Corneal Total and Epithelial Thickness Measured by Sonogage Ultrasound Pachometry and High-resolution Optical Coherence Tomography.

SIGNIFICANCE: This research questions the validity of using the Sonogage ultrasound (US) pachometer to measure corneal epithelial thickness and coincidentally provides confirmation for the conventional view of the mechanism of orthokeratology (OK) based on central epithelial thinning. PURPOSE: The Sonogage (Corneo-Gage Plus 1) pachometer uses A-scan US to measure total corneal thickness. It is claimed that this instrument can also measure corneal epithelial thickness. We sought to validate this claim by comparing total and epithelial thickness measurements with the Sonogage with those obtained with high-resolution optical coherence tomography (OCT). METHODS: Fourteen non-contact lens wearers and 14 subjects who had worn Paragon CRT OK lenses overnight for greater than 1 month were recruited. Three OCT and five US measurements were taken in one eye of each subject. Depending on normality of data, paired t tests or Wilcoxon tests were used to compare total and epithelial thicknesses measured with the Sonogage pachometer and the Tomey Casia OCT. Pearson or Spearman correlation analyses were used to examine relationships between measurements obtained with the two instruments. RESULTS: There was a significant difference in total corneal thickness measurements between the two instruments. Although a significant correlation was found (r = 0.916, P < .001), the Sonogage consistently measured greater total corneal thickness than did the OCT (+19.5 ± 9.2 µm; P < .001). Epithelial thickness using the Sonogage showed little variation (range, 46.4 to 50.0 µm), whereas epithelial thickness using the OCT ranged from 30.7 to 54.7 µm. There was no significant correlation between epithelial thicknesses obtained with the two instruments (r = -0.135, P = .49). Epithelial thickness measured by OCT was significantly thinner in OK wearers (35.8 ± 2.8 µm) than in nonlens wearers (46.7 ± 4.5 µm, P < .001). CONCLUSIONS: The Sonogage is not able to measure epithelial thickness in vivo, returning essentially identical measurements over a range of epithelial thicknesses. Optical coherence tomography measurements confirm the conventional view of the mechanism of OK based on central epithelial thinning.

Impact of the Reference Point for Epithelial Thickness Measurements.

PURPOSE: To analyze the implications of different reference points on the read-out of epithelial thickness mapping. METHODS: A simulation for changing the reference point from normal-to-the-surface tangent to parallel vertical sections quantifying its effect on the read-out of epithelial thickness mapping has been developed. The simulation includes a simple modeling of corneal epithelial profiles and allows the analytical quantification of the differences in the read-out from normal-to-the-surface tangent to parallel vertical sections epithelial thickness mapping. RESULTS: The difference in the read-out between parallel vertical sections and normal-to-the-surface tangent epithelial thickness mapping increases for steeper corneas, but it is not largely affected by asphericity. The difference increases for thicker epithelia. CONCLUSIONS: The reference point for determining the readout of epithelial thickness mapping should be taken into account when interpreting output. Using conventional epithelial thickness map readings (normal-to-the surface tangent) in transepithelial ablations (representing close to parallel vertical sections) may result in induced refractive errors that can be quantified using simple theoretical simulations, because the center-to-periphery progression of the corneal epithelial profile deviates from the progression of the ablated one. Adjustments for the epithelial thickness read-out or, alternatively, for the target sphere can be easily derived. [J Refract Surg. 2020;36(2):200-207.].

[Panorama of limbal alterations (French translation of the article)]. / Panorama des altérations limbiques cornéennes.

The corneal limbus is a privileged region on the border between two quite different microenvironments, where corneal epithelial stem cells, numerous melanocytes, and antigen-presenting cells are all concentrated within a richly vascularized and innervated stroma. This situation within the ocular surface confers on it the key functions of barrier, epithelial renewal and defense of the cornea. As an immunological crossroads and since the corneoscleral limbus is directly exposed to external insults such as caustic agents, ultraviolet radiation, microbial agents, and allergens, it is the potential site of many tumoral, degenerative or inflammatory pathologies and may progress under certain conditions to limbal stem cell deficiency.

Panorama of limbal changes.

