Ординатура / Офтальмология / Английские материалы / Dry Eye and Ocular Surface Disorders_Pflugfelder, Beuerman, Elliot Stern_2004
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tear film proposed by Wolff comprises three distinct components: the mucin layer coating the surface epithelium, the aqueous layer making up the majority of the tear film, and the thin lipid layer sitting on top and slowing evaporation (84). However, several types of measurements question the existence of a free fluid layer beneath the lipid layer (85–87). The currently proposed tear film structure is that of a mucin/aqueous gel decreasing in density toward the lipid layer (Fig. 1) (9,88).
IV. EPITHELIAL SURFACE
The ocular surface is composed of two important, structurally divergent epithelia: on the corneal and on the conjunctival surface (89). Each of these epithelia secretes components which enable the tear film to protect the ocular surface.
Figure 1 Normal tear film structure and components.
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Figure 2 Transmission electron micrograph of the apical layers of the human cornea. The microvilli and microplique are several tenths of a micrometer in length and support small filaments that radiate into the tear layer. They help maintain a regular tear film across the cornea. Tight junctions (zona occludens) are highly resistant to passage of aqueous solution and proteins between the adjacent flattened apical epithelial cells of the cornea. Magnification 17,500.
The corneal epithelium is approximately five to seven cell layers thick. The superficial epithelium contains microvilli that help anchor the tear film (Fig. 2). As cells become senescent and slough, exfoliation holes may be seen by electron microscopy (90). Tear film proteases partially regulate corneal epithelial cell exfoliation. Lacrimal keratoconjunctivitis may disrupt the balance between these proteases and their inhibitors, thereby affecting corneal epithelial thickness. For example, in vitamin A deficiency, decreased production of matrix metalloprotease 9 (MMP-9) correlates with marked thickening of the corneal epithelium, whereas in aqueous tear deficiency, increased protease activity correlates with accelerated desquamation of the corneal epithelium (Fig. 3) (75,91–93).
The barrier function of the corneal epithelium is due to epithelial tight junctions and to the mucins and glycogen coating the apical epithelium. Junctional zonular occludens in the apical corneal epithelium impede pericellular water and protein movement through the corneal epithelium from the tear film or the hygroscopic corneal stroma (94). Epithelial cells synthesize transmembrane mucins, high-molec- ular-weight glycoproteins that coat the hydrophobic cell membranes of the superficial differentiated epithelial cells and extend into the tear film. These mucins lower surface tension and facilitate tear spreading and wetting of the corneal surface. They also serve to anchor the overlaying mucin/aqueous gel to the corneal surface.
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Figure 3 Mechanism of desquamation of the apical corneal epithelium.
The conjunctival epithelium, typically four cell layers thick, contains specialized goblet cells that secrete soluble mucins (95). Soluble mucins wash over the entire epithelial surface and interact with transmembrane mucins anchored to the surfaces of conjunctival and corneal epithelial cells, forming a mucin gel. Additionally, significant numbers of immunocompetent dendritic cells (Langerhans cells) inhabit the conjunctival epithelium and help provide immune protection for this tissue (see Chapter 6 for more detail).
V.MUCIN LAYER
The mucin layer functions as a surfactant for the ocular surface, allowing an evenly spread tear film to wet the hydrophobic epithelium. It is primarily responsible for tear film viscosity, protecting against the shear force of blinking (96) that would otherwise cause abnormal sloughing of epithelial cells, ocular surface irritation, and eventual inflammation. The mucin layer also helps to maintain the ocular surface’s optical purity and refractive power.
