Epi-off-lenticule-on corneal collagen cross-linking in thin keratoconic corneas.

PURPOSE: To evaluate the safety and efficacy of corneal collagen cross-linking (CXL) performed on overlaying a corneal lenticule to thin recipient corneas of progressive keratoconus (KC) patients. METHODS: In this study were enrolled eyes of patients affected by progressive KC with a minimum corneal thickness less than 400 µm, after overlaying a lenticule of human corneal stroma prepared with the femtosecond laser. The lenticules used were 100 µm thick and of 8.5 mm diameter in all the cases. Both the host cornea and the lenticules were subjected to epithelial debridement. CXL was carried out according to the standard protocol. Visual acuity, refraction, slit-lamp examination, endothelial cell density, pachymetry and keratometry, anterior segment tomography (AS-OCT) and confocal microscopy were evaluated preoperatively and at 1, 3, 6 and 12 months postoperatively. RESULTS: CXL was performed in 10 eyes of 8 patients (main age 23), corneal thickness range 379-414 µm, mean 387.6 µm. One patient was lost at follow-up. In all other cases, visual acuity and the endothelial cell density remained stable over a 12-month follow-up. Preoperative mean K1 and mean K2 were 46.91 ± 1.9 and 50.75 ± 2.93, respectively, and at 12 months mean K1 was 47.36 ± 2.66 and mean K2 50.53 ± 3.35. The AS-OCT clearly showed a demarcation line in all patients at 1, 3 (mean depth 283 µm and 267 µm, respectively) and in some cases at 6 months. Reduced keratocyte density and stromal oedema were observed immediately up to 1 month after treatment, while a slight subepithelial haze was present at 1-month and completely disappeared by 6 months. CONCLUSION: This new technique seems to offer a therapeutic opportunity for young patients suffering from progressive KC with very thin corneas, in which the standard treatment is not indicate, and delay or avoid the need for a corneal transplant.

Correlation of central and peripheral keratometric parameters after corneal collagen cross-linking in keratoconus patients.

PURPOSE: To evaluate the difference in the central and peripheral keratometric parameters in patients with keratoconus after corneal collagen cross-linking (CXL). METHODS: Forty-eight eyes of 32 patients (18 males, 16-28 years) affected by progressive keratoconus in different stages of evolution underwent CXL using the standard epithelium-off protocol. Corneal thickness and corneal curvature before CXL and after 6 and 12 months using the Sirius tomographer were analyzed. The values of the mean corneal thickness at the corneal apex (CAT), center of the pupil (PCT), thinnest point (CTTL) and along concentric circles of 2, 4, 6, 8, 8.5, 9, 9.5 and 10 mm diameter were evaluated; the values of the mean curvature at the corneal apex and at the points in which the inferior, superior, nasal and temporal meridians crossed the above-mentioned concentric circles were also evaluated. RESULTS: The mean preoperative values for CAT, PCT and CTTL were 461.4 ± 30.3, 475.3 ± 30.5 and 441 ± 32.0, respectively. The values after 12 months of CXL were 444.6 ± 36.2, 451.6 ± 36.7 and 418.2 ± 41.4. The peripheral corneal thickness at the eight points ranged from 479 to 733 preoperatively. At 12-month post-CXL, the values ranged from 444.6 to 734.1. The mean posterior curvature from apex to periphery ranged from - 4.5 to - 9.1 days preoperatively and from - 4.5 to - 9.2 days at 12 months. These were not statistically significant (ANOVA and unpaired T test). CONCLUSIONS: Our data suggest that CXL over an 8-mm zone can stabilize the peripheral cornea. Longer-term follow-up studies on the peripheral cornea after CXL will provide useful information.

Infant limbus: an immunohistological study.

All published literature to date has identified the human corneo-scleral limbus as the site within which stem cells of the ocular surface reside. Recently we described a unique anatomical structure at the limbus, termed the Limbal Epithelial Crypt (LEC) that has features of a putative stem cell niche. In this study we examined infant limbus tissue (donor age 4 months) for evidence of LEC and performed immunohistological comparison between infant limbus and adult LEC. No defined LEC were detected in the infant limbus. However, the entire infant limbus has characteristics resembling adult LEC. Both infant limbus and LEC demonstrated negative expression for desmoglein 3. p63 and integrin beta1 expressions were located to the distal region of the infant limbus and to the basal region of the LEC. ABCG2 expression was positive throughout most of the infant limbus as was connexin 43. Infant limbus and in particular the distal region, appeared to house cells that are more "stem-like" in nature. The LEC may be a result of normal physiological developmental in order to protect and maintain stem cells at the ocular surface.