The corneal limbus is a privileged region on the border between two quite different microenvironments, where corneal epithelial stem cells, numerous melanocytes, and antigen-presenting cells are all concentrated within a richly vascularized and innervated stroma. This situation within the ocular surface confers on it the key functions of barrier, epithelial renewal and defense of the cornea. As an immunological crossroads and since the corneoscleral limbus is directly exposed to external insults such as caustic agents, ultraviolet radiation, microbial agents, and allergens, it is the potential site of many tumoral, degenerative or inflammatory pathologies and may progress under certain conditions to limbal stem cell deficiency.

Morphological and immunohistochemical characteristics of the equine corneal epithelium.

OBJECTIVE: The morphology of the corneal epithelium in two age groups of horses is described. Distribution patterns of proliferation-, differentiation-, stem cell-associated markers and cell junction proteins were assessed. METHODS: Corneal samples from 12 horses (six foals and six adult horses) were analyzed after H&E staining and immunohistochemistry using the following antibodies: E-cadherin, ß-catenin, Connexin 43 (Cx43), tight junction protein 1 (TJP1), cytokeratin (CK) 14, CK 19, CK 3, CK 10, vimentin, Ki67, p63, nerve growth factor (NGF), ABCG2, and epithelial growth factor receptor. Semiquantitative analysis of crypt, limbal, peripheral, and central zone was performed. Semithin and ultrathin sections were used for ultrastructural evaluation of the epithelium. RESULTS: The height of the epithelium varied between age groups and crypts were consistently present. In the peripheral and central epithelium, three types of basal cells resembling a pseudostratified epithelium were characterized. Potential stem cell markers (CK 14, p63, NGF, and ABCG2) were present in all zones with decreasing frequency toward the center. Cornea-specific differentiation marker CK 3 was not expressed in the most basal cell layer of the limbal epithelium. E-cadherin, ß-catenin, and Cx43 revealed a similar apico-lateral signal pattern throughout the entire epithelium; only TJP1 was additionally seen at the basal surface. CONCLUSIONS: This study presents a systematic semiquantitative evaluation of the equine corneal epithelium, showing the presence of crypts as potential stem cell niche with CK 14, p63, NGF, and ABCG2 as relevant markers for cells with regenerative capacity. The pseudostratified arrangement of the basal layer was a unique finding.

Mapping of Corneal Layer Thicknesses With Polarization-Sensitive Optical Coherence Tomography Using a Conical Scan Pattern.

Purpose: We demonstrate segmentation and mapping of corneal layers (epithelium, Bowman's layer, and stroma) across the entire cornea (limbus to limbus), using additional contrast provided by polarization-sensitive optical coherence tomography (PS-OCT) and analyze the reproducibility of the procedure. Methods: A custom built PS-OCT system operating at 1045 nm central wavelength with conical scanning was used for image acquisition. Conical scanning allows for almost perpendicular beam incidence on the corneal surface and provides good signal quality over the entire field of view. Epithelium, Bowman's layer, and stroma were segmented using the additional contrast provided by PS-OCT. Thickness maps were computed and analyzed in sectors. Both eyes of 20 healthy volunteers were imaged at least three times to test this method and to quantify reproducibility. Results: Thickness maps of the epithelium show significant (P < 0.001) superior thinning and an inferior thickening. Bowman's layer appears homogeneous within the central 7 to 8 mm diameter of the cornea and gets thinner toward the periphery until this layer disappears between 4 and 5.5 mm eccentricity from the center. Intersubject variations of the measured thicknesses of epithelium (coefficient of variation [CV] ∼8%), Bowman's layer (CV∼25%), and stroma (CV∼10%) were observed. Very good reproducibility of thickness measurements of epithelium (CV < 3%), Bowman's layer (CV < 5%), and stroma (CV < 2%) was found. Furthermore, a significant correlation (P < 0.001) between layer thicknesses of the right and left eyes of the same subject was found. Conclusions: PS-OCT with conical scanning is a feasible approach for determining thickness maps of corneal layers on a large field of view with high reproducibility.

Wide Corneal Epithelial Mapping Using an Optical Coherence Tomography.

