Alteration of epithelial paracellular permeability during corneal epithelial wound healing.

We studied the paracellular permeability to mannitol of corneas with epithelium of corneal, limbal, or conjunctival origin. Corneas with epithelial defects reepithelialized by corneal or limbal epithelium were nonvascularized; the corneal permeability was initially increased and returned to normal 3 days later. When epithelial defects extended beyond the limbus, they were healed by conjunctival epithelium. If corneas remained avascular or minimally vascularized, the conjunctiva-derived epithelium underwent a transdifferentiation process into a cornealike morphology in which the corneal permeability was initially increased upon complete reepithelialization, and gradually decreased to a level similar to that of normal cornea, 4 weeks after healing. However, when corneas became vascularized, the conjunctiva-derived epithelium retained its original phenotype, and corneal permeability remained increased throughout the 8-month period of study. The deranged barrier functions noted in the above vascularized cornea were demonstrated further by horseradish peroxidase tracer, which was found in the intercellular spaces of conjunctiva-derived epithelium of vascularized corneas but not in the avascular corneas with epithelia of corneal or limbal origin, or transdifferentiated conjunctival epithelium. To study further the effect of subsequent ocular surface trauma, conjunctival biopsy was performed on transdifferentiated avascular corneas 3 months after initial wounding. The biopsy resulted in extensive vascularization in three of eight previously nonvascularized corneas. Two weeks later, the corneal permeability was increased to a level similar to that of conjunctiva. These results indicate that corneal epithelial paracellular permeability correlates well with the status of the epithelial phenotype.     

Regional distribution of acetylcholine and associated enzymes and their regeneration in corneal epithelium

Acetylcholine and choline acetyltransferase are found in high concentrations in the corneal epithelium of most species, although their function is unknown. We have measured the levels of each in different regions of the rabbit cornea and found that both are much more abundant in central than in peripheral cornea or conjunctival epithelium. Following abrasion of the cornea, epithelial cells from the surrounding cornea or conjunctiva move over rapidly and regenerate. We have assayed choline acetyltransferase and total protein after complete or incomplete abrasion of the corneal epithelium. Acetylcholine-synthesizing activity was not detectable in the regenerating cells until 14-21 days later (depending on the degree of abrasion). Like glycogen and oxidative enzymes, which are also much more abundant in corneal than conjunctival epithelium, choline acetyltransferase regeneration is complete about 28 days after abrasion. In contrast with acetylcholine and choline acetyltransferase, cholinesterase activity is low and its distribution relatively uniform over cornea and conjunctiva. The high ratio of acetylcholine synthesis to cholinesterase activity suggests that acetylcholine released from the corneal epithelium would be able to diffuse to more distant structures within the eye.     

Conjunctival inflammation induces Langerhans cell migration into the cornea

PURPOSE: The virtual absence of Langerhans cells (LC) in donor or recipient corneal epithelium is known to be an important factor in the acceptance of orthotopic corneal allografts. Though it is well known that various types of stimulation to the cornea induce LC migration into the corneal epithelium, resulting in poor graft survival, the influence of conjunctival inflammation on LC migration into the cornea has not been elucidated. Therefore we examined whether LCs migrate into the cornea in the presence of conjunctival inflammation. METHODS: Sixteen BALB/c mice were divided into four groups. Group A: 4 mice with corneal inflammation induced by two 9-0 silk interrupted sutures in the central cornea (positive control); Group B: 4 mice with conjunctival inflammation induced by two 9-0 silk interrupted sutures in the temporal and nasal bulbar conjunctiva 1 mm from the limbus; Group C: 4 mice with conjunctival inflammation by two 10-0 nylon interrupted sutures in the temporal and nasal bulbar conjunctiva 1 mm from the limbus; and Group N: 4 mice with no inflammation (untreated, naive control). Fourteen days after suturing, the mice were sacrificed and LCs migrated into the corneal epithelium were counted by immunofluorescence assay using anti-Ia antibody. RESULTS: In Group A, Ia(+) cells in the cornea totaled 29.4 +/- 3.8 cells/mm(2); in Group B, 7.9 +/- 1.2 cells/mm(2); in Group C, 7.8 +/- 0.7 cells/mm(2); and in Group N, 1. 6 +/- 0.5 cells/mm(2). Significantly greater numbers of Ia(+) cells were detected in Groups A, B and C than in Group N (p < 0.005). CONCLUSIONS: Conjunctival inflammation caused by sutures in the bulbar conjunctiva induced LC migration into the cornea. These results indicate that conjunctival inflammation influences the corneal immunological environment, and may affect the fate of orthotopic corneal allografts.     