[Pathogenesis and epidemiology of neurotrophic keratopathy]. / Pathogenese und Epidemiologie der neurotrophen Keratopathie.

Neurotrophic keratopathy (NK) is a degenerative corneal disease that is based on an impairment of the corneal innervation. The damage to the sensory innervation, which is delivered through the 1st branch of the trigeminal nerve (ophthalmic nerve), can occur throughout the entire length of the nerve from the nucleus in the brainstem, e.g. caused by brain tumors, to the terminal nerve fibers in the cornea, caused for example by refractive corneal surgery (e. g. LASIK). Due to the loss of the sensory innervation, a reduced lacrimation and a reduction in the secretion of trophic factors occur. This in turn inhibits the regeneration potential of the corneal epithelium. In the most severe cases of the disease, the reduction or loss of lacrimation, together with the impaired regeneration potential of the epithelial cells, can lead to persistent epithelial defects, ulcers and corneal perforation. The NK has a prevalence of 5 or fewer individuals per 10,000 and is classified as a rare, i. e. orphan disease (ORPHA137596). A fundamental understanding of the pathogenesis and epidemiology of NK supports the early diagnosis and therefore the initiation of a specific treatment.

Fibrin glue as agent for sealing corneal and conjunctival wound leaks.

PurposeTo describe a novel use of fibrin glue in managing leaking blebs and leaking wounds following trauma or surgery.MethodsInterventional case series.ResultsWe report eight patients, including three where intra-operative or immediate post-penetrating keratoplasty recalcitrant leaks from the graft-host junction and/or openings created by the needle pass, were noted. All three had thin recipient beds in the sector of leak. This was managed by intra-cameral injection of fibrin glue in the affected quadrant. This stopped the leak and allowed the defect to heal. One patient of Descemets-stripping-endothelial-keratoplasty had leak from the surgical wound, which was also sealed with fibrin glue. Two patients with leaking glaucoma-surgery-related blebs were treated with intra-bleb injection of fibrin glue to stop the leak. One patient with a penetrating corneal injury with a metal wire had a brisk leak upon removal of the wire. This was sealed with fibrin glue. Another patient of chemical burn with spontaneous leaks was managed by glue injection in the perforations. Transient rise of intraocular pressure in one patient with a leaking bleb was the only adverse event recorded.ConclusionThis novel adaptation of the application of fibrin glue can help to deal with persistent intra-operative, post-operative and traumatic aqueous and air leaks.

Air pressure changes in the creation and bursting of the type-1 big bubble in deep anterior lamellar keratoplasty: an ex vivo study.

PurposeTo measure the pressure and volume of air required to create a big bubble (BB) in simulated deep anterior lamellar keratoplasty (DALK) in donor eyes and ascertain the bursting pressure of the BB.Patients and methodsTwenty-two human sclera-corneal discs were used. Air was injected into the corneal stroma to create a BB and the pressure measured by means of a pressure converter attached to the system via a side port. A special clamp was designed to prevent air leak from the periphery of the discs. The pressure at which air emerged in the corneal tissue; the bursting pressure measured after advancing the needle into the bubble cavity and injecting more air; the volume of air required to create a BB and the volume of the BB were ascertained.ResultsType-1 BB were achieved in 19 and type-2 BB in 3 eyes. The maximum pressure reached to create a BB was 96.25+/- 21.61 kpa; the mean type-1 intrabubble pressure was 10.16 +/- 3.65 kpa. The mean bursting pressure of a type-1 BB was 66.65 +/- 18.65 kpa, while that of a type-2 BB was 14.77 +/- 2.44 kpa. The volume of air required to create a type-1 BB was 0.54 ml and the volume of a type-1 BB was consistently 0.1 ml.ConclusionsDua's layer baring DALK can withstand high intraoperative pressures compared to Descemet's membrane baring DALK. The study suggests that it could be safe to undertake procedures such as DALK-triple with a type-1 BB but not with a type-2 BB.

Vitreous cavity length in keratoconus: implications for keratoplasty.