Purpose: To map the corneal epithelium using a map measuring 9 mm in diameter and view the effects of age, sex, and axial length. Additionally, we wanted to demonstrate the reproducibility of this technique. Methods: We calculated the epithelial thickness in 220 individuals using an SD-OCT machine with the newly released commercially available algorithm. We included normal eyes with refractive errors between +5 and -6 diopters (D). We excluded patients with an intraocular pressure of >22 mm Hg, history of cataract, previous ocular surgery, or disease and those with corneal pathology. Additionally, we excluded patients with evidence of systemic disease or pregnancy. Lastly, reproducibility was measured in 50 individuals. Results: We found the center of the corneal epithelium to be thicker than the peripheral in all zones except the nasal (P = 0.124). The superior quadrant was found to be the thinnest while the inferior was the thickest. Males had a thicker epithelium in all locations except the superior outer section (P = 0.123). Three zones had a weak correlation with age: outer superior (P = 0.039, R = -0.152); outer temporal (P = 0.042, R = -0.150); and outer superior temporal (P = 0.011, R = -0.187). There was no significant relationship with the axial length. We found good to excellent reproducibility when using this technique in the central as well as the peripheral cornea. Conclusions: We provide a comprehensive study in healthy, normal eyes using a novel algorithm to map the corneal epithelium with a wide diameter. This study can be used as a reference for future research.

Relationship between vessel diameter and depth measurements within the limbus using ultra-high resolution optical coherence tomography.

PURPOSE: To establish a relationship between the diameter and depth position of vessels in the superior and inferior corneo-scleral limbus using ultra-high resolution optical coherence tomography (UHR-OCT). METHODS: Volumetric OCT images of the superior and inferior limbus were acquired from 14 healthy subjects with a research-grade UHR-OCT system. Differences in vessel diameter and depth between superior and inferior limbus were analyzed using repeated measured ANOVA in SPSS and R. RESULTS: The mean (± SD) superior and inferior diameters were 29±18µm and 24±18µm respectively, and the mean (± SD) superior and inferior depths were 177±109µm and 207±132µm respectively. The superior limbal vessels were larger than the inferior ones (RM-ANOVA, p=0.004), and the inferior limbal vessels were deeper than the superior vessels (RM-ANOVA, p=0.041). There was a positive linear association between limbal vessel depth and size within the superior and inferior limbus with Pearson correlation coefficients of 0.803 and 0.754, respectively. CONCLUSION: This study demonstrated that the UHR-OCT was capable of imaging morphometric characteristics such as the size and depth of vessels in the limbus. The results of this study suggest a difference in the size and depth of vessels across different positions of the limbus, which may be indicative of adaptations to chronic hypoxia caused by the covering of the superior limbus by the upper eyelid. UHR-OCT may be a useful tool to evaluate the effect of contact lenses on the microvascular properties within the limbus.

Keratin-14-Positive Precursor Cells Spawn a Population of Migratory Corneal Epithelia that Maintain Tissue Mass throughout Life.

The dynamics of epithelial stem cells (SCs) that contribute to the formation and maintenance of the cornea are poorly understood. Here, we used K14CreERT2-Confetti (Confetti) mice, sophisticated imaging, and computational modeling to trace the origins and fate of these cells during embryogenesis and adult life. We show that keratin-14 (K14+)-expressing progenitors are defined and widely distributed across the E16.5 cornea, after which they undergo cycles of proliferation and dispersal prior to eyelid opening. K14+ clonal patches disappear from the central cornea and are replaced by limbal-derived K14+ streaks, a finding that aligned with bromodeoxyuridine label-retaining studies. We also elucidated the mechanism by which SC clones are lost during life and propose this is due to population asymmetry and neutral drift. Finally, we established that the occurrence of an equatorial migratory mid-line is a consequence of apoptosis in a narrow nasal-temporal region, the site where eyelids meet during blinking.

Using optical coherence tomography to assess the role of age and region in corneal epithelium and palisades of vogt.