Although at first glance the mucin layer appears loosely organized and amorphous, recent findings have demonstrated that it is highly organized in a manner which facilitates its function. Conjunctival and corneal epithelia express
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transmembrane mucins 1, 2, and 4, which form the glycocalyx and anchor the mucin layer to the hydrophobic epithelial cell surface (97–99). The membranespanning domain of MUC1 anchors it to epithelial cells and its extracellular domain extends 200–500 nm into the glycocalyx (100). MUC4, expressed by the stratified conjunctival epithelium (101), forms a sialomucin complex on the surfaces of corneal and conjunctival epithelial cells, and can also be shed into tear fluid as a soluble mucin (48). Conjunctival goblet cells secrete the soluble mucin MUC 5AC (101), which interacts with the membrane-bound mucins and the aqueous layer to form a water-trapping gel. Lacrimal glands secrete MUC-7 into the tear fluid (51,102). MUC1 and MUC4 have been shown to prevent inflammatory cell adhesion (103,104), suggesting that the mucin layer may function, in general, to prevent adherence of inflammatory cells, bacteria, or debris to the ocular surface (99). In summary, expression of mucins on epithelial cell surfaces and the chemical interactions with the soluble mucins promote tear film spreading and ocular surface wetting. An intact and hydrated mucin gel helps protect the epithelium from environmental insult and minimizes shear forces during blinking (8,96,105).
VI. AQUEOUS LAYER
The aqueous layer of the tear film, or the more aqueous portion of the mucin gel, contains dissolved oxygen, electrolytes, and multiple proteins including growth factors (see Tables 1–3), which help maintain a trophic and protective environment for the ocular surface epithelium. The health of ocular surface epithelial tissues depends on growth factors such as EGF (66,106,107), HGF, and KGF (63,64). Immunoglobulins and other proteins such as lactoferrin (108), lysozyme (109), defensins (110), and immunoglobulin A (111) protect the ocular surface from infection by bacteria and viruses. Still other proteins, such as the interleukin 1 receptor antagonist, help minimize ocular surface inflammation (23,112,113).
Tear film electrolytes, such as Na+, K+, Cl–, Ca2+, and others, present in concentrations similar to those found in serum, result in a normal tear osmolarity of about 300 mOsm/L (11,18), which helps maintain normal epithelial cell volume. Ions help solubilize proteins, and in some cases are essential for enzymatic activity. Proper osmolarity is also required for maintenance of normal corneal nerve membrane potential and for cellular homeostasis and secretory function.
The roles of the main and accessory lacrimal glands in tear secretion and the pathogenesis of dry eye disease remain unresolved. Most of the normal daily tear flow comes from the accessory lacrimal glands (the glands of Wolfring and Krause) located in the superior conjunctival fornix and in the upper lid just superior to the meibomian glands. The main lacrimal gland is thought to function primarily as a reservoir supplying the ocular surface with copious amounts of
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fluid to wash out infectious or irritating particles that threaten epithelial integrity (114). Removal of the main lacrimal glands from squirrel monkeys resulted in no damage to the cornea, conjunctiva, or eyelid, indicating that accessory lacrimal gland function was sufficient to maintain a healthy ocular surface (115). The same procedure performed in cats and humans led to signs of keratoconjunctivitis sicca (KCS) (116), suggesting an important role for the main lacrimal glands in maintaining ocular surface health in these species (117).
Most tear secretion is driven, through the lacrimal functional unit, by stimulation of the afferent sensory nerves from the ocular surface. These signals are integrated in the central nervous system and may be modified or altered by cortical functions such as emotion. Tear production decreases by as much 66% under topical anesthesia (118), or to below detectable levels under general anesthesia when all emotional and afferent sensory stimulus for tear secretion is removed (119). Anesthesia of the nasal mucosa decreased tear secretion in the ipsilateral eye by about 34% (120). The effects of anesthesia underscore the importance of neuronal control for normal tear production.
VII. THE LIPID LAYER
The lipid layer, secreted by the meibomian glands whose ducts exit just anterior to the mucocutaneous junction of the lids, minimizes tear evaporation (121) and facilitates tear film spreading over the corneal surface. In addition, the lipid layer prevents skin-surface fatty acids from entering and disrupting the tear film at the lid margins (122).