PurposeTo compare axial length (AL) with vitreous cavity length (VCL) in patients with keratoconus and to ascertain whether graft size can be tailored to reduce myopic refractive error in patients with keratoconus undergoing penetrating keratoplasty (PK).Patients and methodsThe AL and VCL were measured prospectively in patients with keratoconus not undergoing PK (Group 1) and in normal phakic, emmetropic individuals (Group 2). A retrospective analysis of these measurements in patients with keratoconus who had undergone PK (Group 3) was also performed. The postoperative spherical equivalent (SE) was then correlated to size of donor buttons.ResultsKeratoconus patients tended to have a longer mean VCL than emmetropic normal individuals. The mean VCL of these patients (Group 1) was 16.49 mm±SD 1.13 compared to the mean VCL of 15.94 mm±SD 0.56 in normals (Group 2, P<0.0001). Patients with keratoconus who had an undersized graft showed reduced myopic refractive error compared to those with same size or oversized grafts.ConclusionVCL measurement is more accurate than AL measurement in deciding upon graft-host size disparity for corneal graft in patients with keratoconus. In patients with increased VCL, undersizing the donor button helps in reducing postoperative myopia. We recommend VCL measurement as part of the routine workup in all keratoconus patients undergoing corneal transplants.

Tacrolimus immunosuppression in high-risk corneal grafts.

BACKGROUND: Unlike the immune privilege enjoyed by low-risk corneal grafts, high-risk corneal grafts experience rejection rates comparable to liver and kidney transplants. Systemic immunosuppression reduces the risk of rejection in high-risk corneal grafts. METHODS: Systemic tacrolimus, a specific T cell inhibitor, was used at a mean daily dose of 2.5 mg to immunosuppress 43 patients undergoing high-risk corneal transplantation. Immunosuppression was continued for a period of 18-24 months after the high-risk corneal graft. RESULTS: During a mean follow-up period of 33.7 months, clarity of the graft was maintained in 65% of patients. Eight patients experienced rejection episodes while on tacrolimus, and this led to graft failure in five patients. CONCLUSION: Tacrolimus is relatively safe and effective in reducing rejection and prolonging graft survival in patients with high-risk keratoplasty compared with other series where similar immunosuppression was not used.

Expression of haematopoietic stem cell markers, CD133 and CD34 on human corneal keratocytes.

AIM: To study the expression of CD133 and CD34 antigens on cultured human keratocytes over time. METHODS: Primary cultures of human corneal stromal cells were established from explants derived from cadaver eye donors. The cultures were sorted for CD133+ and CD34+ cells using magnetic beads. Both the primary cultures and secondary passages of sorted cells were further analysed by flow cytometry and western blot analysis for expression of the same antigens over time. RESULTS: Four different cell populations-namely, CD133+, CD133-, CD34+ and CD34-, were identified in the culture samples. Two further specific subgroups were identified by flow cytometry: CD133+/CD34- cells and CD133+/CD34+ cells. Expression of CD133 declines more than CD34 with time in cell cultures. Although most cells lost expression of these markers, small populations retained staining up to 5 weeks in culture. CONCLUSION: Human keratocytes express the haematopoietic stem cell markers CD133 and CD34. This expression decreases with time in culture, with most but not all cells losing expression. On the basis of these markers, the corneal stroma shows a heterogeneous population of cells. Expression or down regulation of expression of these molecules could represent different stages of activation of these cells.

Pre-Descemets endothelial keratoplasty: the PDEK clamp for successful PDEK.

PurposeTo design and produce a clamp that enables good handling of donor sclera-corneal disks, allows air to be injected in the corneal stroma and consistently provides pre-Descemets endothelial keratoplasty (PDEK) tissue by creation of a type-1 big bubble (BB) avoiding a type-2BB by shutting the fenestrations in the periphery of the pre-Descemets layer and preventing escape of air.Patients and methodsA clamp with spring loaded handles attached to two rings of 9 mm diameter and 1 mm width with a side port for insertion of a needle for air injection was designed and produced. The clamp was tested in 20 human donor sclera-corneal disks and refined over three prototypes. The final design was used for PDEK in 8 patients.ResultsThe clamp was able to prevent any escape of air or the formation of a type-2BB. By preventing air leak, it provided better control over the volume of air injected allowing for consistent formation of a type-1BB, which could be expanded to its full diameter by advancing the tip of the needle into the bubble cavity. Centration of tissue in the clamp was important. The clamp was used successfully to obtain PDEK tissue from 8 donor sclara-corneal disks for transplant in three eyes with pseudophakic bullous keratopathy and five eyes with pseudophakia and Fuchs' endothelial dystrophy.ConclusionThe PDEK clamp is a simple and useful instrument that will give surgeons the confidence of consistently obtaining PDEK tissue with ease and without the risk of separating the Descemets membrane in a type-2BB.

