mast cells and eosinophils after allergen challenge in allergic conjunctivitis

PURPOSE

The purpose of the study is to evaluate in a double-blind, randomized study the efficacy of lodoxamide tromethamine 0.1% versus placebo.

METHODS

Signs and symptoms, tear tryptase, and tear fluid cytology were evaluated in 20 asymptomatic subjects with allergic conjunctivitis. The study included three allergen challenges in skin test-positive patients. At the first visit, a threshold dose of allergen was established. At the second visit, a bilateral ocular challenge was performed without pretreatment. At the third visit, either lodoxamide or placebo eye drops were used for 1 week before ocular challenge.

RESULTS

Lodoxamide significantly reduced tryptase levels (P < 0.01), neutrophils (P < 0.04), and eosinophils (P < 0.01) in the tear fluid and significantly inhibited ocular itching (P < 0.02) when compared with that of placebo.

CONCLUSIONS

Lodoxamide is effective in reducing tryptase levels and the recruitment of inflammatory cells in the tear fluid after allergen challenge.

Flow cytometry tear analysis in patients with chronic allergic conjunctivitis

Tears from patients with chronic allergic conjunctivitis were analyzed with flow cytometry to determine the function of the T lymphocyte-related immunological reactions in the disease pathogenesis. Twenty-eight patients and 22 normal volunteers were studied; tears were obtained with capillary tubes. T helper/T suppressor ratios and the percentages of HLA DR+, CD23+, and CD3+ cells were significantly higher in patients than in controls. This study provides support for the hypothesis that chronic allergic conjunctivitis results from T lymphocyte-related immunological reactions.

Assessment of the tear film with tandem scanning confocal microscopy

PURPOSE

To evaluate the confocal microscopic appearance of the tear film by correlating the interference pattern created by the lipid surface with physiologic measurements of tear function and with clinic diagnosis.

METHODS

A series of 53 patients was examined with a battery of tests to evaluate the tear film and with confocal microscopy to evaluate the interference pattern created by the lipid surface of the tear film. Assessment of the tear-film appearance involved five nonparametric scales: degree of debris in the tear film, variability of the interference pattern over time, linearity of the interference pattern, number of dry spots seen, and relative thickness of the lipid layer. For 10 other subjects, the confocal appearance of the tear film and evaporation from the ocular surface was tested for before and after meibomian gland expression.

RESULTS

The confocal appearance correlated with multiple physiologic variables and clinical diagnosis. Patients with seborrheic meiboman gland dysfunction had a thicker lipid layer, greater variability, and more debris than did patients with obstructive meibomiam gland dysfunction. After meibomian gland expression, lipid thickness and linearity increased, whereas pattern variability and evaporation decreased significantly. The mean reduction in evaporation was 48%.

CONCLUSIONS

Confocal microscopy can be used to examine the tear film, and its appearance correlates significantly with physiologic variables in normal-individuals and in disease states. The confocal appearance of the lipid interference patterns and evaporation from the ocular surface are altered after lipid expression from meibomian glands.

[Exfoliative conjunctival cytology in Sjögren's syndrome]

The study has been performed in order to give a contribution to the knowledge of the conjunctival cytology in Sjögren's syndrome. Exfoliated cells were obtained by a tampon from the inferior conjunctival sac of 11 patients affected by primary or secondary Sjögren's syndrome; the material was placed onto a microscope slide and it was stained with May-Grumwald-Giemsa, after the ferning test had been performed. Tear production had been previously assessed in all patients by Schirmer's I test and basic secretion test that showed an evident tear fluid hyposecretion. Cytoanalysis of tear fluid showed, in all patients, a marked lymphocytosis, presence of epithelial cells (sometimes cell-adhesion) and a number of "snake-cells", with a nucleus-cytoplasm ratio in favour of cytoplasm. Some of those cells have a thicker cytoplasmic membrane and/or a spindle-shaped peripheral nucleus, characteristic finding of keratoconjunctivitis sicca. In all cases we found mucus and fibrin (sometimes with included cells), organic debris and big crystals, some of which aggregated.

