IOVS
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

(Investigative Ophthalmology and Visual Science. 2004;45:423-428.)
© 2004 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.03-0947
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (10)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Song, I.-K.
Right arrow Articles by Joo, C.-K.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Song, I.-K.
Right arrow Articles by Joo, C.-K.

Morphological and Functional Changes in the Rat Cornea with an Ethanol-Mediated Epithelial Flap

In-Kyung Song and Choun-Ki Joo

From the Laboratory of Ophthalmology and Visual Science, College of Medicine, The Catholic University of Korea, Seoul, Korea.


   Abstract
Top
 Abstract
Materials and Methods
 Results
 Discussion
 References
 
PURPOSE. To establish morphologic and functional changes in the rat cornea after 20% ethanol-mediated epithelial flap creation.

METHODS. The epithelial flap was detached using 20% ethanol and then repositioned. On the other eye, the corneal epithelium was mechanically scraped. The morphologic changes were examined by light and transmission electron microscopy (LM and TEM). Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assays and proliferative cell nuclear antigen (PCNA) staining were performed to detect apoptosis and proliferation, respectively. The number of central stromal keratocytes was counted during wound-healing periods. A Y-chromosome-specific gene (SRY) was detected in genomic DNA obtained from female epithelial cells with male epithelial flap. Expressions of matrix metalloproteinase (MMP)-9, aquaporine5 (AQP5), and mucin1 (MUC1) mRNAs were examined by reverse transcription-polymerase chain reaction.

RESULTS. After flap repositioning, damaged basal cells were observed until day 1, and the death of epithelial cells and stromal keratocytes loss peaked at day 1. Cell proliferation and MMP-9 mRNA levels peaked from days 1 to 3. By contrast, after mechanical scraping, the denuded stromal surface was covered with multilayer epithelial cells at day 3, cell death peaked at 4 hours, cell proliferation peaked from 12 hours to day 1, stromal keratocytes loss peaked at 8 hours, and MMP-9 mRNA was widely expressed from 12 hours to day 3. In both corneas, hemidesmosomes were not observed until day 1, and they continued to be present at day 3. The SRY gene was detected in female epithelial cells at day 1 after transplantation, but not at day 3. Expressions of AQP5 and MUC1 mRNAs were unchanged.

CONCLUSIONS. Ethanol-mediated flap creation induces less keratocyte loss and a slower wound-healing process than mechanical scraping. The flap protecting the underlying stromal surface may have epithelial function. In addition, this Sprague-Dawley rat model may be useful in the study of the wound-healing response after LASEK.

Laser subepithelial keratomileusis (LASEK) is a relatively new refractive surgical technique that makes up for the disadvantages of laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).1 2 LASEK theoretically produces faster visual recovery, less postoperative pain, reduced stromal haze, and faster epithelial healing than PRK.3 4 It also offers the advantages of avoiding the flap- and interface-related complications of LASIK, the difficulty in predicting flap thickness and ablation depth, and posterior ectasia.5 6

Ethanol was initially used in refractive surgery to assist in the removal of epithelium before PRK and has recently been shown to enhance epithelial flap creation without a significant loss of flap viability in LASEK.7 8 9 Exposure times to ethanol of more than 1 minute result in significant damage to superficial corneal epithelium10 and treatment with 20% alcohol for 30 seconds results in reproducible epithelial flap creation in the chick cornea and in relatively low levels of stromal and epithelial cell death after surgery.11 Alcohol delamination of corneal epithelium results in a very smooth cleavage at the level of the hemidesmosomal attachments, including the superficial lamina lucida.12

The cornea is covered by nonkeratinized, stratified epithelium that is responsible for maintaining ocular surface integrity and is essential for vision. This provides an initial barrier to tears and to the intraocular environment.13

The integrity of the epithelium can be monitored from the presence of aquaporins (AQPs), a family of water channels, which are water-selective transporting proteins. A special role of AQP5 in water transport is to prevent dehydration or to secrete watery products, and the epithelium thus contributes to corneal hydration homeostasis.14 15 16 On the ocular surface, mucin (MUC) is the main component of the innermost layer of the tear film. Mucin1 (MUC1) mRNA has been detected in all cell layers of the corneal epithelium, and MUC1 plays a protective role against the adherence of pathogens.17

After PRK or LASEK surgery, the anterior stroma including Bowman’s layer is removed, and an ethanol-induced epithelial flap is replaced on the ablated stromal layer. After surgery, the corneal epithelial flap is attached to the stroma and undergoes wound healing.2 We developed an available animal model for LASEK study using the eyes of Sprague-Dawley rat (although rat cornea does not have Bowman’s layer). In the present study, corneal morphologic, biochemical, and functional changes were studied after 20% ethanol-induced epithelial flap creation and reposition and compared with mechanical scraped cornea.