Corneal confocal scanning laser microscopy in patients with dry eye disease treated with topical cyclosporine.

PurposeTo investigate the effect of cyclosporine on corneal ultrastructure and on major signs and symptoms of patients with dry eye disease.Patients and methodsIn this prospective cohort study, patients with dry eye disease were treated with a drop of cyclosporine 0.05% twice daily. Clinical evaluation was carried out at baseline and at months 1, 3, and 6. All patients completed the Ocular Surface Disease Index (OSDI) questionnaire, and tear film break-up time (BUT), fluorescein and lissamine green staining, and Schirmer test were carried out. In vivo confocal microscopy was also performed and epithelial cellular density, keratocyte activation, and subbasal plexus morphology were assessed.ResultsA total of 40 patients completed the study. After 6 months, OSDI, BUT, and fluorescein and lissamine green staining showed a clinically significant improvement. During the 6-month follow-up, density of intermediate epithelial cells increased from 1969.5±85.4 cell/mm2 to 4881.2±175.7 cell/mm2 (P<0.01); average grade of keratocyte activation decreased from 3.6±0.1 to 1.8±0.1 (P<0.001); average grade of number of subbasal nerves decreased from 5.3±0.2 to 2.6±0.2 (P<0.001); average grade of nerve reflectivity decreased from 3.8±0.1 to 2.1±0.2 (P<0.001); and average grade of nerve tortuosity decreased from 3.8± 0.1 to 2.2±0.2 (P<0.001).ConclusionCyclosporine was effective in controlling symptoms and signs of dry eye disease. In vivo confocal microscopy showed an increase in cell density of intermediate epithelium cells, a decrease in hyperreflective keratocytes, and a decrease in density, tortuosity, and reflectivity of nerve fibers.

Detecting glaucoma with RADAAR: the Bridlington Eye Assessment Project.

AIMS: To develop a diagnostic algorithm using rim/disc area asymmetry ratio (RADAAR) to discriminate between normal and glaucomatous eyes using the Heidelberg retina tomograph (HRT). METHODS: RADAAR was calculated by dividing rim/disc area ratio of the larger disc by that of the smaller disc. RADAAR normal tolerances in each disc sector were generated using 611 population based normal elderly individuals (mean age 72.5 years). Overall status for each individual was equal to the worst disc sector. Sensitivity was assessed in 45 patients with open angle glaucoma. RESULTS: RADAAR variance was significantly greater in glaucoma compared with normality in all disc sectors (p<0.001). Normal RADAAR limits were not dependent on age or sex. Overall, at the 99th percentile limit, the diagnostic algorithm's specificity was 95.1% with a sensitivity of 55.6%. Males with glaucoma showed greater RADAAR variance than females. Corresponding sensitivity of the diagnostic algorithm was 63.0% and 44.4% in males and females respectively, although this difference was not statistically significant (p = 0.43). CONCLUSION: The RADAAR diagnostic algorithm detected glaucoma with moderate sensitivity but was limited by significant numbers of glaucoma patients with symmetrical optic disc changes. RADAAR may have greater sensitivity in males than in females although this requires confirmation in further studies.

A novel use of amniotic membrane in the management of tube exposure following glaucoma tube shunt surgery.

AIM: To report a novel technique using amniotic membrane to cover exposed glaucoma tube shunts. METHODS: A consecutive series of three cases that underwent drainage tube shunt surgery with the Ahmed valve for intractable glaucoma. All three patients developed exposure of the tube secondary to necrosis of the overlying bovine pericardial patch and conjunctiva. Repair of the defect was carried out with a double layer of amniotic membrane, the inner one acting as a graft and the outer as a patch. Autologous serum was used to promote epithelial growth. RESULTS: Successful lasting closure of the conjunctival defect was achieved in all cases. CONCLUSION: Erosion of the drainage tube following shunt surgery is a potentially serious problem. It can be successfully managed using a double layer of amniotic membrane.