Secretory immune system of rabbit ocular adnexa.

A distinct system of immunity in a variety of animals is located subjacent to epithelial surfaces and is typified by the predominance of immunoglobulin A (IgA) and secretory component (SC) in various external secretions, including tears. The present study examined normal rabbit lacrimal gland, conjunctiva, and cornea for the presence of immunoglobulin and SC. IgA-staining plasma cells predominated within lacrimal gland and conjunctival stroma, and SC was found in the epithelial cells of both these tissues but not within corneal epithelium. These observations are consistent with findings for other secretory sites in both rabbits and humans and establish lacrimal gland and conjunctiva as integral parts of the rabbit secretory immune system.     

Conjunctival transdifferentiation is due to the incomplete removal of limbal basal epithelium

Previous studies have shown that using n-heptanol to create a total corneal epithelial defect beyond the limbus results in two different healing patterns with an unpredictable incidence. Between 14-68% of these wounded rabbit corneas (n = 287, combining various reports) showed extensive vascularization and conjunctivalization, whereas the remaining were not vascularized and had conjunctival transdifferentiation with a cornea-like epithelium. To investigate the role of the limbal epithelium in these two healing patterns, the authors treated rabbit eyes for various durations with n-heptanol and additional scraping. Histology showed that treatment for up to 120 seconds removed both the corneal and conjunctival epithelia but left the limbal basal cells intact. To prove viability, they cultured the treated limbal explants on collagen gel. After 14 days of culture, increased stratification of the limbal epithelium and an epithelial outgrowth onto the corneal stroma was observed. The latter was proven to be of corneal origin (positive to AE-5 but negative to AM-3 monoclonal antibody staining). The authors then surgically removed the entire limbal zone including 2 mm of peripheral cornea and 3 mm of adjacent conjunctiva in addition to n-heptanol debridement of the entire corneal epithelium in 54 rabbit eyes and observed a high incidence (96%) of corneal vascularization and conjunctivalization of the resultant epithelial phenotype (positive to AM-3, but negative to AE-5 monoclonal antibody staining). These results support the hypothesis that corneal epithelial stem cells are located in the limbus and indicate that an incomplete removal of the basal limbal epithelium by n-heptanol leads to unvascularized corneas with conjunctival transdifferentiation. Conversely, complete removal of such cells results in corneal vascularization and conjunctivalization.     

Lipophilicity influence on conjunctival drug penetration in the pigmented rabbit: a comparison with corneal penetration.

The influence of lipophilicity on the conjunctival penetration of beta blockers in the pigmented rabbit was investigated and compared with that on corneal penetration. The beta blockers were hydrophilic sotalol, atenolol, nadolol, pindolol, and acebutolol; lipophilic metoprolol, timolol, oxprenolol, levobunolol, labetalol, and alprenolol; and the very lipophilic propranolol and betaxolol. Drug penetration was evaluated by using the isolated pigmented rabbit conjunctiva and cornea in the modified Ussing chamber and was monitored by reversed phase HPLC. The conjunctiva was more permeable to all the beta blockers than was the cornea. A sigmoidal relationship, rather than the familiar parabolic relationship, best described the influence of lipophilicity on both conjunctival and corneal drug penetration. The ratio of corneal to conjunctival permeability coefficients was most sensitive to changes in log PC within the region of 1.5 and 2.5. Outside of this region, the ratio was relatively independent of changes in lipophilicity. For several beta blockers, their intrinsic sympathomimetic activity may play a minor role in influencing their conjunctival and corneal penetration.     