[Ultrastructural study on rat precorneal tear film by quick freezing-freeze substitution method]

The ultrastructure of rat precorneal tear film was visualized by the quick freezing-freeze substitution (QF-FS) method and the conventional fixative method under the scanning electron microscope (SEM). In the QF-FS method, the eyeballs of rats were quickly frozen with an isopentane-propane mixture cooled by liquid nitrogen applied directly to the eyes. After enucleating the eyes and fracturing the frozen corneas, the corneas were prepared for SEM observation. In contrast to the conventional fixative method, by which the microvilli of the surface epithelial cells could be visualized clearly, a very thin membrane-like structure was observed to cover the corneal surface in the eyes prepared by the QF-Fs method. Between the membrane-like structure and corneal surface, a homogeneous and fine network-like structure was observed. The results suggest that the structure of the tear film might be different from the one we have believed to consist of three layers until now. The QF-FS method is considered to be useful for the morphological study of the precorneal tear film.

Oxidative reactions in the tear fluid of patients suffering from dry eyes

PURPOSE

To evaluate whether products of oxidative and inflammatory reactions are detectable in the tear fluid of patients suffering from dry eyes.

METHODS

The tear fluid of 217 patients (397 eyes) was sampled. Criteria for grouping of the patients were (1) basic secretion test ("sicca 1": BST = 0-5 mm, n = 78 eyes; "sicca 2": BST = 6-10 mm, n = 109 eyes) and (2) subjective symptoms (normal BST, burning, foreign body sensations, tearing, dryness of the eyes: n = 78 eyes). One group of healthy patients (normal BST, n = 132 eyes) served as controls. Lipid peroxide levels and myeloperoxidase activity, as parameters for oxidative tissue damage and inflammatory activity, were determined in the tear fluid. Those patients whose consent could be obtained were subjected to the rose bengal test (sicca 1: 56 eyes; sicca 2: 97 eyes; subjective symptoms: 44 eyes; controls: 49 eyes). The correlation between BST and rose bengal test results was calculated.

RESULTS

Lipid peroxides were significantly (P < 0.05) higher in the groups sicca 1 and subjective symptoms than in healthy controls, as was the inflammatory activity in groups sicca 1, sicca 2 and subjective symptoms. Additionally, the inflammatory activity in the group sicca 1 was significantly (P < 0.05) higher than in the groups sicca 2 and subjective symptoms. No evidence of a significant correlation between BST and rose bengal test results was observed.

CONCLUSIONS

Both oxidative tissue damage and polymorphonuclear leukocytes indicating an oxidative potential occur in the tear film of patients suffering from dry eyes. These reactions lead to severe damage of the involved tissue. Free radicals and inflammation may be involved in the pathogenesis or in the self-propagation of the disease.

Eosinophil cationic protein in tears of normal subjects and patients affected by vernal keratoconjunctivitis

The objectives of this study were to determine: 1) levels of tear eosinophil cationic protein (ECP) in patients with vernal keratoconjunctivitis (VKC); 2) the effect of pharmacologic therapy on ECP release; and 3) the correlation of this mediator with the severity of the disease. Tears were collected from 10 controls and 20 VKC patients before and after therapy for cytologic analysis and ECP measurement by radioimmunoassay. Ocular signs and symptoms were evaluated before tear collection. Mean ECP levels in controls were 7.5 +/- 0.4 microgram/l, and in VKC patients, 988.3 +/- 128 micrograms/l before therapy (P < 0.001) and 566.3 +/- 121 micrograms/l after therapy (P < 0.005). In dexamethasone (Dex) 0.1% or cyclosporin A (CsA) 2% patients (five per group), tear ECP decreased significantly after 7-14 days of treatment. Disodium cromoglycate (DSCG) 4% (five patients) for 14 days did not significantly affect ECP levels. ECP levels were significantly correlated with allergic signs (P < 0.001), symptoms (P < 0.001), and the number of eosinophils in tears (P < 0.005). The results of this study suggest that tear ECP levels accurately reflect the clinical status of VKC patients. The measurement of ECP may prove useful not only in the diagnosis and monitoring of allergic disease, but also as an objective parameter for the evaluation of new antiallergic therapies.

[Effect of prostaglandin inhibitors on leukocyte infiltration and on leukocytes in tears after experimental injury of the corneal epithelium]

The effect of drug inhibiting prostaglandin biosynthesis on permeation of leucocytes to tears and cornea after experimental removal of the epithelium was observed. It was shown that 1% indomethacin solution and 0.5% ibuprofen solution considerably decreased the number of leucocytes in tears and their infiltration to the proper substance and negatively affected regeneration of corneal epithelium. The investigations have shown that prostaglandin inhibitors inhibit leucocyte infiltration and may, in effect, decrease the number of proteolytic enzymes that are secreted by these cells.