The lipid layer varies in composition. Polar lipids such as phospholipids, sphingomyelin, ceramides, and cerebrosides are found adjacent to the aqueous phase of the tear film (123), while nonpolar lipids, including wax and cholesterol esters, triglycerides, and free fatty acids, associate with the polar lipids and form the lipid–air interface (124). Even spreading of the lipid layer is important, because accumulation of lipid in thick patches, especially the nonpolar oils, may contaminate the mucin layer, rendering it unwettable (125,126). Blinking helps spread the lipid layer evenly over the tear film surface (127). Uniform tear film spreading is also due partly to the low surface tension of the lipid–air interface, about half that of an aqueous–air interface (128).
VIII. CONCLUSION
The tear film provides an environment that protects the ocular surface and maintains its health. The tear film’s components interact to form a stable structure composed of mucin, aqueous solution, and lipid that protects the ocular surface
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and provides a medium for delivery of supportive and protective proteins. Alteration of any one of the tear film components can destabilize the tear film, interfere with its normal functions, and lead to lacrimal keratoconjunctivitis. Although a reduced quantity of tear production is commonly associated with dry eye disease, alteration of the normal tear film composition is an equally, if not more important, cause of ocular irritation and ocular surface disease.
REFERENCES
1.Rolando M, Zierhut M. The ocular surface and tear film and their dysfunction in dry eye disease. Surv Ophthalmol 2001; 45:S203–S210.
2.Rieger G. The importance of the precorneal tear film for the quality of optical imaging. Br J Ophthalmol 1992; 76:157–158.
3.Rolando M, Iester M, Macri A, Calabria G. Low spatial-contrast sensitivity in dry eyes. Cornea 1998; 17:376–379.
4.Tutt R, Bradley A, Begley C, Thibos LN. Optical and visual impact of tear breakup in human eyes. Invest Ophthalmol Vis Sci 2000; 41:4117–4123.
5.Liu Z, Pflugfelder SC. Corneal surface regularity and the effect of artificial tears in aqueous tear deficiency. Ophthalmology 1999; 106:939–943.
6.De Paiva CS, Harris LD, Pflugfelder SC. Keratoconus-like topographic changes in keratoconjunctivitis sicca. Cornea 2003; 22:22–24.
7.De Paiva CS, Lindsey JL, Pflugfelder SP. Assessing the severity of keratitis sicca with videokeratoscopic indices. Ophthalmology 2003; 110:1102–1109.
8.Doane MG. Interactions of eyelids and tears in corneal wetting and the dynamics of the normal human eyeblink. Am J Ophthalmol 1980; 89:507–516.
9.Dilly PN. Structure and function of the tear film. Adv Exp Med Biol 1994; 350:239–247.
10.Ren H, Wilson G. The effect of a shear force on the cell shedding rate of the corneal epithelium. Acta Ophthalmologica Scand 1997; 75:383–387.
11.Gilbard JP. Human tear film electrolyte concentrations in health and dry-eye disease. Int Ophthalmol Clin 1994; 34:27–36.
12.Tsubota K, Tseng TCG, Nordlund ML. Anatomy and physiology of the ocular surface. In: Holland EJ, Mannis MJ, eds. Ocular Surface Disease New York: Springer-Verlag; 2002.
13.Chen R, Jin Z, Colon LA. Analysis of tear fluid by CE/LIF: a noninvasive approach for glucose monitoring. J Capillary Electrophor 1996; 3:243–248.
14.Ubels JL, MacRae SM. Vitamin A is present as retinol in the tears of humans and rabbits. Curr Eye Res 1984; 3:815–822.
15.Gluud BS, Jensen OL, Krogh E, Birgens HS. Prostaglandin E2 level in tears during postoperative inflammation of the eye. Acta Ophthalmol (Copenh) 1985; 63:375–379.
16.Arnold RR, Cole MF, McGhee JR. A bactericidal effect for human lactoferrin. Science 1977; 197:263–265.