Inhibition of conjunctival scarring and contraction by a porous collagen-glycosaminoglycan implant

PURPOSE: To study the healing processes of full-thickness wounds in the adult rabbit conjunctiva after grafting with a porous collagen-glycosaminoglycan (CG) copolymer matrix. METHODS: A 7-mm trephine was used to produce lesions of the bulbar conjunctiva down to the level of the bare sclera. Full-thickness removal of the conjunctiva and Tenon's capsule created a reproducible wound bed. Wounds either remained ungrafted (control) or were grafted with CG matrix. In previous studies, this CG matrix has induced partial regeneration of the dermis in the human, the swine, and the guinea pig. Healing of the conjunctival epithelium and underlying stroma was evaluated by histology, immunohistochemistry, and measurement of wound contraction kinetics. RESULTS: By 28 days, ungrafted wounds had closed by contraction (26.4% +/- 5.0% fornix shortening) and the formation of scarlike tissue comprising an aligned array of dense collagen populated with occasional fibroblasts. Grafting of identical defects with CG copolymer matrix resulted in inhibition of wound contraction (6.8% +/- 3.2% fornix shortening) and the formation of a tissue that resembled normal conjunctival stroma, being composed of a loose network of collagen fibers and fibroblasts. Contractile fibroblasts (myofibroblasts) were identified at the edge of both ungrafted and grafted wounds during the period of active contraction. Both ungrafted and grafted wounds were completely re-epithelialized by 28 days. CONCLUSIONS: Implantation of CG copolymer matrix drastically reduced contraction and promoted the formation of a nearly normal subconjunctival stroma.     

The effect of rigid gas permeable contact lens wear on proliferation of rabbit corneal and conjunctival epithelial cells.

PURPOSE: To study the effect of rigid contact lens oxygen transmissibility on cell proliferation of the corneal, limbal, and conjunctival epithelium in vivo following 2 days of extended wear in the rabbit model. METHODS: Fourteen adult New Zealand White rabbits were divided equally into two groups. Each group was assigned to one of two test rigid gas permeable (RGP) contact lenses (Dk/Ltotal = 10 and 97) with uniform thickness (0.15 mm) and diameter (14.0 mm). One eye of each rabbit randomly received a contact lens for two days (48 hrs) extended wear, and the fellow eye was used as a control. Rabbits were injected intravenously with 5-bromo-2-deoxyuridine (200 mg/kg) in sterile phosphate buffered saline (pH 7.4) 24 hours before being killed. Corneas with a limbal rim of episclera and overlying conjunctiva were fixed in situ and excised. Nuclei labeled with BrdU were detected with a monoclonal anti-BrdU antibody and an FITC-conjugated secondary antibody. Digital images were collected and BrdU-labeled nuclei of whole-mount corneas were counted from superior limbus to inferior limbus using epifluorescence microscopy. RESULTS: Twenty-four hours after intravenous injection of BrdU, labeled nuclei were confined to and appeared as pairs in the basal epithelial layer. The density of BrdU-labeled nuclei were found to be 258 +/- 42, 167 +/- 43, 372 +/- 64, and 310 +/- 46 (pairs/mm2, mean +/- SD, n = 14) in normal controls for adjacent conjunctiva, limbus, peripheral cornea, and central cornea, respectively. By contrast,there was significant 81.35% (low Dk)and 22.46% (ultra-high Dk) suppression of cell proliferation in the central cornea after two days lens wear (n = 7). In addition, significant increases in the labeling of limbal and conjunctival epithelium were also noted. CONCLUSIONS: Significantly less BrdU labeling of epithelial cells at the normal rabbit limbus was noted as compared to the peripheral and central cornea (P < 0.05) and is consistent with the presence of slow-cycling limbal basal cells and the limbal stem cell theory; however, this is the first report of up-regulation of limbal cell proliferation induced by contact lens wear. This study also revealed, for the first time, that short-term extended wear of RGP lenses inhibits central corneal epithelial cell proliferation. This effect was significantly more pronounced for a low-oxygen vs. a hyper-oxygen transmissible test lens. This data also suggests that corneal epithelial layer thinning seen following extended contact lens wear may be explained, in part, by suppression of basal epithelial cell proliferation. Further study is clearly necessary to validate and extend these preliminary findings.     