Epithelial proliferative potential of organ cultured corneoscleral rims; implications for allo-limbal transplantation and eye banking.

AIMS: To determine the epithelial proliferative capacity of organ cultured limbal tissue and correlate this with various donor and eye banking factors. METHODS: 24 corneoscleral limbal (CSL) rims left over from penetrating keratoplasty were split in half and set up as in vitro explant cultures. Corneal epithelial proliferative potential (CEPP) was assessed by the number of "cycles" of growth achieved before explants underwent exhaustion and failure to generate an epithelium to subconfluence. The dependence of CEPP on the age of the donor, time of death to enucleation, time of enucleation to organ culture, and time in organ culture in the eye bank was determined. RESULTS: CSL rims were capable of up to four cycles of culture with a wide variation between tissue samples. Of the various factors examined, death to enucleation time was the only statistically significant factor affecting the CEPP (regression coefficient: -0.062 (cycles/hour), CI -0.119 to -0.004, p = 0.037). Time in organ culture had little effect on CEPP. CONCLUSIONS: Preselected organ cultured CSL rims from eye banks may offer a viable alternative tissue source for use in allo-limbal transplantation.

Choroidal detachment in perforated corneal ulcers: frequency and management.

AIMS: To determine the frequency of choroidal detachment (CD) in eyes with non-traumatic corneal ulcer perforation and, also, to assess the efficacy and safety of cyanoacrylate glue in sealing corneal perforations. METHODS: 18 eyes of 17 patients were studied. Inclusion criterion was any patient with a non-traumatic perforated corneal ulcer. All patients had a thorough history taken and complete ophthalmic examination including B-scan ultrasonography. Patient demographics, presence of CD, and efficacy of corneal gluing were assessed. RESULTS: Eight of the 18 eyes (44%) were documented to have a CD. Among perforations of >2 mm2, six eyes (75%) were documented to have CD compared with two eyes (20%) with perforations of

Fibrin glue inhibits migration of ocular surface epithelial cells.

PurposeFibrin glue has been used successfully in numerous ophthalmic surgical procedures. Recently, fibrin glue has been used in limbal stem cell transplantation to reduce both operative time and to negate the need for sutures. The aim of this study was to determine the effects of fibrin glue on epithelial cell migration in vitro.MethodsCorneoscleral rims were split to retain the epithelial layer, Bowman's layer, and anterior stroma. Rims were cut into eight equal-sized pieces and were placed directly on culture plates or affixed with fibrin glue. Rims were maintained in culture for 25 days and epithelial cell growth was monitored. Cells were photographed to measure area or growth and immunofluorescence staining of explants for fibrin was performed.ResultsExplants that were glued demonstrated significantly delayed epithelial cell growth and migration as compared with explants without glue. By day 16, all fibrin glue had dissolved and coincided with onset of cell growth from glued explants. Cell growth commenced between days 3 and 4 for control explants without glue and around days 14-16 for explants with fibrin glue.ConclusionsFibrin glue delays epithelial cell migration by acting as a physical barrier and can potentially interfere with explant-derived limbal epithelial cell migration on to the corneal surface. We propose that glue should be used to attach the conjunctival frill of the limbal explant but care should be taken to ensure that the glue does not wrap around the explant if used to secure the explant as well. Strategic use of glue, to attach the recessed conjunctiva, can be advantageous in delaying conjunctival cell migration and reducing the need for sequential sector conjunctival epitheliectomy.

Optical coherence tomography characteristics of different types of big bubbles seen in deep anterior lamellar keratoplasty by the big bubble technique.

PurposeTo define optical coherence tomography (OCT) characteristics of type-1, type-2, and mixed big bubbles (BB) seen in deep anterior lamellar keratoplasty.MethodsHuman sclero-corneal discs were obtained from UK (30) and Canada (16) eye banks. Air was injected into corneal stroma until a BB formed. UK samples were fixed in formalin before scanning with Fourier-domain (FD-OCT). One pair of each type of BB was scanned fresh. All BB obtained from Canada were scanned fresh with time-domain (TD-OCT). For each OCT machine used, type-1 BB from which Descemets membrane (DM) was partially peeled, were also scanned. The morphological characteristics of the scans were studied.ResultsFD-OCT of the posterior wall of type-1 (Dua's layer (DL) with DM) and type-2 BB (DM alone) both revealed a double-contour hyper-reflective curvilinear image with a hypo-reflective zone in between. The anterior line of type-2 BB was thinner than that seen with type-1 BB. In mixed BB, FD-OCT showed two separate curvilinear images. The anterior image was a single hyper-reflective line (DL), whereas the posterior image, representing the posterior wall of type-2 BB (DM) was made of two hyper-reflective lines with a dark space in between. TD-OCT images were similar with less defined component lines, but the entire extent of the BB could be visualised.ConclusionOn OCT examination the DM and DL present distinct features, which can help identify type-1, type-2, and mixed BB. These characteristics will help corneal surgeons interpret intraoperative OCT during lamellar corneal surgery.