Using spectral-domain optical coherence tomography (OCT) to observe the morphology and epithelial thickness (ET) of the palisades of Vogt (POV), and to evaluate the role of age and region on these structures.One hundred twelve eyes of 112 healthy subjects were enrolled and divided into 4 groups: A (0-19), B (20-39), C (40-59), and D (≥60 years old). RTvue-100 OCT was applied on the cornea and the limbus. The morphology of the subepithelial stroma underneath the epithelium of POV was classified into typical and atypical types. Maximum ET of POV was measured manually from OCT images.The positive rate of typical POV in superior, nasal, temporal, and inferior limbus was: Group A: 100%, 69.2%, 65.4%, 100%; Group B: 100%, 73.5%, 61.8%, 94.1%; Group C: 95.8%, 41.7%, 37.5%, 83.3%; Group D: 67.9%, 0%, 3.6%, 25%, showing a significant decreasing tendency with age. The maximum ET of POV in superior, nasal, temporal, and inferior limbus was: Group A: 103.5 ±â€Š10.1 um, 89.2 ±â€Š9.7 um, 87.9 ±â€Š13.6 um, 104.7 ±â€Š14.1 um; Group B: 111.4 ±â€Š15.8 um, 85.3 ±â€Š9.9 um, 88.2 ±â€Š8.6 um, 112.6 ±â€Š19.7 um; Group C: 116.4 ±â€Š16.4 um, 82.8 ±â€Š11.6 um, 87.0 ±â€Š11.6 um, 120.0 ±â€Š25.6 um; Group D: 96.3 ±â€Š17.9 um, 73.8 ±â€Š15.9 um, 79.2 ±â€Š16.7 um, 87.4 ±â€Š18.5 um. Age-dependent change was observed. In general, the maximum ET of POV in superior/inferior quadrants was thicker than the other 2 quadrants.Spectral-domain OCT is a useful tool to observe the limbal microstructure and provide invaluable information. Aging and anatomic regions had significant effects on the microstructure of these areas.

Identifying the Palisades of Vogt in Human Ex Vivo Tissue.

PURPOSE: The Palisades of Vogt (POV) constitute the corneal epithelial stem cell niche, but identification of this region in ex vivo tissue is difficult. Here we introduce a simple, direct method of identifying the POV in unsectioned, ex vivo human tissue. METHODS: Twenty-two eyes were studied, four whole and eighteen rims. Orientation of whole eyes was determined and the eyes were marked to maintain their cardinal orientation prior to dissection. Samples were imaged with brightfield, linearly polarized light and transmitted circularly polarized light (CPL), and optical coherence tomography (OCT) volumes were acquired in all twelve clock hrs around the limbus. Five samples were also fluorescently labeled to identify the epithelial basement membrane, and whole mounts were imaged with laser scanning confocal microscopy. Images were compared to confirm that the structures visible with polarized light were POV. RESULTS: Under CPL the POV presented as amber radial ridges visible in the superior and inferior regions of the tissue. Identification of POV was confirmed by correlating the structures seen under CPL, OCT and laser-scanning confocal microscopy. CONCLUSIONS: CPL can be used to quickly identify POV regions in donor tissue. This technique can assist in targeted harvesting of stem cell regions for research and tissue for limbal transplant.

Comparison of very-high-frequency ultrasound and spectral-domain optical coherence tomography corneal and epithelial thickness maps.

PURPOSE: To compare corneal thickness and epithelial thickness measurements in maps obtained using the RTVue spectral domain optical coherence tomography (SD-OCT) system and the Artemis 2 immersion arc-scanning very-high-frequency ultrasound (VHF-US) system. SETTING: Department of Ophthalmology, Columbia University Medical Center, New York, New York, USA. DESIGN: Comparative study. METHODS: Eyes of normal volunteers were scanned with the SD-OCT system followed by the VHF-US system and then again by the SD-OCT system. On each map, the minimum corneal thickness and mean values of corneal thickness and epithelial thickness in the 3.0 mm radius zone and in 0.5 mm wide concentric annuli of up to a 3.0 mm radius around the corneal vertex were determined. RESULTS: Both eyes of 12 normal volunteers were scanned. The corneal thickness values from both devices were highly correlated (R > 0.96); in the 3.0 mm radius zone, they were not statistically significantly different. There was no statistically significant change in epithelial thickness or corneal thickness in SD-OCT measurements taken before versus after immersion US. Although highly correlated (R > 0.76), the SD-OCT epithelial thickness values were systematically thinner (1.7 ± 2.1 µm) than the VHF-US measurements in the 3.0 mm radius zone (P < .01). CONCLUSIONS: The corneal thickness measurements in the 3.0 mm radius zone in normal eyes were equivalent between the 2 systems. Although correlated, the VHF-US epithelial thickness measurements were systemically thicker than the SD-OCT values. FINANCIAL DISCLOSURES: Drs. Silverman and Reinstein have a commercial interest in Arcscan, Inc. Dr. Reinstein is a consultant to Carl Zeiss Meditec. None of the other authors has a financial or proprietary interest in any material or method presented.