纤维蛋白胶对眼表的作用

目的观察以纤维蛋白胶（FG）为载体构建兔角膜上皮、基质和内皮细胞片,以及FG对兔结膜和角膜的黏合作用。方法在培养板孔底制作薄层和厚层FG,将培养兔角膜3种细胞分别接种于FG表面,观察细胞生长情况。FG对兔结膜的黏合研究：分为A组（结膜原位粘合组）,B组（结膜移位粘合组）,对照组（单纯切除结膜植片）,每组5只眼,观察结膜组织黏合情况。FG对兔角膜的黏合研究：3例兔前板层角膜移植术,先用10—0尼龙线间断缝合4针固定植片,再用FG黏合,将植片固定于植床。结果角膜3种细胞在薄层和厚层FG表面生长良好：角膜上皮细胞可形成单层和复层;角膜基质细胞间网状连接;角膜内皮细胞排列紧密。对兔结膜的黏合研究：A组和B组植片与植床对位良好,紧密黏合。组织切片显示术后第4周A组和B组结膜上皮完整,炎症基本消退。对照组结膜上皮有局部脱落区,纤维层局部显示瘢痕组织结构特征。兔前板层角膜移植术后3个月,植片与受体角膜愈合良好。结论FG可作为角膜3种细胞的生长载体构建细胞片。FG对兔结膜和角膜组织有良好的黏合作用。     

Keratoepithelioplasty in rat: development of a model and histological study.

A model for keratoepithelioplasty (KEP) was developed using the Lewis rat, and histological studies were performed using this model. The entire corneal epithelium was removed mechanically and a 1.5-mm width of the conjunctiva including the limbus was excised. An oval corneal lamellar graft (3 x 1.5 mm) with an intact epithelium taken from another Lewis rat was transplanted on the denuded limbus. Biomicroscopic observations showed much less vascular invasion in the part of the cornea adjacent to the lenticule than in other parts of the cornea, and the cornea remained clear adjacent to the lenticule. Histologically, a few vessels were observed in the corneal stroma under the lenticule. Epithelial cells on the lenticule specimens showed histological characteristics of the corneal epithelium. These findings indicate that one of the functions of KEP is to block neovascularization in the newly developing corneal epithelium by transplanting the lenticule between the corneal epithelium and conjunctival vessels. The present study also confirmed that this model is useful in the research of the pathophysiological mechanism of KEP.     

Wistar rat palpebral conjunctiva contains more slow-cycling stem cells that have larger proliferative capacity: implication for conjunctival epithelial homeostasis