   Materials and Methods
Top
 Abstract
 Materials and Methods
Results
 Discussion
 References
 
Animals and Procedures
Adult Sprague-Dawley rats (250–280 g) were used for this study. All animal procedures conformed to Institutional Guidelines and to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. The rats were anesthetized with an intramuscular mixture of ketamine hydrochloride (20 mg/kg) and xylazine hydrochloride (5 mg/kg).

A total of 252 eyes were divided into four groups: group 1 for paraffin block (n = 5 at each time point); group 2 for Epon block (n = 5 at each time point); group 3 for RNA extraction (n = 6 at each time point); and group 4 for genomic DNA extraction (six male eyes and six female eyes).

For a model of ethanol-detached repositioning of the epithelium, the central corneas were incised using 2-mm trephines and were exposed to 20% ethanol in distilled water for 30 to 40 seconds in 3-mm markers placed on the corneal surface. The ethanol was then removed from the corneal surface using a dry sponge (Weckel or Merocel; Medtronics-Xomed, Jacksonville, FL), and the cornea was thoroughly rinsed with a balanced salt solution. The loosened epithelium was lifted and rolled as a flap toward the 12 o’clock position with a detaching spatula. The epithelial flap was then repositioned on the stroma and allowed to dry for 1 minute (Fig. 1) .



View larger version (147K):
[in this window]
[in a new window]
  		FIGURE 1. Epithelial flap creation using 20% ethanol on the rat eye. Epithelial trephination is performed using a 2-mm diameter trephine. (A) A 3-mm diameter marker is placed on the eye and the cornea is exposed to 20% ethanol for 30 to 40 seconds. (B) The central epithelial flap is detached with a spatula. (C) The detached flap is rolled to the 12 o’clock position. (D) The epithelial flap is replaced on the stroma with a repositioning spatula.

 
To produce mechanically scraped cornea, we incised rat eyes with 2-mm trephines and removed the central epithelium with a spatula.

For ethanol-exposed corneas, the eyes were incised and exposed to 20% ethanol for 30 seconds. The ethanol was then removed by dry sponge and the eyes rinsed with a balanced salt solution. The rats were killed at 0, 4, 8, and 12 hours and 1, 3, and 7 days after each procedure, and the corneoscleral rim of each eye was removed using surgical scissors for examination.

Light Microscopy and Transmission Electron Microscopy
The corneoscleral rims were fixed in 2% paraformaldehyde/2.5% glutaraldehyde fixative solution in 0.1 M phosphate buffer overnight at 4°C and postfixed in 1% osmium tetroxide for 40 minutes. After dehydration in a graded ethanol series, the tissues were embedded in epoxy resin and polymerized at 60°C for 48 hours. One-micrometer sections were stained with toluidine blue and examined by light microscope (LM; Olympus, Tokyo, Japan). Subsequent 80-nm ultrathin sections were mounted on copper grids, stained with 2% uranyl acetate-lead citrate and examined by transmission electron microscope (TEM; model 1200EX; JEOL, Tokyo, Japan).

Cell Death Detection by TUNEL Assay
Corneoscleral rims were fixed in 4% paraformaldehyde fixative solution in 0.1 M phosphate buffer for 4 hours at 4°C and then dehydrated through a graded ethanol series, cleared in xylene, and embedded in paraffin. Sections (5 µm) were placed on 0.2% gelatin-coated slides, rehydrated through graded ethanol, and rinsed with phosphate-buffed saline (PBS). To detect fragmentation of DNA associated with apoptosis, an in situ cell death detection kit for the TUNEL assay was purchased from Roche Diagnostics Corp. (Indianapolis, IN). The fluorescence-based assay was used according to the manufacturer’s instructions, and negative control slides were included in each assay. The slides were then rinsed with PBS and incubated for 30 minutes at 37°C with proteinase K (20 µg/mL in 10 mM Tris/HCl [pH 7.4]). The slides were rinsed again with PBS and incubated for 2 minutes on ice in permeabilization solution (0.1% Triton X-100 in 0.1% sodium citrate). The slides were rinsed with PBS, and 50-µL aliquots of the TUNEL reaction mixture were placed on the sections, followed by incubation for 40 minutes at 37°C in a humidified atmosphere in the dark. The slides were then rinsed three times with PBS, and the nuclei were counterstained with propidium iodide. The sections were mounted in an aqueous mounting medium and observed using a fluorescence microscope (Axiovert model S100; Carl Zeiss Meditec, Oberkochen, Germany). In negative control experiments, sections were incubated with fluorescein-labeled dNTP without enzyme.