Corneal sensation after myopic and hyperopic LASIK: clinical and confocal microscopic study.

AIM: To assess the long term (1 year) effect of myopic and hyperopic LASIK on corneal sensation and innervation. METHODS: 83 eyes of 43 patients having LASIK were evaluated. According to the preoperative spherical equivalent, the eyes were divided into three groups: group 1, myopia from -0.75 to -6.00 D; group 2, myopia from -6.25 to -11.50 D; and group 3, hyperopia from 1.25 to 5.00 D. Corneal sensation was measured and in vivo confocal microscopy (IVCM) was done at the central cornea before, and at 1 month, 3 months, 6 months, and 1 year after LASIK. RESULTS: The mean corneal sensation in group 1 was greater than in groups 2 and 3 at all postoperative measurements. The difference between group 1 on one hand and groups 2 and 3 on the other hand was statistically significant at 1 month and 3 months after LASIK and was not statistically significant afterwards. IVCM study of 27 eyes revealed that the number and length of nerve fibre bundles in the sub-basal region decreased after LASIK and was significantly lower at all times after surgery despite the return of corneal sensation to preoperative level. CONCLUSION: After LASIK, central corneal sensitivity is decreased for as long as 6 months or more. The results suggest that lamellar cutting of the cornea during LASIK impairs corneal sensitivity and is related to the ablation depth. The diameter of ablation too may contribute to this drop in sensitivity. The return of corneal sensations does not directly correlate with the regeneration of nerve fibres as determined by confocal imaging. Sensations return to normal values before complete restoration of normal innervation if this indeed ever occurs.

Impression cytology of the ocular surface.

Impression cytology refers to the application of a cellulose acetate filter to the ocular surface to remove the superficial layers of the ocular surface epithelium. These cells can then be subjected to histological, immunohistological, or molecular analysis. Proper technique is essential as the number of cells sampled can vary considerably. Generally two to three layers of cells are removed in one application but deeper cells can be accessed by repeat application over the same site. Applications for impression cytology include diagnosing a wide range of ocular surface disorders, documenting sequential changes in the conjunctival and corneal surface over time, staging conjunctival squamous metaplasia, and monitoring effects of treatment. It is also a useful investigational tool for analysing ocular surface disease with immunostaining and DNA analysis. It is non-invasive, relatively easy to perform, and yields reliable information about the area sampled with minimal discomfort to the patient. Major ophthalmic centres should develop and introduce this technique into routine clinical practice. This is best achieved with a team approach including the ophthalmologist, pathologist, microbiologist, and the immunologist.

Limbal epithelial crypts: a novel anatomical structure and a putative limbal stem cell niche.

BACKGROUND/AIMS: There is substantial evidence that mammalian epithelial stem cells are located within well defined niches. Although the corneoscleral limbus is acknowledged as the site of corneal epithelial stem cells no anatomical niche for such cells has yet been described. The authors undertook to re-evaluate the microanatomy of the limbus in order to identify possible sites that may represent a stem cell niche. METHODS: Systematic serial 5-7 microm sections of human corneoscleral segments obtained from cadaver donors, were examined. The sections were stained with haematoxylin and eosin or toludine blue. Sections with specific areas of interest were further examined immunohistologically for the corneal epithelial marker cytokeratin 14 and the "stem cell" marker ABCG2 transporter protein. RESULTS: Distinct anatomical extensions from the peripheral aspect of the limbal palisades were identified. These consist of a solid cord of cells extending peripherally or circumferentially. The cells stained positive for CK14 and ABCG2. CONCLUSIONS: A novel anatomical structure has been identified at the human limbus, which demonstrates characteristics of being a stem cell niche. The authors have termed this structure the limbal epithelial crypt.