PURPOSE: To determine the location of conjunctival epithelial stem cells. METHODS: Wistar rats received daily injection of 5-bromo-2-deoxyuridine (BrdU) at a dose of 5 mg/100 g for 2 weeks followed by a 1-month BrdU-free period before death. After the rats were sacrificed, the orbital contents and eyelids were exenterated en bloc, fixed in buffer formalin, and embedded in paraffin. To compare the proliferative capacity of ocular epithelial cells, 1.0% phorbol myristate (TPA) in petrolatum was topically applied once daily to both eyes of Wistar rats for 12 days. After 6, 12, 18, and 24 hours and 2, 4, 6, 8, 10, and 12 days of TPA treatment, rats were administered BrdU intraperitoneally 7 hours before they were sacrificed. The ocular epithelium was fixed and processed for immunochemistry, and the labeling index (LI) of every epithelial zone was determined. RESULTS: Slow-cycling cells, detected as label-retaining cells (LRCs), were found in bulbar, fornical, and palpebral epithelia and mucocutaneous junctions, as well as in limbal epithelia. The greatest numbers of LRCs were identified in palpebral epithelium. Under normal situations, in conjunctiva the LI was lowest in palpebral epithelium (2.1 +/- 0.5) compared with bulbar (2.2 +/- 0.5), fornical (2.3 +/- 0.4) epithelia and mucocutaneous junction (3.4 +/- 0.9), respectively. In cornea, the LI was lowest in limbal epithelium (1.8 +/- 0.7) compared with central corneal epithelium (3.5 +/- 0.6). Twenty-four hours after TPA treatment, an 8.2-fold increase in the palpebral epithelial basal cell labeling index was noted compared with 4.7-fold, 5.7-fold, and 3.8-fold increases in bulbar, fornical, and mucocutaneous junction epithelial basal cell labeling indices, and a sevenfold increase in the limbal basal cell labeling indices compared with a 2.1-fold increase in the corneal basal cell labeling index, respectively. Limbal and palpebral epithelia maintained a significantly greater proliferative response (5.5-to 6.3-fold increase, respectively) during chronic stimulation than corneal, bulbar, fornical epithelia, and mucocutaneous junction (0.6- to 2.3-fold increase, respectively). CONCLUSIONS: In Wistar rat conjunctiva, slow-cycling cells are primarily located in palpebral epithelium, which has greater proliferative capacity than other conjunctival epithelia. This finding means that, in the Wistar rat, the conjunctival epithelial stem cells are mainly located in palpebral epithelium. These data open new perspectives in ocular epithelial development and are relevant in conjunctival wound repair.     

Nociceptive sensation and sensitivity evoked from human cornea and conjunctiva stimulated by CO2

PURPOSE: To compare sensation and sensitivity evoked from human cornea and conjunctiva stimulated by CO2. METHODS: Twenty healthy participants were recruited for the study. Central corneal and temporal conjunctival chemical sensation and sensitivity of only one eye of each subject were evaluated. Air mixed with different concentrations of CO(2) was delivered by a modified Belmonte pneumatic esthesiometer. The ascending method of limits was used to determine the sensitivity and subjects were required to characterize the sensation at threshold. RESULTS: The sensations evoked by CO(2) in the cornea and conjunctiva were stinging or burning. The sensation evoked by mechanical stimulation was that of irritation. The corneal and conjunctival chemical thresholds were 31% +/- 2% and 54% +/- 5% CO(2) (mean +/- SE), respectively. The corneal and conjunctival mechanical thresholds were 80 +/- 6 and 140 +/- 10 mL/min (mean +/- SE), respectively. The corneal sensitivity was significantly higher for both mechanical and chemical stimuli (P < 0.05). CONCLUSIONS: The results suggest that CO(2) stimulates similar corneal and conjunctival nociceptors in that the interpretations were the same (i.e., nociceptive). The central cornea had a higher sensitivity to CO(2) than the temporal conjunctiva, which may reflect a different peripheral innervation, such as different nerve density or different receptor characteristics. Sensations evoked by mechanical and chemical stimulation were different, which suggests that at the peripheral level, the two modalities stimulate two different kinds of molecular receptors or channels and that this information is somehow retained within the nociceptive system.     