Counting of Anterior Stromal Keratocytes and Statistical Analysis
Keratocyte numbers in the anterior stroma of unwounded and wounded corneas were determined by counting the total number of cells in 400x magnified fields of hematoxylin-eosin-stained corneas (n = 4). The position of each field across the diameter of the cornea was randomly selected within the central cornea and the total number of cells in four fields of each cornea was determined.

Data were analyzed on computer (Excel 2002; Microsoft, Seattle, WA) and presented as the mean ± SD. Probabilities were considered significant if P <= 0.05.

Immunohistochemistry
Sections (5 µm) from paraffin-embedded tissues were placed on 0.2% gelatin-coated slides, rehydrated through a graded ethanol series, and rinsed with PBS. The primary antibody, mouse monoclonal anti-PCNA, was purchased from Santa Cruz Biotechnology (Santa Cruz, CA) and the secondary antibody, Alexa Fluor 488 goat anti-mouse IgG, was purchased from Molecular Probes (Eugene, OR). To block nonspecific staining, the sections were incubated with 3% bovine serum albumin (BSA) in PBS for 30 minutes at room temperature. The sections were incubated with the dilution of primary antibody (1:100) in 1% BSA-PBS for 1 hour at room temperature. The slides were then rinsed three times with PBS, incubated with a dilution of the secondary antibody (1:500) in 1% BSA-PBS for 1 hour at room temperature, and rinsed three times with PBS, and the nuclei were counterstained with propidium iodide. The sections were mounted in an aqueous mounting medium and observed by fluorescence microscopy (n = 4). In the negative control experiments, preimmune mouse serum was substituted for the primary antibody.

Identification of Ethanol-Induced Epithelial Flap Attachment
Corneal epithelial flaps were created in the eyes of male and female rats, by exposure to 20% ethanol for 30 seconds. The female epithelial flap was removed and a male donor epithelial flap was placed on the denuded recipient female stromal surface. Epithelial cells were collected at days 1 and 3 after surgery, and genomic DNA was extracted using the phenol-chloroform method. For identification of flap attachment, a Y-chromosome-specific gene (SRY) was detected in genomic DNA obtained from female corneal epithelial cells after the transfer of male epithelial flaps, using the polymerase chain reaction (PCR).

Reverse Transcription-Polymerase Chain Reaction
After each surgery, corneal epithelial cells were collected, and total RNA was isolated with an extraction reagent (TRIzol; Invitrogen-Gibco, Gaithersburg, MD). A 5-µg portion of total RNA was used for reverse-transcription (RT) with reverse transcriptase (SuperScript II; Invitrogen-Gibco). Then, 50 ng of reverse-transcribed DNA and 80 ng of genomic DNA were amplified using PCR with 1.0 U of Taq DNA polymerase (Perkin Elmer, Boston, MA) in a mixture containing 0.4 µM of each primer, 0.2 mM dNTPs, and 1.5mM MgCl2. Primer sequences were as follows: MMP-9 (380 bp) sense, 5'-AAC TCA GCC TTT GAG GAT CC-3', and antisense, 5'-CAG TAT CCA GTG CAT CCG GT-3'; SRY (351 bp) sense, 5'-TGG CTC AAC AGA ATC CCA GC-3', and antisense, 5'-TGG GGA TGT CCA CAG GCT GT-3'; AQP5 (730 bp) sense, 5'-C AAG GCG GTG TTC GCA GAG TTC C-3', and antisense, 5'-C CTC TCG ATG ATC TTC CCA GTC C-3'; MUC1 (286 bp) sense, 5'-TCG ACA GGC AAT GGC AGT AG-3', and antisense, 5'-TCT GAG AGC CAC CAC TAC CC-3'; and ß-actin (350 bp) sense, 5'-AGG CCA ACC GCG AGA AGA TGA CC-3', and antisense, 5'-GAA GTC CAG GGC GAC GTA GCA C-3'. ß-Actin was used to control for RNA quality. The amplification reaction was performed in a thermal cycler (PTC-100; MJ Research, Watertown, MA). The conditions consisted of an initial denaturation for 5 minutes at 94°C, followed by 30 cycles (25 cycles for AQP5) of denaturation for 30 seconds at 94°C, amplification for 1 minute at 58°C (at 55°C for AQP5), and extension for 1 minute at 72°C. A final extension was performed for 10 minutes at 72°C. The PCR products were analyzed by electrophoresis on a 1% agarose gel and viewed using ethidium bromide staining.