17.Broekhuyse RM. Tear lactoferrin: a bacteriostatic and complexing protein. Invest Ophthalmol 1974; 13:550–554.
56 |
Stern et al. |
18.Craig JP, Simmons PA, Patel S, Tomlinson A. Refractive index and osmolality of human tears. Optom Vis Sci 1995; 72:718–724.
19.Jensen OL, Gluud BS, Birgens HS. The concentration of lactoferrin in tears of normals and of diabetics. Acta Ophthalmol (Copenh) 1986; 64:83–87.
20.Kijlstra A, Jeurissen SH, Koning KM. Lactoferrin levels in normal human tears. Br J Ophthalmol 1983; 67:199–202.
21.Kuizenga A, van Haeringen NJ, Kijlstra A. Inhibition of hydroxyl radical formation by human tears. Invest Ophthalmol Vis Sci 1987; 28:305–313.
22.Seal DV. The effect of ageing and disease on tear constituents. Trans Ophthalmol Soc U K 1985; 104(Pt 4):355–362.
23.Solomon A, Dursun D, Liu Z, Xie Y, Macri A, Pflugfelder SC. Proand antiinflammatory forms of interleukin-1 in the tear fluid and conjunctiva of patients with dry-eye disease. Invest Ophthalmol Vis Sci 2001; 42:2283–2292.
24.Fleming A. Allison VD. Observations on a bacteriolytic substance (“lysozyme”) found in secretions and tissues. Br J Exp Pathol 1922; 3:252–260.
25.Vinding T, Eriksen JS, Nielsen NV. The concentration of lysozyme and secretory IgA in tears from healthy persons with and without contact lens use. Acta Ophthalmol (Copenh) 1987; 65:23–26.
26.Aho VV, Paavilainen V, Nevalainen TJ, Peuravuori H, Saari KM. Diurnal variation in group IIa phospholipase A2 content in tears of contact lens wearers and normal controls. Graefes Arch Clin Exp Ophthalmol 2003; 241:85–88.
27.Qu XD, Lehrer RI. Secretory phospholipase A2 is the principal bactericide for staphylococci and other gram-positive bacteria in human tears. Infect Immun 1998; 66:2791–2797.
28.Saari KM, Aho V, Paavilainen V, Nevalainen TJ. Group II PLA(2) content of tears in normal subjects. Invest Ophthalmol Vis Sci 2001 Feb; 42(2):318–320.
29.Crouch RK, Goletz P, Snyder A, Coles WH. Antioxidant enzymes in human tears. J Ocul Pharmacol 1991; 7:253–258.
30.Kitaoka M, Nakazawa M, Hayasaka S. Lysosomal enzymes in human tear fluid collected by filter paper strips. Exp Eye Res 1985; 41:259–265.
31.Coyle PK, Sibony PA. Tear immunoglobulins measured by ELISA. Invest Ophthalmol Vis Sci 1986; 27:622–625.
32.Sand B, Jensen OL, Eriksen JS, Vinding T. Changes in the concentration of secretory immunoglobulin A in tears during post-operative inflammation of the eye. Acta Ophthalmol (Copenh) 1986; 64:212–215.
33.Coyle PK, Sibony PA, Johnson C. Electrophoresis combined with immunologic identification of human tear proteins. Invest Ophthalmol Vis Sci 1989; 30:1872–1878.
34.Imanishi J, Takahashi F, Inatomi A, Tagami H, Yoshikawa T, Kondo M. Complement levels in human tears. Jpn J Ophthalmol 1982; 26:229–233.
35.Kerenyi A, Nagy G, Veres A, Varga L, Fust A, Nagymihany A, Czumbel N, Suveges I, Fust G. C1r-C1s-C1inhibitor (C1rs-C1inh) complex measurements in tears of patients before and after penetrating keratoplasty. Curr Eye Res 2002; 24:99–104.
36.Willcox MD, Morris CA, Thakur A, Sack RA, Wickson J, Boey W. Complement and complement regulatory proteins in human tears. Invest Ophthalmol Vis Sci 1997; 38:1–8.