Sensitivity and neural organization of the cat cornea

The innervation of the adult cat cornea was investigated both psychophysically and histologically. Mean corneal touch threshold (CTT) for 25 adult domestic cats was 43 +/- 9 mg in the center of the cornea and 100 +/- 32 and 94 +/- 33 mg in the superior and inferior cornea, respectively. Gold chloride impregnation showed that the cat cornea is innervated by 16-20 radial nerve trunks that enter the mid-posterior stroma at various sites around the corneal circumference. As these trunks travel anteriorly toward the center of the cornea they give off collaterals that form the anterior stromal and subepithelial plexus. Fibers from the subepithelial plexus penetrate the epithelial basement membrane and give off numerous long fibers that ramify in the basal epithelial layer. Intraepithelial terminals arise from these, penetrating between the epithelial cells, ending with a terminal enlargement at the wing cell level. A distinct pattern of neural organization was found in the periphery of the cat cornea. This consisted of finer nerve fibers that entered the cornea at the subepithelial and basal epithelial levels at numerous sites around the corneal circumference. These fibers branched after a short distance in the cornea and appeared to innervate the anterior stroma and epithelium in the periphery of the cornea. This study thus provides direct evidence of two types of neural organization in the cornea of the domestic cat. Stromal nerves appear to be the main source of innervation to the epithelium in the center of the cornea while conjunctival nerves supply the peripheral epithelium.     

Change of paracellular permeability of ocular surface epithelium by vitamin A deficiency

Dietary vitamin A deficiency in young rabbits caused advanced squamous metaplasia with keratinization of conjunctival epithelium and concomitant reduced paracellular permeability to 3H-mannitol. Both morphologic and permeability changes were reversed with systemic administration of vitamin A. In adult rabbits, vitamin A deficiency caused milder changes of goblet cell loss and increased cellular stratification in conjunction with reduced permeability in the conjunctiva-like epithelium that covers the vascularized cornea after chemical injury with n-heptanol. Topically applied retinoid (tretinoin 0.1%) did not affect the morphology and permeability of the normal corneal or conjunctival epithelium of rabbits that were not vitamin A deficient. These studies showed that altered permeability is associated with the epithelial abnormality during vitamin A deficiency and helped clarify the physiologic function of retinoids in the ocular surface epithelia in the nondeficient state.     

Immunohistochemical localization of hyaluronan synthase in cornea and conjunctive of cynomolgus monkey.

Distribution of hyaluronan synthase was investigated in cornea and conjunctiva of Cynomolgus monkeys (Macaca fascicularis) using polyclonal antibodies against the streptococcal enzyme. Strong immunoreaction was found in the cell membranes of the corneal endothelium, corneal epithelium, and most of the conjunctival epithelium. In the corneal epithelium all cells except the basal ones stained. In the conjunctiva all cylindrical cells stained, whereas among the goblet cells one type showed intense membrane staining, the other remained unstained. In the limbal portion of the conjunctival epithelium, which in many other respects differs morphologically and functionally from the remaining conjunctiva, all membranes of the different layers of the stratified epithelium except the most superficial ones, appeared unstained. Staining was also seen in all stromal fibroblasts and capillary endothelial cells.     

角膜缘干细胞的研究

角膜缘干细胞是位于角膜缘基底上皮层底的一类特殊类型的上皮细胞,随着细胞培养技术的发展,角膜缘干细胞体外培养后移植用于治疗由于角膜缘干细胞缺乏或者功能不全引起的眼表疾病成为研究的热点。本文就其解剖学定位、生物学特性、组织工程化角膜的基础性研究及其临床应用做一综述。     

Movement of fluorescein monoglucuronide in the rabbit cornea. Diffusion in the stroma and endothelial permeability

The movement of fluorescein monoglucuronide, a fluorescent metabolite of fluorescein, was studied in the rabbit cornea in vitro and in vivo. A stromal strip was exposed to fluorescein monoglucuronide, and the diffusion rate and the distribution in the stroma were measured every hr for 24 hr. The diffusion coefficient was 0.94 +/- 0.11 (+/- S.D.) X 10(-6) cm2/sec, and the saline/stroma distribution ratio was in a range of 0.67 to 0.69. The concentration of fluorescein monoglucuronide in the anterior chamber and the cornea was measured every hr for 8 hr following intravenous administration. The endothelial permeability was 4.7 +/- 1.0 X 10(-4) cm/min, and the aqueous/cornea distribution ratio was 0.56 +/- 0.05. It appears that the corneal endothelial permeability in the living eye determined hitherto from systemic administration of fluorescein is most likely the permeability to fluorescein monoglucuronide.     

Development of cultured rabbit corneal epithelium for drug permeation studies: a comparison with excised rabbit cornea.

The aim of this study was to prepare and test an artificial corneal epithelium (reconstituted rabbit corneal epithelium, RRCE) exhibiting barrier characteristics and paracellular permeability similar to those of native rabbit cornea. The RRCE was obtained from a rabbit corneal epithelium (RCE) cell line grown for 8 days in submerged culture, then for 7 days in air-interface conditions on Snapwell polyester membranes. Permeation studies on the RRCE were carried out in comparison with rabbit excised corneas in vitro, using timolol maleate (TM) as the test drug, alone and in association with the following ocular permeation enhancers: benzalkonium chloride, ethylene-diaminetetraacetic acid sodium salt, polyethoxylated castor oil, polyoxyethylene stearyl ether, sodium deoxycholate, and escin. The integrity of the RRCE was assessed by measuring the transepithelial electrical resistance (TEER) during culture time and after every permeation experiment. When TM was tested alone, the permeation parameters (apparent permeability coefficient, lag time) obtained with the RRCE were similar to those of excised rabbit corneas. The artificial epithelium, however, was less sensitive than native cornea to the effect of permeation enhancers.     

蚕蚀性角膜溃疡角膜及邻近球结膜的组织病理和免疫组化研究

目的：研究蚕蚀性角膜溃疡角膜和邻近球结膜的组织病理和免疫病理变化,探讨巨噬细胞、Ｔ细胞、ＶＣＡＭ－１等与其发病的关系。方法：接受球结膜切除术及板层角膜移植术的１８进行性蚕蚀性角膜溃疡患者,取病变角膜及邻近球结膜作组织病理研究,并采用链酶生物系（ｌａｂｅｌｌｅｄ ｓｔｒｅｐｔａｖｉｄｉｎ ｂｉｏｔｉｎ,ＬＳＡＢ）方法作免疫组化研究。检测的抗体包括：ＣＤ６８、ＣＤ３、ＣＤ４、ＣＤ８、ＶＣＡＭ－１、ＣＤ     

The conjunctiva in corneal epithelial wound healing

BACKGROUND/AIMS—During the healing of corneal epithelial wounds with limbal involvement, conjunctival epithelium often migrates across the denuded limbus to cover the corneal surface. It is believed that, over a period of time, conjunctival epithelium covering the cornea assumes characteristics of corneal epithelium by a process referred to as conjunctival transdifferentiation. The purpose of this study was to examine, clinically, the fate of conjunctival epithelial cells covering the cornea and to assess the healing of corneal epithelial wounds when the conjunctival epithelium was removed or actively prevented from crossing the limbus and extending onto the cornea.
METHODS—10 patients with conjunctivalisation of the cornea were followed for an average of 7.5 months. Five patients in this group had their conjunctival epithelium removed from the corneal surface and allowed to heal from the remaining intact corneal epithelium. In another four patients with corneal epithelial defects, the conjunctival epithelium was actively prevented from crossing the limbus by mechanically scraping it off.
RESULTS—The area of cornea covered by conjunctival epithelium appeared thin, irregular, attracted new vessels and was prone to recurrent erosions. Conjunctivalisation of the visual axis affected vision. Removal of conjunctival epithelium from the cornea allowed cells of corneal epithelial phenotype to cover the denuded area with alleviation of symptoms and improvement of vision. It was also established that migration of conjunctival epithelium onto corneal surface could be anticipated by close monitoring of the healing of corneal epithelial wounds, and prevented by scraping off conjunctival epithelium before it reached the limbus.
CONCLUSION—This study shows that there is little clinical evidence to support the concept that conjunctival transdifferentiation per se, occurs in humans. "Replacement" of conjunctival epithelium by corneal epithelial cells may be an important mechanism by which conjunctival "transdifferentiation" may occur. In patients with partial stem cell deficiency this approach can be a useful and effective alternative to partial limbal transplantation, as is currently practised.

Keywords: corneal epithelium; conjunctiva; stem cells; transdifferentiation