   Results
Top
 Abstract
 Materials and Methods
 Results
Discussion
 References
 
Light Microscopic Analysis of Corneas after 20% Ethanol-Detached Epithelial Flap Repositioning and Mechanical Scraping
The cornea is composed of the epithelium, stroma with resident keratocytes, and endothelium. The normal corneal epithelium has five to seven cell layers, including the superficial squamous layer, the wing layer, and a single basal layer of columnar cells. The corneal stroma forms approximately 90% of the cornea (Fig. 2A) .



View larger version (109K):
[in this window]
[in a new window]
  		FIGURE 2. Light micrographs of unwounded central cornea (A) and corneas after 20% ethanol-detached flap repositioning (B–H), mechanical scraping (I–K), and 20% ethanol treatment for 30 seconds (L, M). After flap repositioning, damaged corneal basal cells are observed (B–F), and superficial cells peel off gradually (D–F). At day 3 after flap repositioning (G), damaged basal cells and peeling superficial cells were not observed. At day 7, epithelial morphology was normal (H). In mechanically scraped corneas, epithelial cells cover the denuded stroma at day 3 after the operation (K). Minimal changes in ethanol-treated corneal epithelium are observed (L, M). E, epithelium; B, basement membrane; S, stroma. Toluidine blue staining; magnification, x400.

 
After ethanol-detached epithelial flap repositioning, damaged corneal basal cells were observed, and superficial cells were observed peeling off gradually through day 1. The epithelial cell nuclei were vacuolated. At day 3, damaged basal cells and peeling superficial cells were not observed, and swollen basal epithelial cells and death of stromal keratocytes were evident. At day 7, epithelial and stromal morphology were normal (Figs. 2B 2C 2D 2E 2F 2G 2H) .

After mechanical scraping, corneal epithelial cells proliferated and migrated over the denuded stroma. Basal epithelial cells were swollen by day 1 and a stratified epithelium, five to seven layers thick, covered the stroma by day 3 (Figs. 2I 2J 2K) .

In the corneas that were exposed only to 20% ethanol, no significant changes were seen (Figs. 2L 2M) .

Death of Epithelial Cells and Stromal Keratocytes
After ethanol-detached flap repositioning, TUNEL-positive cells were detected in the superficial cells and the wound-edge epithelial cells by 4 hours and in the central corneal basal epithelial cells by day 1 (Figs. 3A 3B 3C 3D) . After mechanical scraping, TUNEL-positive cells were detected in the central anterior stromal keratocytes and in the migratory edge of epithelial cells at 4 hours, but were not detected at day 1 (Figs. 3E 3F 3G 3H) .



View larger version (78K):
[in this window]
[in a new window]
  		FIGURE 3. Apoptosis of epithelial cells (A–J) and stromal keratocytes (K). After ethanol-detached flap repositioning, TUNEL-positive cells were detected in the superficial epithelial cell (A) and the edge (B) at 4 hours and in basal epithelial cells at day 1 (C, D). After mechanical scraping, TUNEL-positive cells were detected in the central anterior stromal keratocytes (E) and in the migratory epithelial cells of the edge (F) at 4 hours and are not detected at day 1 (G, H). Similar results were observed in at least three other corneas. Arrows: edges of the flaps. Bar, 50 µm.

 
The number of central stromal keratocytes during the wound-healing period after each surgery decreased maximally (5.65 ± 1.67 cells) at day 1 after ethanol-detached epithelial flap repositioning and at 8 hours (3.27 ± 1.87 cells) after mechanical scraping. There were fewer corneal keratocytes after mechanical scraping than after ethanol-detached flap repositioning (P < 0.05: Fig. 4 ).