The Normal Tear Film and Ocular Surface |
57 |
37.Uchio E, Ono SY, Ikezawa Z, Ohno S. Tear levels of interferon-gamma, interleukin (IL) -2, IL-4 and IL-5 in patients with vernal keratoconjunctivitis, atopic keratoconjunctivitis and allergic conjunctivitis. Clin Exp Allergy 2000; 30:103–109.
38.Pflugfelder SC, Jones D, Ji Z, Afonso A, Monroy D. Altered cytokine balance in the tear fluid and conjunctiva of patients with Sjogren’s syndrome keratoconjunctivitis sicca. Curr Eye Res 1999; 19:201–211.
39.Tishler M, Yaron I, Geyer O, Shirazi I, Naftaliev E, Yaron M. Elevated tear interleukin-6 levels in patients with Sjogren syndrome. Ophthalmology 1998; 105:2327–2329.
40.Tuominen I, Vesaluoma M, Teppo AM, Gronhagen-Riska C, Tervo T. Soluble Fas and Fas ligand in human tear fluid after photorefractive keratectomy. Br J Ophthalmol 1999; 83:1360–1363.
41.Tuominen IS, Tervo TM, Teppo AM, Valle TU, Gronhagen-Riska C, Vesaluoma MH. Human tear fluid PDGF-BB, TNF-alpha and TGF-beta1 vs corneal haze and regeneration of corneal epithelium and subbasal nerve plexus after PRK. Exp Eye Res 2001; 72:631–641.
42.Vesaluoma M, Teppo AM, Gronhagen-Riska C, Tervo T. Increased release of tumour necrosis factor-alpha in human tear fluid after excimer laser induced corneal wound. Br J Ophthalmol 1997; 81:145–149.
43.Vesaluoma MH, Tervo TT. Tenascin and cytokines in tear fluid after photorefractive keratectomy. J Refract Surg 1998; 14:447–454.
44.Sitaramamma T, Shivaji S, Rao GN. HPLC analysis of closed, open, and reflex eye tear proteins. Indian J Ophthalmol 1998; 46:239–245.
45.Jumblatt JE, Cunningham LT, Li Y, Jumblatt MM. Characterization of human ocular mucin secretion mediated by 15(S)-HETE. Cornea 2002; 21:818–824.
46.Arango ME, Li P, Komatsu M, Montes C, Carraway CA, Carraway KL. Production and localization of Muc4/sialomucin complex and its receptor tyrosine kinase ErbB2 in the rat lacrimal gland. Invest Ophthalmol Vis Sci 2001; 42:2749–2756.
47.Argueso P, Balaram M, Spurr-Michaud S, Keutmann HT, Dana MR, Gipson IK. Decreased levels of the goblet cell mucin MUC5AC in tears of patients with Sjogren syndrome. Invest Ophthalmol Vis Sci 2002; 43:1004–1011.
48.Pflugfelder SC, Liu Z, Monroy D, Li DQ, Carvajal ME, Price-Schiavi SA, Idris N, Solomon A, Perez A, Carraway KL. Detection of sialomucin complex (MUC4) in human ocular surface epithelium and tear fluid. Invest Ophthalmol Vis Sci 2000; 41:1316–1326.
49.Jumblatt MM, McKenzie RW, Jumblatt JE. MUC5AC mucin is a component of the human precorneal tear film. Invest Ophthalmol Vis Sci 1999; 40:43–49.
50.Zhao H, Jumblatt JE, Wood TO, Jumblatt MM. Quantification of MUC5AC protein in human tears. Cornea 2001; 20:873–877.
51.Jumblatt MM, McKenzie RW, Steele PS, Emberts CG, Jumblatt JE. MUC7 expression in the human lacrimal gland and conjunctiva. Cornea 2003; 22:41–45.