Ophthalmic variables in rehabilitated juvenile Kemp's ridley sea turtles (Lepidochelys kempii).

OBJECTIVE: To determine central corneal thickness (total corneal thickness [TCT], epithelial thickness [ET], and stromal thickness [ST]), anterior chamber depth (ACD), and intraocular pressure (IOP) in Kemp's ridley sea turtles (Lepidochelys kempii). DESIGN: Prospective cross-sectional study. ANIMALS: 25 healthy rehabilitated juvenile Kemp's ridley sea turtles. PROCEDURES; Body weight and straight-line standard carapace length (SCL) were recorded. All turtles underwent a complete anterior segment ophthalmic examination. Central TCT, ET, ST, and ACD were determined by use of a spectral-domain optical coherence tomography device. Intraocular pressure was determined with a rebound tonometer; the horse setting was used to measure IOP in all 25 turtles, and the undefined setting was also used to measure IOP in 20 turtles. For each variable, 3 measurements were obtained bilaterally. The mean was calculated for each eye and used for analysis purposes. RESULTS: The mean ± SD body weight and SCL were 3.85 ± 1.05 kg (8.47 ± 2.31 lb) and 29 ± 3 cm, respectively. The mean ± SD TCT, ET, ST, and ACD were 288 ± 23 µm, 100 ± 6 µm, 190 ± 19 µm, and 581 ± 128 µm, respectively. Mean ± SD IOP was 6.5 ± 1.0 mm Hg when measured with the horse setting and 3.8 ± 1.1 mm Hg when measured with the undefined setting. CONCLUSIONS AND CLINICAL RELEVANCE: Results provided preliminary reference ranges for objective assessment of ophthalmic variables in healthy juvenile Kemp's ridley sea turtles.

Effects of Central Corneal Stromal Thickness and Epithelial Thickness on Intraocular Pressure Using Goldmann Applanation and Non-Contact Tonometers.

PURPOSE: To investigate whether corneal thickness parameters measured by optical coherence tomography (OCT), such as central corneal thickness (CCT), central corneal stromal thickness (CCST), and central corneal epithelial thickness (CCET), influence the intraocular pressure (IOP) difference measured by Goldmann applanation tonometry (GAT) and non-contact tonometry (NCT). METHODS: In total, 50 eyes from 50 subjects without glaucomatous defects were included in this retrospective, cross-sectional study. We measured IOP using GAT and NCT and calculated the difference between the two methods. CCT was measured by a Cirrus HD-OCT device using anterior segment imaging. The basement membrane of the epithelium, which was seen as a high-reflection line in the OCT image, was taken as a reference line to measure CCST and CCET. RESULTS: The mean IOP measured by GAT and NCT was 16.7 ± 3.0 and 18.1 ± 3.8 mmHg, respectively. The mean IOP difference was 1.5 ± 1.7 mmHg, and the IOP measured by NCT was 8.4% ± 11.3% higher than that measured by GAT. The CCET and CCST were 57.9 ± 5.6 and 501.7 ± 33.8 µm, respectively. CCT showed a positive correlation with both GAT IOP (r = 0.648, P < 0.001) and NCT IOP (r = 0.676, P < 0.001). Although CCST showed a significant correlation with GAT IOP and NCT IOP, CCET did not. The difference between GAT IOP and NCT IOP increased with CCT (r = 0.333, P = 0.018), and CCET was positively correlated with the IOP difference between GAT and NCT (r = 0.435, P = 0.002). CONCLUSIONS: IOP increased with greater CCT, and CCST seemed to have a more important role than CCET. CCET also increased with greater CCT, and this may be a possible explanation for the increasing difference in IOP between GAT and NCT with increasing CCT.

Corneal Epithelial Thickness Measured by Manual Electronic Caliper Spectral Domain Optical Coherence Tomography: Distributions and Demographic Correlations in Preoperative Refractive Surgery Patients.

