Non-invasive in vivo measurement of the tear film using spatial autocorrelation in a live mammal model

Measurement of In Vivo Three-Dimensional Corneal Cell Density and Size Using Two-Photon Imaging in C57BL/6 Mice

PURPOSE

To measure the cell size and cell density in five layers of the central cornea in the widely used inbred C57BL/6 mouse strain using in vivo three-dimensional (3D) two-photon (2PH) imaging.

METHODS

Corneas were scanned using a 2PH laser scanning fluorescence microscope after staining with plasma membrane stain and Hoechst 33342. Good quality 3D images were selected for the cell density and cell size analysis. Cell density was determined by counting the cell nuclei in a predefined cube of 3D images. Cell size measurements, including cell surface area, cell volume, nuclear surface area and nuclear volume, were automatically quantified using the Imaris software. The cell and nuclear surface-area-to-volume ratio (S:V ratio) and the cell nuclear-cytoplasmic ratio (N:C ratio) were calculated.

RESULTS

The highest cell density was observed in the basal epithelium and the lowest in the posterior stroma. The highest cell surface area was found in the anterior stroma, and the highest cell volume was observed in the superficial epithelium. The lowest cell surface area and cell volume were both found in the basal epithelium. The highest S:V ratio was observed in the basal epithelium and the lowest in the superficial epithelium. The highest cell nuclear surface area and volume were both observed in the superficial epithelium and the lowest in the basal epithelium. The highest cell nuclear S:V ratio was observed in the basal epithelium and the lowest in the superficial epithelium. The highest N:C ratio was found in the basal epithelial cells and the lowest in the posterior keratocytes.

CONCLUSIONS

We are the first to quantify the cell density and size parameters, including cell surface area and volume, cell nuclear surface area and volume, and the S:V ratio, in the five layers of the central cornea. These data provide important cell morphology features for the study of corneal physiology, pathology and disease in mice, particularly in C57BL/6 mice.

Mean central corneal thickness and corneal power measurements in pigmented and white rabbits using Visante optical coherence tomography and ATLAS corneal topography

OBJECTIVE

To document central corneal thickness (CCT) and power measurements in pigmented and white rabbits using Visante anterior segment optical coherence tomography (AS-OCT) and the Humphrey ATLAS Corneal Topography System (HACTS).

ANIMAL STUDIED

Fourteen female rabbits (seven pigmented and seven white rabbits) were involved in this study.

PROCEDURE

Twenty-eight eyes underwent AS-OCT and HACTS examination. Central corneal thickness, corneal power in the steepest and flattest meridians, astigmatism, pupil diameter (PD), and white-to-white (WTW) were calculated.

RESULTS

The CCT was 390 ± 14.2 and 373 ± 7.2 μm for pigmented and white rabbits, respectively. The CCT values showed statistical difference between the two breeds (P = 0.017). The corneal power in the steepest meridian was 44.6 ± 1.9 Diopter (D) in pigmented rabbits and 47.8 ± 1.5 D in white rabbits. The corneal power in the flattest meridian was 44.0 ± 2.1 D in pigmented rabbits and 47.4 ± 1.5 D in white rabbits. The astigmatism, PD, and WTW values were 0.6 ± 0.22 D, 7.9 ± 0.9, and 14.5 ± 0.1 mm in pigmented rabbits, respectively. The corresponding values in white rabbits were 0.4 ± 0.23 D, 7.5 ± 0.5, and 13.5 ± 0.2 mm. There was a statistically significant difference in corneal power in the steepest and flattest meridians and for WTW. No such difference was observed for astigmatism or PD between them.

CONCLUSIONS

Differences exist in the CCT, corneal power, and WTW between 5-month-old pigmented and white rabbits. Such difference should be considered when designing cornea-related experiments in rabbits.

Normal corneal thickness measurements in pigmented rabbits using spectral-domain anterior segment optical coherence tomography

OBJECTIVE

To document the thickness of the central cornea in pigmented rabbits using spectral-domain anterior segment optical coherence tomography (AS-OCT).

ANIMALS STUDIED

Seventeen pigmented rabbits (6 male, 11 female, both eyes) were involved in this study.

PROCEDURES

Thirty-four eyes from healthy pigmented rabbits underwent a complete ophthalmologic examination, including AS-OCT. Eight radial scans, 6 mm in length and centered on the cornea, were obtained using the AS-OCT. Corneal thickness was automatically calculated using pachymetry software. Measurements were displayed as the mean and standard deviation for each of the 17 regions defined by the software. The 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 eight 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

There was no statistically significance difference in corneal thickness between gender, eye, and the eight directions (P = 0.804, P > 0.05, P > 0.05). There was a statistical difference between the thickness in areas a, b, and c (P < 0.05). The corneal thickness increased gradually from the center to the periphery of the 6 mm measured. The center corneal thickness was 387 ± 19.8 μm for OD and 384 ± 20.2 μm for OS. The corneal thicknesses of the thinnest point of the right eyes (OD) and the left eyes (OS) were 376 ± 20.2 and 370 ± 16.8 μm, respectively. There was positive correlation between the thinnest point and area a in both the right eyes (r = 0.892, P < 0.001) and the left eyes (r = 0.832, P < 0.001).

CONCLUSIONS

This is the first documentation of the rabbit corneal thickness in vivo using the spectral-domain AS-OCT. Pigmented rabbit corneas were almost 150 μm thinner than human corneal values. Gender and eye were not associated with any statistical differences in central corneal thickness in this study.

Pre-corneal tear film thickness in humans measured with a novel technique

PURPOSE

The purpose of this work was to gather preliminary data in normals and dry eye subjects, using a new, non-invasive imaging platform to measure the thickness of pre-corneal tear film.

METHODS

Human subjects were screened for dry eye and classified as dry or normal. Tear film thickness over the inferior paracentral cornea was measured using laser illumination and a complementary metal-oxide-semiconductor (CMOS) camera. A previously developed mathematical model was used to calculate the thickness of the tear film by applying the principle of spatial auto-correlation function (ACF).

RESULTS

Mean tear film thickness values (±SD) were 3.05 μm (0.20) and 2.48 μm (0.32) on the initial visit for normals (n=18) and dry eye subjects (n=22), respectively, and were significantly different (p<0.001, 2-sample t-test). Repeatability was good between visit 1 and 2 for normals (intraclass correlation coefficient [ICC]=0.935) and dry eye subjects (ICC=0.950). Tear film thickness increased above baseline for the dry eye subjects following viscous drop instillation and remained significantly elevated for up to approximately 32 min (n=20; p<0.05 until 32 min; general linear mixed model and Dunnett's tests).

CONCLUSIONS

This technique for imaging the ocular surface appears to provide tear thickness values in agreement with other non-invasive methods. Moreover, the technique can differentiate between normal and dry eye patient types.