View larger version (38K):
[in this window]
[in a new window]
  		FIGURE 4. The number of central stromal keratocytes. During wound healing, anterior keratocytes decreased maximally at day 1 after ethanol-detached epithelial flap repositioning and at 8 hours after mechanical scraping. The number of keratocytes at 8 hours after scraping decreased more than that of keratocytes at day 1 after flap repositioning. *P < 0.05.

 
Proliferation of Epithelial Cells
PCNA staining was performed to identify the proliferation of epithelial cells. After ethanol-detached epithelial flap repositioning, proliferation peaked in the central cornea at day 3 and in the peripheral cornea at day 1, continuing in the peripheral cornea until day 7 (Figs. 5A 5B 5C 5D 5E 5F 5G 5H 5I 5J) . After mechanical scraping, cell proliferation peaked in the peripheral cornea at 12 hours. In the unwounded cornea, PCNA-positive cells were rarely detected in basal cells, and PCNA-positive cells were not detected in the negative control (Figs. 5K 5L 5M 5N 5O 5P 5Q 5R 5S 5T) .



View larger version (60K):
[in this window]
[in a new window]
  		FIGURE 5. Proliferation of epithelial cells. After ethanol-detached flap repositioning (A–J), cell proliferation peaked in the central cornea at day 3 and in the peripheral cornea at day 1. Proliferation continued in the peripheral cornea until day 7. After mechanical scraping, cell proliferation peaked in the peripheral cornea at 12 hours. Similar results were observed in at least three other corneas. Arrows: the edges of the flaps. Bar, 50 µm.

 
After each surgery, mRNA levels of MMP-9, which is synthesized and secreted by basal corneal epithelial cells migrating to resurface a wound, were examined using RT-PCR. MMP-9 mRNA levels in the corneas after ethanol-detached flap repositioning peaked at days 1 to 3, and in corneas subjected to mechanical scraping, they were widely expressed from 4 hours to 3 days (Fig. 6) .



View larger version (21K):
[in this window]
[in a new window]
  		FIGURE 6. Expression of MMP-9 mRNA in corneal epithelial cells. In the corneas after ethanol-mediated flap repositioning, MMP-9 mRNA levels peaked between days 1 to 3, and after mechanical scraping, MMP-9 was widely expressed in the corneas from 4 hours to day 3. The presented data are from one of four independent experiments with similar results.

 
Attachment of Corneal Epithelial Flap after Each Surgery
One day after both types of surgery, the basal lamina was not well defined, there was no space between the basal lamina and the basal epithelial cell layer, and there were no hemidesmosomes present in the cell membranes of basal epithelial cells (Figs. 7A 7C) . In the basal epithelial layer of flap after ethanol-detached flap repositioning, surviving basal epithelial cells had migrated to the position of dead cells at day 1 (Fig. 7A , inset). At day 3, the basal lamina was well defined, and hemidesmosomes were present (Figs. 7B 7D) . In ethanol-only-treated corneas, the basal lamina was well defined, and hemidesmosomes were detected at all time points (Figs. 7E 7F) .



View larger version (170K):
[in this window]
[in a new window]
  		FIGURE 7. Transmission electron micrographs of rat central corneal basement membrane. These micrographs demonstrate results at day 1 (A) and day 3 (B) after ethanol-mediated flap repositioning, day 1 (C) and day 3 (D) after mechanical scraping, and 0 hours (E) and 4 hours (F) after 20% ethanol treatment for 30 seconds. After both types of surgery, the basal lamina was not well defined: There was no space between the basal lamina and the basal epithelial layer and no hemidesmosomes in the cell membrane of the basal epithelial cell at day 1 (A, C). In the basal epithelial layer of the ethanol-mediated flap, surviving basal epithelial cells were seen migrating to the position of dead cells at day 1 (A, inset). At day 3, the basal lamina is defined well, and hemidesmosomes are seen (B, D). In the ethanol-only treated cornea, the basal lamina is well defined and hemidesmosomes are seen at all time points (E, F). Similar results were observed in at least three other corneas. Arrows: hemidesmosomes. Bar, 1 µm.