PURPOSE: The aim of this study was to report the distributions and demographic correlations of corneal epithelial thickness measured by manual electronic caliper spectral domain optical coherence tomography in preoperative refractive surgery patients. DESIGN: This was a retrospective review. METHODS: The charts of 218 consecutive patients (413 eyes) who presented for refractive surgery evaluation from April 2013 through September 2013 were retrospectively reviewed. RESULTS: The mean corneal epithelial thickness was 51.0 µm with a range of 43 to 61 µm. Corneal epithelial thickness was significantly correlated with sex (P < 0.0001), corneal keratometry (P = 0.01), and underlying corneal thickness excluding the epithelium (P = 0.0268). No significant associations were identified in which corneal epithelial thickness correlated with either age (P = 0.0760) or existing refractive status of the eye (P = 0.5135). CONCLUSIONS: Corneal epithelial thickness measured by manual electronic caliper optical coherence tomography in preoperative refractive surgery patients is comparable with the findings for the general population using other measurement techniques, the awareness of which may be useful in the preoperative assessment of these patients.

Reproducibility and Daytime-Dependent Changes of Corneal Epithelial Thickness and Whole Corneal Thickness Measured With Fourier Domain Optical Coherence Tomography.

PURPOSE: To evaluate the reproducibility of Fourier domain anterior segment optical coherence tomography (RTVue) based on repeated measurements of corneal thickness (CT) and epithelial thickness (ET) and to test daytime-dependent changes of these parameters. METHODS: Twenty-three eyes from 23 healthy volunteers were included in this prospective study. Three clinical observers performed 3 consecutive measurements each of CT and ET in 3 sessions using RTVue, resulting in 9 measurements per session. Session 1 was performed at 9 AM ± 1 hour, session 2 at 4 PM ± 1 hour on the same day, and session 3 at 9 AM ± 1 hour 2 days later. CT and ET were assessed in 9 areas: corneal vertex, 4 paracentral zones, and 4 peripheral zones. RESULTS: The mean intraclass correlations (ICCs) for intraobserver and interobserver reproducibility were almost perfect (0.81-0.99) for CT in all corneal zones and for ET in all but 2 peripheral zones. Intraobserver ICC was substantial (0.61-0.8) for superior peripheral area and interobserver ICC for superior and inferior peripheral positions. CT and ET decreased significantly from morning to afternoon sessions [-3.15 µm (P < 0.0001) and -0.52 µm (P < 0.0001), respectively]. ET correlated weakly to moderately with CT. CONCLUSIONS: RTVue yields almost perfect reproducibility for CT and ET in central, paracentral, and peripheral lateral zones. Central CT and ET were significantly thinner in the afternoon compared with the morning.

Evaluation of Total Corneal Thickness and Corneal Layers With Spectral-Domain Optical Coherence Tomography.

PURPOSE: To evaluate total corneal thickness and corneal layers in healthy young adults using spectral-domain optical coherence tomography and to describe its repeatability and reproducibility. METHODS: Eighty-six eyes from 86 healthy volunteers were prospectively and consecutively enrolled. Manual measurements of central corneal thickness (CCT) and central thickness of epithelium, Bowman's layer, stroma, and the Descemet-endothelium complex were performed using Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany). To assess the reliability of the repeated measurements, intraclass correlation coefficients and coefficients of variation were used. RESULTS: Mean CCT, epithelium, Bowman's layer, stroma, and Descemet-endothelium values were 555.50 ± 29.64, 54.60 ± 4.25, 16.70 ± 1.73, 467.51 ± 28.91, and 16.74 ± 1.66 µm, respectively. The intraclass correlation coefficients ranged from 0.746 (Bowman's layer) to 0.999 (CCT and stroma) and from 0.483 (Bowman's layer) to 0.995 (CCT) and 0.998 (stroma) for intraobserver repeatability and interobserver reproducibility, respectively. The measurements showed coefficients of variation lower than 11% in all cases. CONCLUSIONS: This study establishes a normal database for corneal thickness and all its layers in healthy young adults with spectral-domain optical coherence tomography. This device exhibited a high degree of intraobserver repeatability and interobserver reproducibility for all regions except Bowman's layer.

Corneal thickness, epithelial thickness and axial length differences in normal and high myopia.

BACKGROUND: Corneal biometric parameters can possibly be influenced by high myopia (HM). The influence of HM on corneal thickness (CT), epithelial thickness (ET) has not yet been clearly established. The aim of this study is to observe ET, CT and axial length (AL) differences between in normal and subjects with HMs and to investigate factors influencing the corneal biometric parameters and AL, such as age and gender. METHODS: A total of 97 normal subjects (97 eyes) and 48 HM subjects (48 eyes) were included. The ET and CT of the central 6-mm diameter (17 regions) and the AL data were captured. The 17 corneal and epithelial regions were the center (1 mm radius, area a), the inner ring (2.5 mm radius, area b), the outer ring (3 mm radius, area c) and the 8 radial scan lines in eight directions (Superior (1) , SN (2), Nasal (3), IN (4), Inferior (5), IT (6), Temporal (7), ST (8)) with an angle of 45° between each consecutive scan line (a, b 1-8, c 1-8). RESULTS: The ALs were increased about 4 mm in the HMs (P < 0.001). No differences in ET were observed; in contrast, significantly thicker CTs were observed in the HMs in 16 regions except the b5 subregion. In normal group, age was negatively correlated with AL but not CCT and CET and gender was correlated with CET. In HM group, age was not correlated with CCT , AL or CET and gender was correlated with AL and CCT but not CET. CONCLUSIONS: CT was thicker in the HMs but not ET. Age and gender should be considered for AL, CT and ET in both normal and HM group.

Maintenance of the corneal epithelium is carried out by germinative cells of its basal stratum and not by presumed stem cells of the limbus

The purpose of this investigation was to analyze the proliferative behavior of rabbit corneal epithelium and establish if any particular region was preferentially involved in epithelial maintenance. [3H]-thymidine was injected intravitreally into both normal eyes and eyes with partially scraped corneal epithelium. Semithin sections of the anterior segment were evaluated by quantitative autoradiography. Segments with active replication (on) and those with no cell division (off) were intermingled in all regions of the tissue, suggesting that the renewal of the epithelial surface of the cornea followed an on/off alternating pattern. In the limbus, heavy labeling of the outermost layers was observed, coupled with a few or no labeled nuclei in the basal stratum. This suggests that this region is a site of rapid cell differentiation and does not contain many slow-cycling cells. The conspicuous and protracted labeling of the basal layer of the corneal epithelium suggests that its cells undergo repeated cycles of replication before being sent to the suprabasal strata. This replication model is prone to generate label-retaining cells. Thus, if these are adult stem cells, one must conclude that they reside in the corneal basal layer and not the limbal basal layer. One may also infer that the basal cells of the cornea and not of the limbus are the ones with the main burden of renewing the corneal epithelium. No particular role in this process could be assigned to the cells of the basal layer of the limbal epithelium.

Vertical and horizontal corneal epithelial thickness profile using ultra-high resolution and long scan depth optical coherence tomography.

PURPOSE: To determine the vertical and horizontal thickness profiles of the corneal epithelium in vivo using ultra-long scan depth and ultra-high resolution spectral domain optical coherence tomography (SD-OCT). METHODS: A SD-OCT was developed with an axial resolution of ∼ 3.3 µm in tissue and an extended scan depth. Forty-two eyes of 21 subjects were imaged twice. The entire horizontal and vertical corneal epithelial thickness profiles were evaluated. The coefficient of repeatability (CoR) and intraclass correlation (ICC) of the tests and interobserver variability were analyzed. RESULTS: The full width of the horizontal epithelium was detected, whereas part of the superior epithelium was not shown for the covered super eyelid. The mean central epithelial corneal thickness was 52.0 ± 3.2 µm for the first measurement and 52.3 ± 3.4 µm for the second measurement (P>.05). In the central zone (0-3.0 mm), the paracentral zones (3.0-6.0 mm) and the peripheral zones (6.0-10.0 mm), the mean epithelial thickness ranged from 51 to 53 µm, 52 to 57 µm, and 58 to 72 µm, respectively. There was no difference between the two tests at both meridians and in the right and left eyes (P>.05). The ICCs of the two tests ranged from 0.70 to 0.97 and the CoRs ranged from 2.5 µm to 7.8 µm from the center to the periphery, corresponding to 5.6% to 10.6% (CoR%). The ICCs of the two observers ranged from 0.72 to 0.93 and the CoRs ranged from 4.5 µm to 10.4 µm from the center to the periphery, corresponding to 8.7% to 15.2% (CoR%). CONCLUSIONS: This study demonstrated good repeatability of ultra-high resolution and long scan depth SD-OCT to evaluate the entire thickness profiles of the corneal epithelium. The epithelial thickness increases from the center toward the limbus.