Effects of nitrous acid exposure on human mucous membranes

Nitrous acid (HONO) is formed both indirectly from the reaction of nitrogen dioxide (NO2) with water on indoor surfaces, and directly during combustion. This gaseous pollutant may be a previously unrecognized causal factor in assessments of nitrogen oxide exposure effects. The present study is the first attempt to evaluate exposure effects of HONO on the human airways and the mucous membranes of the eyes and nose. Fifteen healthy adult nonsmokers were exposed for 3.5 h in a double-blind, balanced protocol to clean air, 77, and 395 ppb HONO. Each exposure was preceded by a 1-h baseline measurement period, and exposures were separated by 1 wk. There was a 10-min exercise period during exposure. Effects measurements included assessment of bronchial reactivity, measurement of specific airway conductance, spirometry, acoustic rhinometry, nasal lavage, tear-fluid cytology, a CO2 eye-provocation test, evaluation of eye redness, and subjective sensations. Effects of HONO exposure on the eyes were found as exposure-related changes in tear-fluid cytology. In particular, the number of squamous cells increased by 20, 67, and 80% following exposure to clean air, 77, and 395 ppb HONO, respectively (p = 0.004). Possible indications of exposure effects on sensitivity to CO2 eye provocation and on specific airway conductance were also measured. For specific airway conductance there was an approximate 10% decrease in conductance following exercise in association with HONO exposure, compared with a 2% decrease with clean air (p = 0.038).

Vernal keratoconjunctivitis: a model of 5q cytokine gene cluster disease

Clinical studies of vernal keratoconjunctivitis (VKC) patients show that total IgE serum levels are increased even in the absence of IgE antibodies to common allergens. Activated eosinophils are also a constant feature of VKC at both the circulation (cytofluorimetry) and tissue (tear cytology and conjunctival scrapings) levels. Moreover, allergen challenge induces a prolonged inflammatory reaction with a prevalent participation of eosinophils, lymphocytes and possibly basophils. Immunohistochemical studies of VKC biopsies show a multicellular inflammatory infiltrate with prevalence of activated eosinophils, mast cells and CD4 lymphocytes in both epithelium and subepithelium. Mediator studies indicate that eosinophil products (eosinophil peroxidase, eosinophinal cationic protein and eosinophil-derived neurotoxin/eosinophil protein X) are increased in both serum and tears, where tryptase and interleukin (IL)-5 are also detectable in higher amounts than in controls. On the basis of these findings, we postulate that VKC can represent a phenotypic model of up-regulation of the cytokine gene cluster on chromosome 5q which through its products (IL-3, IL-4, IL-5 and granulocyte/macrophage-colony-stimulating factor) regulates Th2 prevalence, IgE production as well as mast cell and eosinophil growth and function in VKC.

Tear Fluid Cellular Fibronectin Levels After Photorefractive Keratectomy

BACKGROUND

Fibronectin is supposed to have an important role in wound healing. The extradomain A-containing cellular fibronectin (EDAcFn) refers to fibronectin, which instead of being a hepatocyte derived component of blood plasma or body fluids, is produced locally. The present study was undertaken to clarify the possible changes in excretion of EDAcFn in tears following excimer laser photorefractive keratectomy (PRK).

METHODS

An immunoassay was used to determine EDAcFn concentrations in human tear fluid samples of 11 eyes after PRK. Tear fluids were collected with scaled microcapillaries preoperatively as well as 1, 2, and 7 days after PRK. The time used to collect a known volume of tears was registered. This was done to estimate the dilution effect related to the hypersecretion of tears after PRK.

RESULTS

The mean preoperative tear fluid EDAcFn concentration was 0.28 +/- 0.07 ng/microliter with a wide range (0.05 to 0.63). The tear fluid flow-corrected excretion of EDAcFn was 1.36 +/- 0.35 ng/min (range, 0.145 to 3.50). There was a significant increase in both postoperative tear fluid flow and excretion of EDAcFn on days 1 and 2. The elevation of the mean EDAcFn concentration did not decrease in spite of reflex tearing. The mean excretion of EDAcFn in tears was 28-fold on the first and 17-fold on the second postoperative day. Normal level was reached by day 7.

CONCLUSION

There is a rapid increase in excretion of EDAcFn in tears following PRK. This seems to last only as long as an epithelial defect persists. The epithelium of all eyes healed in 3 to 4 days in spite of wide interindividual variations in both tear fluid flow and EDAcFn excretion.

Effects of rigid lens extended wear on lactate dehydrogenase activity and isozymes in rabbit tears

Effects of and recovery from continuous wear of four rigid gas permeable (RGP) contact lenses was assessed by noninvasive measurement of lactate dehydrogenase (LDH) activity and isozyme pattern in rabbit tears. Oxygen transmissibility (Dk/L) of lenses used was 27, 44, 84, and 97 x 10(-9) (cm/s)(ml O2/ml mm Hg); lens thickness (0.15 mm) and diameter (14.0 mm) were standardized. Lenses were worn continuously for 90 days; recovery was assessed 30 days after cessation of lens wear. LDH activity was measured by UV rate assay; isozyme subtypes were determined by agarose gel electrophoresis. Light and scanning electron microscopy (LM, SEM) were used with the determination of total protein as additional measures of lens effects. LDH levels were inversely correlated with lens Dk/L values; low Dk/L values increased the anaerobic (LDH4,5)/aerobic (LDH1,2,3) subtypen ratio indicating in vivo metabolic shift. SEM observations were consistent with these results. There was no significant difference in the total cell content of tears or total tear protein levels between control and RGP test-wear groups. Measurement of tear LDH activity and isozyme ratios appears to provide a sensitive, noninvasive assessment of the effects of RGP lens-induced hypoxia over time on the corneal surface. A level of Dk/L of > or = 84 appears best for maintaining corneal physiology during extended wear. Recovery from chronic lens-induced hypoxia is characterized by a return to normal tear LDH levels and isozyme subtypes.

Sampling methods for tear stix tests

Four different methods for transferring conjunctival fluid to stix test pads were studied. One hundred and twenty eyes from 120 cataract-extracted patients (35 in the operated eye 0-5 days post-operatively and 85 in the contralateral eye) were included in the study. Transfer of fluid by a glass rod from the lateral part of the inferior fornix was demonstrated to be the most suitable method, showing the highest sensitivity to leucocyte-esterase (83%, N = 120, p < 0.05) and with a specificity equal to that of the other methods tested (cotton, spongostan, Schirmer paper strip). The glass rod method is suitable for detecting blood, nitrite, albumin, pH, and glucose, and is a both quick and unintrusive procedure. All four transferring methods may, however, cause an increased desquamation of epithelial cells and an increased amount of mucus.

Effectiveness of nedocromil sodium 2% eyedrops on clinical symptoms and tear fluid cytology of patients with vernal conjunctivitis

A double-masked, randomised, placebo-controlled study was conducted to evaluate the effectiveness of nedocromil 2% eyedrops, a mast cell stabilizer, in 20 symptomatic patients with vernal conjunctivitis. A 1-week baseline period was followed by 6 weeks of treatment. Clinical examination and cytological evaluation of tear fluid were performed weekly, and the patients recorded their subjective assessment on a daily diary card. The nedocromil group showed significantly less hyperaemia in the course of treatment than did the placebo group, and significantly less itching at all visits compared with baseline itching. In the nedocromil-treated group, but not in the placebo group, the number of neutrophils, eosinophils and lymphocytes in tears decreased significantly during some treatment weeks when compared with baseline. The overall assessment of treatment efficacy by both clinician and patient was significantly in favour of nedocromil treatment over placebo.

Ocular surface alteration after long-term treatment with an antiglaucomatous drug

BACKGROUND

This study was undertaken to see whether long-term locally applied ocular medications produced any alterations in the ocular surface, and, in particular, whether it caused damage to the mucus layer of the tear film.

METHODS

The authors studied the ocular surface of 40 control subjects (group 1), 21 patients (group 2) chronically treated with a commercial preparation of 0.5% timolol maleate, and 20 previously untreated glaucomatous patients (group 3) in need of treatment with the same drug. Parameters studied were Schirmer's test, lacrimal meniscus height, break-up time, fluorescein and rose Bengal stains, conjunctival impression cytology, mucus staining, and the ferning test.

RESULTS

Patients in groups 2 and 3 showed a significant decrease (P less than 0.001) in the number of normal Schirmer's and break-up time tests. All had positive vital stains. Results showed a significant decrease (P less than 0.001) in goblet-cell density, mucus granules, and reticular sheets, and an increase (P less than 0.001) in pathologic crystallization patterns.