 
One day after transplantation, the SRY gene was detected in the female recipient corneal epithelium after attachment of the donor ethanol-detached male epithelial flap, but it was not detected at day 3 (Fig. 8) .



View larger version (16K):
[in this window]
[in a new window]
  		FIGURE 8. Detection of SRY gene in female corneal epithelium after receiving an ethanol-mediated male epithelial flap. The SRY gene was detected in female corneal epithelial cells at day 1 after transplantation but was not detected at day 3.

 
Functional Capacity of Corneal Epithelium after Ethanol Detachment
The mRNA levels of AQP5 and MUC1, which are related to normal corneal epithelial function, were unchanged in both epithelial cells in the flap after ethanol-exposed flap repositioning and regenerated corneal epithelial cells after mechanical scraping (Fig. 9) .



View larger version (24K):
[in this window]
[in a new window]
  		FIGURE 9. Expression of AQP-5 and MUC1. There were no changes in the mRNA levels of AQP5 and MUC1 in epithelial cells with ethanol-mediated flap or regenerated corneal epithelial cells after mechanical scraping. The presented data are from one of four independent experiments with similar results.

 

   Discussion
Top
 Abstract
 Materials and Methods
 Results
 Discussion
References
 
Surgery for the correction of refractive errors has been performed for many years, and its use accelerated with the development of the excimer laser.18 However, one problem with the study of LASEK has been the lack of a suitable experimental animal model in which to create reproducible epithelial flaps. Many studies have therefore evaluated the toxic effects of dilute ethanol on the corneal surface. These have demonstrated that treatment with 20% ethanol for less than 1 minute results in effective epithelial flap creation with decreased cell damage.10 13 19

Lee et al.11 have developed an animal model for LASEK in the eyes of White Leghorn chicks (which have a Bowman’s layer similar to that of humans). The animal used in the current model was in Sprague-Dawley rats, which have no Bowman’s layer. After LASEK surgery in humans, the anterior stroma including Bowman’s layer is removed with excimer laser ablation, the ethanol-induced epithelial flap is replaced on the stroma, and wound-healing occurs between epithelial flap and stroma. Although the Sprague-Dawley rat eye has no Bowman’s layer, it seems to be a useful model for examining the wound-healing process after LASEK surgery.

Ethanol is frequently used during PRK to facilitate removal of the epithelium, and removal with ethanol is faster to perform and provides sharper edges and a smoother Bowman’s layer surface than mechanical scraping.7 8 9 Dilute ethanol is also used in LASEK to make a corneal epithelial flap effectively. Espana et al.20 reported that the cleavage plane of the ethanol-induced corneal epithelial flap is located between the lamina lucida and the lamina densa of the basement membrane, where integrin ß4 interacts with laminin 5 to form hemidesmosomes.

Corneal reepithelialization involves necessary basement membrane remodeling and cell migration. MMP-9 is the primary MMP synthesized and secreted by basal corneal epithelial cells migrating to resurface a wound. The pattern of MMP-9 synthesis is consistent with the timing of basement membrane degradation—a rapid increase in expression within a day of wounding.21

The present study compared the wound-healing process between ethanol-mediated flap repositioning and mechanical scraping. After epithelial flap repositioning, damaged basal epithelial cells and detachment of superficial cells were observed until day 1, and the death rate of epithelial cells and loss of stromal keratocytes peaked at day 1. Epithelial cell proliferation peaked at day 3, and MMP-9 mRNA was also expressed from days 1 to 3. By contrast, in the mechanically scraped cornea, the denuded stromal surface was covered with multilayers of epithelial cells at day 3, cell death peaked at 4 hours, cell proliferation peaked from 12 hours to day 1, and MMP-9 mRNA was widely expressed from 12 hours to day 3. Thus, the cornea with ethanol-mediated flap repositioning had a slower wound-healing process and less keratocyte loss than the mechanically scraped cornea.