52.Fukuda M, Fullard RJ, Willcox MD, Baleriola-Lucas C, Bestawros F, Sweeney D, Holden BA. Fibronectin in the tear film. Invest Ophthalmol Vis Sci 1996; 37:459–467.
53.Fukuda M, Wang HF. Dry eye and closed eye tears. Cornea 2000; 19(3 suppl):S44–S48.
58 |
Stern et al. |
54.Jensen OL, Gluud BS, Eriksen HO. Fibronectin in tears following surgical trauma to the eye. Acta Ophthalmol (Copenh) 1985; 63:346–350.
55.Virtanen T, Ylatupa S, Mertaniemi P, Partanen P, Tuunanen T, Tervo T. Tear fluid cellular fibronectin levels after photorefractive keratectomy. J Refract Surg 1995; 11:106–112.
56.Thakur A, Willcox MD. Cytokine and lipid inflammatory mediator profile of human tears during contact lens associated inflammatory diseases. Exp Eye Res 1998; 67:9–19.
57.Van Setten G, Schultz G. Transforming growth factor-alpha is a constant component of human tear fluid. Graefes Arch Clin Exp Ophthalmol 1994; 232:523–526.
58.Van Setten GB, Macauley S, Humphreys-Beher M, Chegini N, Schultz G. Detection of transforming growth factor-alpha mRNA and protein in rat lacrimal glands and characterization of transforming growth factor-alpha in human tears. Invest Ophthalmol Vis Sci 1996; 37:166–173.
59.Gupta A, Monroy D, Ji Z, Yoshino K, Huang A, Pflugfelder SC. Transforming growth factor beta-1 and beta-2 in human tear fluid. Curr Eye Res 1996; 15:605–614.
60.Yoshino K, Garg R, Monroy D, Ji Z, Pflugfelder SC. Production and secretion of transforming growth factor beta (TGF-beta) by the human lacrimal gland. Curr Eye Res 1996; 15:615–624.
61.Kokawa N, Sotozono C, Nishida K, Kinoshita S. High total TGF-beta 2 levels in normal human tears. Curr Eye Res 1996; 15:341–343.
62.Li DQ, Tseng SC. Three patterns of cytokine expression potentially involved in epithelial-fibroblast interactions of human ocular surface. J Cell Physiol 1995; 163:61–79.
63.Wilson SE, Chen L, Mohan RR, Liang Q, Liu J. Expression of HGF, KGF, EGF and receptor messenger RNAs following corneal epithelial wounding. Exp Eye Res 1999; 68:377–397.
64.Wilson SE, Liang Q, Kim WJ. Lacrimal gland HGF, KGF, and EGF mRNA levels increase after corneal epithelial wounding. Invest Ophthalmol Vis Sci 1999; 40:2185–2190.
65.van Setten GB. Basic fibroblast growth factor in human tear fluid: detection of another growth factor. Graefes Arch Clin Exp Ophthalmol 1996; 234:275–277.
66.Van Setten GB, Viinikka L, Tervo T, Pesonen K, Tarkkanen A, Perheentupa J. Epidermal growth factor is a constant component of normal human tear fluid. Graefes Arch Clin Exp Ophthalmol 1989; 227:184–187.
67.Vesaluoma M, Teppo AM, Gronhagen-Riska C, Tervo T. Platelet-derived growth factor-BB (PDGF-BB) in tear fluid: a potential modulator of corneal wound healing following photorefractive keratectomy. Curr Eye Res 1997; 16:825–831.
68.Rocha EM, Cunha DA, Carneiro EM, Boschero AC, Saad MJ, Velloso LA. Identification of insulin in the tear film and insulin receptor and IGF-1 receptor on the human ocular surface. Invest Ophthalmol Vis Sci 2002; 43:963–967.
69.Fujishima H, Takeyama M, Takeuchi T, Saito I, Tsubota K. Elevated levels of substance P in tears of patients with allergic conjunctivitis and vernal keratoconjunctivitis. Clin Exp Allergy 1997; 27:372–378.
The Normal Tear Film and Ocular Surface |
59 |
70.Varnell RJ, Freeman JY, Maitchouk D, Beuerman RW, Gebhardt BM. Detection of substance P in human tears by laser desorption mass spectrometry and immunoassay. Curr Eye Res 1997; 16:960–963.
71.Yamada M, Ogata M, Kawai M, Mashima Y, Nishida T. Substance P and its metabolites in normal human tears. Invest Ophthalmol Vis Sci 2002; 43:2622–2625.
72.Mertaniemi P, Ylatupa S, Partanen P, Tervo T. Increased release of immunoreactive calcitonin gene-related peptide (CGRP) in tears after excimer laser keratectomy. Exp Eye Res 1995 Jun; 60(6):659–665.
73.Berta A, Tozser J, Holly FJ. Determination of plasminogen activator activities in normal and pathological human tears. The significance of tear plasminogen activators in the inflammatory and traumatic lesions of the cornea and the conjunctiva. Acta Ophthalmol (Copenh) 1990; 68:508–514.
74.Virtanen T, Konttinen YT, Honkanen N, Harkonen M, Tervo T. Tear fluid plasmin activity of dry eye patients with Sjogren’s syndrome. Acta Ophthalmol Scand 1997; 75:137–141.
75.Sathe S, Sakata M, Beaton AR, Sack RA. Identification, origins and the diurnal role of the principal serine protease inhibitors in human tear fluid. Curr Eye Res 1998; 17:348–362.
76.Ahn CS, McMahon T, Sugar J, Zhou L, Yue BY. Levels of alpha1-proteinase inhibitor and alpha2-macroglobulin in the tear film of patients with keratoconus. Cornea 1999; 18:194–198.
77.Reitz C, Breipohl W, Augustin A, Bours J. Analysis of tear proteins by oneand two-dimensional thin-layer iosoelectric focusing, sodium dodecyl sulfate electrophoresis and lectin blotting. Detection of a new component: cystatin C. Graefes Arch Clin Exp Ophthalmol 1998; 236:894–899.
78.Barro CD, Romanet JP, Fdili A, Guillot M, Morel F. Gelatinase concentration in tears of corneal-grafted patients. Curr Eye Res 1998; 17:174–182.
79.Sobrin L, Liu Z, Monroy DC, Solomon A, Selzer MG, Lokeshwar BL, Pflugfelder SC. Regulation of MMP-9 activity in human tear fluid and corneal epithelial culture supernatant. Invest Ophthalmol Vis Sci 2000; 41:1703–1799.
80.Holopainen JM, Moilanen JA, Sorsa T, Kivela-Rajamaki M, Tervahartiala T, Vesaluoma MH, Tervo TM. Activation of matrix metalloproteinase-8 by membrane type 1-MMP and their expression in human tears after photorefractive keratectomy. Invest Ophthalmol Vis Sci 2003; 44:2550–2556.
81.Margrini L, Bonini S, Centofanti M, Schiavone M, Bonini S. Tear tryptase levels and allergic conjunctivitis. Allergy 1996; 51:577–581.
82.Tseng SC, Hatchell D, Tierney N, Huang AJ, Sun TT. Expression of specific keratin markers by rabbit corneal, conjunctival, and esophageal epithelia during vitamin A deficiency. J Cell Biol 1984; 99:2279–2286.
83.Nakamura T, Nishida K, Dota A, Matsuki M, Yamanishi K, Kinoshita S. Elevated expression of transglutaminase 1 and keratinization-related proteins in conjunctiva in severe ocular surface disease. Invest Ophthalmol Vis Sci 2001; 42:549–556.
84.Wolff E. Anatomy of the eye and orbit 1954; New York: Blakiston.
85.Nichols BA, Chiappino ML, Dawson CR. Demonstration of the mucous layer of the tear film by electron microscopy. Invest Ophthalmol Vis Sci 1985; 26:464–473.