CONCLUSION

These results demonstrate that chronic application of a commercial preparation of timolol maleate damaged the ocular surface, especially the mucus layer of the tear film.

Immunologically induced purulent anterior segment inflammation of the guinea pig eye

A single conjunctival application of ovalbumin to inbred guinea pigs (IMM/S 209) immunized with the same antigen in Freund's complete adjuvant provoked an acute purulent inflammation of the anterior segment of the eyes with a duration of up to 1 week. Intense conjunctival injection and chemosis were followed by a purulent discharge. A corneal haze was observed regularly, and a considerable proportion of the animals developed a pronounced pannus and corneal ulcers. Tear fluid cytology revealed a rapid increase in cell concentration, from the normal level (less than 10(8)/l) to greater than 10(11)/l. Seventy to 95% of the cells were polymorphonuclear leukocytes. Histological examination revealed an acute inflammatory reaction which radiated from the conjunctival fornices to the entire anterior segments of the eyes. The process was characterized by an intense oedema, vasodilation and perivascular aggregations of polymorphonuclear leukocytes, and to a lesser extent eosinophilic granulocytes which characteristically infiltrated and penetrated the epithelial layers. Neovascularization could be observed early after challenge in the stroma of all parts of the outer eye. Ulcerations of the conjunctival and corneal epithelia were observed frequently. After a number of reiterations of the antigenic challenge, a marked infiltration with lymphocytes and basophils/mast cells was observed, and significant scarring of the conjunctival mucosa developed. In several animals, a slight, but significant co-reaction of the contra-lateral, non-challenged eye was observed.

Clinical and cytologic aspects of ocular late-phase reaction in the guinea pig

Conjunctival tissue of 12 guinea pigs (6 experimental animals, 6 controls) was sensitized with IgG1 (PCA titer 1:2,000) antisera to dinitrophenol carrier and challenged topically with hapten. Periorbital swelling, conjunctival edema and conjunctival redness were evaluated clinically at 0.5 h and hourly for 12 h, and again at 24 and 48 h. Tears were collected at each time point for cytology. Results showed an early-phase reaction (EPR) peaking at 0.5 h and a late-phase reaction (LPR) peaking at 5-7 h. Periorbital swelling was minimal 4 h after appearance of the EPR. Conjunctival edema and redness increased in both the early and late phases, but without two distinct peaks. However, individual animals in both groups did show biphasic reactions. Tear cytology showed an abnormal increase in neutrophils and eosinophils in the later time periods and was a significant way to detect ocular anaphylaxis. In conclusion, if the presence of two significant peaks of clinical inflammation after one antigen challenge is taken as the narrowest definition of a late phase in anaphylaxis, our results show an ocular LPR occurring in our animal model.

[Effect of drugs inhibiting prostaglandin activity on the appearance of leukocytes in the tears after experimental injury to the corneal epithelium]

The investigations were carried out on 3 groups of rabbits in which the corneal epithelium was removed and into the conjunctival sac were instilled drugs inhibiting the activity of prostaglandins such as indomethacin solution or a 0.5 p.c. solution of ibuprofen. It was demonstrated that these drugs lead in the period of the first 8 hours to reduction of the number of leucocytes in tears from 2 to 20 times in comparison with the control group. The presented observation show that prostaglandin inhibitors may be a successful mean leading to decrease of number of leucocytes in tears after corneal injury.

Quantitative tear meniscus cytology versus quantitative conjunctival fluid cytology

A technique for quantitative cytology of tear fluid collected by a micropipette from the tear meniscus is presented. This technique has been compared with that using the standardized Norn-pipette collecting fluid from the lateral part of the inferior fornix. The two methods show the same magnitude of sensitivity and specificity, the same coefficient of variation, as regards neutrophilic leucocytes and erythrocytes (200 samples). The normal tear meniscus contains less than 40 leucocytes or erythrocytes per microliter. The tear cell concentration is about 200 times lower in the meniscus than in the fornix inferior in normal eyes. This observation supports the compartment-theory of a rapid flow in the tear meniscus and a slow flow with insignificant admixture in the conjunctival fluid in the fornices. The tear meniscus cytology is of use in association with testing of other parameters in the tears, for instance. The conjunctival fluid cytology is quicker and more convenient for the patient. In addition, it is necessary in connection with observation for pathological epithelial desquamation, because the tear meniscus cytology brings about an artificial increase of the epithelial cell concentration.