Furthermore, epithelial cells in the repositioned flap protecting the denuded stromal surface were alive, proliferated, and proceeded to a wound-healing response. Through sex cross-transplantation, we demonstrated that the epithelial cells in the repositioned flap regenerated within 3 days, similar to the renewal time of normal corneal epithelium: 5 to 7 days.22

The corneal epithelium forms a barrier between the environment and the stroma of the cornea, and through its interaction with the tear film, forms a smooth refractive surface on the cornea.22 In this study, AQP5 and MUC1 mRNA expression was used to demonstrate corneal epithelial functions after ethanol-mediated flap repositioning. AQP5 is one of a family of water channel proteins, selectively localized on the surface of corneal epithelium in the eye. Its mRNA expression is enriched in the cornea and a major role of AQP5 in the corneal epithelium is to prevent dehydration or to secrete watery products and the epithelium can contribute to corneal hydration homeostasis.15 23

Mucin is normally produced by goblet cells, but also by the corneal and conjunctival epithelia, in particular the transmembrane mucin, MUC1.24 MUC1 mRNA has been detected in all cell layers of the corneal epithelium, and MUC1 protein is present in the apical membranes of the superficial cells of the cornea and conjunctiva.17 MUC1 has been shown to play a protective role against the adherence of pathogens.25 In this study, AQP5 and MUC1 mRNA expression levels were unchanged in corneas with ethanol-induced epithelial flap repositioning and in mechanically scraped corneas. Consequently, corneal epithelium may maintain its function after both types of surgery.

In conclusion, extrapolating from the results of this study, it appears that ethanol-mediated epithelial flap repositioning during LASEK induces less keratocyte loss and a slower wound-healing process than mechanical scraping in PRK. The repositioned epithelial flap protecting the underlying stromal surface may have an epithelial function and allow for slow replacement of damaged epithelial cells by peripheral epithelial cells during the wound healing process. In addition, this Sprague-Dawley rat eye model may be useful to study the wound-healing response after LASEK.


   Footnotes
 
Supported by Grant 2000-N-NL-01-C-121 from the National Research Laboratory Program.

Submitted for publication August 31, 2003; revised October 8, 2003; accepted October 13, 2003.

Disclosure: I.-K. Song, None; C.-K. Joo, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Choun-Ki Joo, Laboratory of Ophthalmology and Visual Science, College of Medical, The Catholic University of Korea, 505 Banpo-dong, Soecho-ku, Seoul 137-040, Korea; ckjoo{at}catholic.ac.kr.


   References
Top
 Abstract
 Materials and Methods
 Results
 Discussion
 References
 

  1. Dastjerdi MH, Soong HK. LASEK (laser subepithelial keratomileusis). Curr Opin Ophthalmol. 2002;13:261–263.[CrossRef][Medline][Order article via Infotrieve]
  2. Azar DT, Ang RT, Lee JB, et al. Laser subepithelial keratomileusis: electron microscopy and visual outcomes of flap photorefractive keratectomy. Curr Opin Ophthalmol. 2001;12:323–328.[CrossRef][Medline][Order article via Infotrieve]
  3. Lee JB, Seong GJ, Lee JH, et al. Comparison of laser epithelial keratomileusis and photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg. 2001;27:565–570.[CrossRef][ISI][Medline][Order article via Infotrieve]
  4. Litwak S, Zadok D, Garcia-deq Uevedo V, Robledo N, Chayet AS. Laser-assisted subepithelial keratectomy versus photorefractive keratectomy for the correction of myopia: a prospective comparative study. J Cataract Refract Surg. 2002;28:1330–1333.[CrossRef][ISI][Medline][Order article via Infotrieve]
  5. Autrata R, Rehurek J. Laser-assisted subepithelial keratectomy for myopia: two-year follow-up. J Cataract Refract Surg. 2003;29:661–668.[CrossRef][ISI][Medline][Order article via Infotrieve]
  6. Scerrati E. Laser in situ keratomileusis vs. laser epithelial keratomileusis (LASIK vs. LASEK). J Refract Surg. 2001;17:S219–S221.[ISI][Medline][Order article via Infotrieve]
  7. Abad JC, Talamo JH, Vidaurri-Leal J, Cantu-Charles C, Helena MC. Dilute ethanol versus mechanical debridement before photorefractive keratectomy. J Cataract Refract Surg. 1996;22:1427–1433.[ISI][Medline][Order article via Infotrieve]
  8. Abad JC, An B, Power WJ, et al. A prospective evaluation of alcohol-assisted versus mechanical epithelial removal before photorefractive keratectomy. Ophthalmology. 1997;104:1566–1574.[ISI][Medline][Order article via Infotrieve]
  9. Carones F, Fiore T, Brancato R. Mechanical vs. alcohol epithelial removal during photorefractive keratectomy. J Refract Surg. 1999;15:556–562.[ISI][Medline][Order article via Infotrieve]
  10. Kim SY, Sah WJ, Lim YW, Hahn TW. Twenty percent alcohol toxicity on rabbit corneal epithelial cells: electron microscopic study. Cornea. 2002;21:388–392.[CrossRef][ISI][Medline][Order article via Infotrieve]
  11. Lee JB, Javier JA, Chang JH, et al. Confocal and electron microscopic studies of laser subepithelial keratomileusis (LASEK) in the white leghorn chick eye. Arch Ophthalmol. 2002;120:1700–1706.[Abstract/Free Full Text]
  12. Browning AC, Shah S, Dua HS, et al. Alcohol debridement of the corneal epithelium in PRK and LASEK: an electron microscopic study. Invest Ophthalmol Vis Sci. 2003;44:510–513.[Abstract/Free Full Text]
  13. Chen CC, Chang JH, Lee JB, Javier J, Azar DT. Human corneal epithelial cell viability and morphology after dilute alcohol exposure. Invest Ophthalmol Vis Sci. 2002;43:2593–2602.[Abstract/Free Full Text]
  14. Funaki H, Yamamoto T, Koyama Y, et al. Localization and expression of AQP5 in cornea, serous salivary glands, and pulmonary epithelial cells. Am J Physiol. 1998;275:1151–1157.
  15. Hamann S, Zeuthen T, La Cour M, et al. Aquaporins in complex tissues: distribution of aquaporins 1-5 in human and rat eye. Am J Physiol. 1998;274:1332–1345.
  16. Patil RV, Saito I, Yang X, Wax MB. Expression of aquaporins in the rat ocular tissue. Exp Eye Res. 1997;64:203–209.[CrossRef][ISI][Medline][Order article via Infotrieve]
  17. Watanabe H. Significance of mucin on the ocular surface. Cornea. 2002;21:S17–S22.[CrossRef][Medline][Order article via Infotrieve]
  18. Ambrosio R, Jr, Wilson S. LASIK vs LASEK vs PRK: advantages and indications. Semin Ophthalmol. 2003;18:2–10.[Medline][Order article via Infotrieve]
  19. Gabler B, Winkler von Mohrenfels C, Dreiss AK, Marshall J, Lohmann CP. Vitality of epithelial cells after alcohol exposure during laser-assisted subepithelial keratectomy flap preparation. J Cataract Refract Surg. 2002;28:1841–1846.[CrossRef][ISI][Medline][Order article via Infotrieve]
  20. Espana EM, Grueterich M, Mateo A, et al. Cleavage of corneal basement membrane components by ethanol exposure in laser-assisted subepithelial keratectomy. J Cataract Refract Surg. 2003;29:1192–1197.[CrossRef][ISI][Medline][Order article via Infotrieve]
  21. Sivak JM, Fini ME. MMPs in the eye: emerging roles for matrix metalloproteinases in ocular physiology. Prog Retin Eye Res. 2002;21:1–14.[CrossRef][ISI][Medline][Order article via Infotrieve]
  22. Gipson IK. Anatomy of the conjunctiva, cornea and limbus. Smolin G Thoft RA eds. The Cornea. 1994; 3rd Ed. 3–46. Little, Brown New York.
  23. Verkman AS. Role of aquaporin water channels in eye function. Exp Eye Res. 2003;76:137–143.[CrossRef][ISI][Medline][Order article via Infotrieve]
  24. Watanabe H, Fabricant M, Tisdale AS, et al. Human corneal and conjunctival epithelia produce a mucin-like glycoprotein for the apical surface. Invest Ophthalmol Vis Sci. 1995;36:337–344.[Abstract/Free Full Text]
  25. Kardon R, Price RE, Julian J, et al. Bacterial conjunctivitis in Muc1 null mice. Invest Ophthalmol Vis Sci. 1999;40:1328–1335.[Abstract/Free Full Text]



This article has been cited by other articles:


Home page
 Am. J. Physiol. Cell Physiol.Home page
D. Reigada and C. H. Mitchell
Release of ATP from retinal pigment epithelial cells involves both CFTR and vesicular transport
Am J Physiol Cell Physiol, January 1, 2005; 288(1): C132 - C140.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (10)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Song, I.-K.
Right arrow Articles by Joo, C.-K.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Song, I.-K.
Right arrow Articles by Joo, C.-K.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS