Assessment of corneal endothelial cell parameters using specular microscopy in previously infected SARS-CoV-2 patients

PURPOSE

To examine the long-term effects of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection on the corneal endothelium.

METHODS

This was a comparative, cross-sectional study that included subjects who had recovered from SARS-CoV-2 infection for at least 6 months (group 1) and a group of age- and sex-matched controls with no prior symptomatology or documentation of SARS-CoV-2 infection (group 2). After full ophthalmological evaluation, specular microscopy was used to examine the endothelial cell parameters, including endothelial cell density, coefficient of variation, hexagonality, average area, and central corneal thickness.

RESULTS

Sixty-four and 53 right eyes were included in groups 1 and 2, respectively. No statistically significant differences were detected in any of the examined specular parameters between the two groups.

CONCLUSION

SARS-CoV-2 infection may have no delayed sequel on the corneal endothelium. Future prospective studies with repeated examinations in the same subjects would be useful.

Assessment of the reliability (repeatability) of corneal thickness measurements in soft contact lens wearers using a non-contact specular microscope

PURPOSE

To assess variability across 3 measures of central corneal thickness (CCT) obtained with a non-contact specular microscope and taken over a few minutes from habitual soft contact lens wearers.

METHODS

One eye from 200 healthy adults (with an average age of 21 y, half of whom had a 3.5 ± 2.1 year history of successful daily wear of soft contact lenses while the control group had nominally normal eyes) were assessed using the auto-focus Topcon 2000P instrument to obtain an image of the endothelium and CCT.

RESULTS

The individual CCT values encountered in the 200 subjects ranged from 0.449 mm to 0.591 mm, with the average of 3 measures ranging from 0.459 to 0.591 mm in the control group and between 0.449 and 0.585 mm for the SCL wearers. The group mean CCT values were the same for both groups (at 0.524 mm), but the group mean SD value was marginally higher (at 0.028 mm) for the SCL group as compared to controls (SD = 0.026 mm). The normalized intra-subject variability (as the group-mean coefficient of variation, COV value) was 0.843 ± 0.401 for the control group and higher at 1.08 ± 0.546 for the SCL group (p < 0.001).

CONCLUSIONS

Repeat measures of central corneal thickness, using a non-contact specular microscope, is very similar to those taken on age-matched non-contact lens wearers. These results may not equally apply to similar pachymetry measures in patients wearing RGP lenses or for older patients wearing soft contact lenses.

Multiple count sampling of goblet cells in microscope high-power fields using conjunctival impression cytology

BACKGROUND

Published studies indicate that assessments of goblet cell density using conjunctival impression cytology has provided very variable results, but the reasons for this are unclear. Systematic analyses of the sources of variability are required.

METHODS

From 20 healthy young adults, conjunctival impression cytology specimens were obtained using a supported filter unit applied to the superior bulbar conjunctiva. The filters were stained with Giemsa and 10 non-overlapping, randomly selected high-power field images were obtained from each specimen and the numbers of goblet cells per high-power field counted.

RESULTS

From all 200 high-power fields assessed, the numbers of goblet cells ranged from zero to 74, with an overall mean value of 11.6 ± 14.8 per high-power field. From each successive set of 10 microscope field images from all individuals, the average number of goblet cells ranged from 23.2 in the first high-power field that obviously included numerous goblet cells down to 6.2 per high-power field. As the outcome from multiple counts/individual was systematically increased, these averages progressively decreased from 23.2 to 11.6 per high-power field, and while the standard deviation values also progressively declined (from 7.9 to 5.5 per high-power field), the relative variability (as the co-efficient of variation) did not, and increased to averaged values of over 100 per cent.

CONCLUSIONS

These analyses indicate that there is a benefit of making multiple counts of goblet cells from different high-power fields, but that there is no obvious benefit of using more than five to seven high-power fields for any particular specimen.

Paracrine Interactions between the Conjunctival and Corneal Epithelial Cells Regulate Microenvironmental Homeostasis during Artificially Induced Inflammation

Purpose/Aim of the study: The corneal and conjunctival epithelium interact with each other and reciprocally modulate the levels of soluble mediators to maintain balance in the ocular surface. The aim of the present study was to analyze paracrine interactions between the corneal and conjunctival epithelium in an inflamed microenvironment (LPS or PMA induction) to test the levels of pro- and anti-inflammatory cytokines and nitric oxide released by the epithelia.

MATERIALS AND METHODS

The corneal (pRSV-T) and conjunctival (HC0597) epithelial cell cultures and their indirect co-cultures were treated for 2 h with LPS (E. coli) or for 30 min with phorbol 12-myristate-13-acetate (PMA) to induce inflammation. Cytokine expression (IL-1β, IL-6, IL-10) and the level of apoptosis were analyzed by ELISA, and the nitric oxide (NO) level by Griess reaction. Cells were incubated for 24 h.

RESULTS

The apoptosis of the corneal and conjunctival epithelia decreased (by 43% and 53%, respectively) in co-cultures compared to corresponding monocultures. The conjunctival epithelium produced lower amounts (23%) of NO than the corneal epithelium. PMA and LPS had comparable effects on the levels of NO in mono- and co-cultures. The levels of the tested cytokines changed depending on the type of cell culture and culture conditions (mono- vs. co-cultures and inflammation). The most striking changes were observed for IL-6 expression.

CONCLUSIONS

Paracrine interactions between the corneal and conjunctival epithelia may regulate microenvironmental homeostasis during artificially induced inflammation among others by balancing the levels of NO, cytokines, and the viability of cells.

On the regional variability of averaged cell area estimates for the human corneal endothelium in relation to the extent of polymegethism

Purpose

To assess variability in the coefficient of variation (COV) in cell area estimates when using different numbers of cells for endothelial morphometry.

Methods

Using non-contact specular microscopy images of the corneal endothelium, 4 sets of 20 cases were selected that included 200 cells and had overall (global) COV values of less than 30 (group 1), 31–40 (group 2), 41–50 (group 3) and over 50% (group 4). Subjects could be normal, or had ophthalmic disease (such as diabetes), a history of rigid or soft contact lens wear or were assessed after cataract surgery. A step-wise analysis was undertaken, 20 cells at a time, of the variability in cell area estimates when using different numbers of cells for the calculations.

Results

Variability in the average cell area values was higher if only 20–60 cells were used in the calculations and then tended to decrease. The standard deviation values on these average cell area values and the calculated COV showed the same overall trends and were more than twice as large for endothelia with marked polymegethism. Using more than 100 cells/image in markedly polymegethous endothelia only increased the variability in the calculations.

Conclusions

These analyses indicate that substantial region variability in cell area values can be expected in polymegethous endothelia. The analysis further confirm that using only small numbers of cells (e.g. less than 50/image) in such cases is likely to yield far less reliable estimates of COV.

On the Variability in Goblet Cell Density in Human Bulbar Conjunctival Samples Collected by Impression Cytology with Millicell-CM Biopore Membrane Units

PURPOSE

To assess whether or not the numbers of goblet cells were different across regions of a conjunctival impression cytology (CIC) specimen.

METHODS

CIC specimens were obtained from the exposed nasal bulbar conjunctiva from 22 healthy young Caucasian adults, stained with Giemsa and evaluated at a final magnification of 200× (medium power fields). Up to 14 different non-overlapping microscope fields, depending on whether or not a field was without goblet cells, were randomly selected.

RESULTS

A microscope field could contain between 0 and 145 goblet cells. If only fields that included goblet cells were analyzed, the mean goblet cell counts (22 specimens, 10 fields/specimen) ranged from 37.8 to 44.8/field (equivalent to 259-313 goblet cells/mm²). When 3, 5 or 10 fields were used to calculate average goblet cell counts for each specimen, the standard deviation values were between 15.8/field and 21.7/field, with the lower values obtained if 10 fields were used. The resultant averaged inter-sample variability, as the coefficient of variation (COV), ranged from 40.0% to 57.5%, while the averaged intra-sample variability in counts ranged from 52.1% to 73.9%. If fields without goblet cells were also used, the resultant mean goblet cell count (from 10 fields/specimen) was statistically lower at 33.1/field (232 goblet cells/mm²).

CONCLUSIONS

These analyses confirm various subjective comments made by early CIC investigators; the distribution of goblet cells across a CIC filter may be highly variable.

Assessment of size and nucleo-cytoplasmic characteristics of the squamous cells of the corneal epithelium

PURPOSE

The aim was to objectively assess size, nucleus and nucleo-cytoplasmic ratio features of squamous cells from the corneal epithelium

METHODS

The corneas of recent post-mortem sheep eyes were either glutaraldehyde-fixed for transmission electron microscopy or impression cytology samples taken, glutaraldehyde-fixed and stained with Giemsa. From the specimens for impression cytology, a representative region was photographed from 12 different samples taken from the central region and 16 different samples taken from mid-peripheral regions of the corneal epithelium. Images were subjected to morphometry after overlays were generated.

RESULTS

Electron microscopy revealed a very distinctive stratified corneal epithelium with several superficial layers, confirming the squamous phenotype. Impression cytology from such superficial layers revealed a cell size of 60.1 ± 4.8 μm, nucleus dimension of 12.3 ± 1.5 μm, cell area of 2,419 ± 416 μm(2) and nucleus area of 131 ± 31 μm(2) . A nucleo-cytoplasmic ratio based on nucleus-to-cell length had a mean of 0.207 ± 0.022, while a cytoplasm-to-nucleus length ratio was 3.975 ± 0.474. Estimates of the nucleo-cytoplasmic ratio based on areas had a mean value of 0.059 ± 0.011. Very similar results were found for mid-peripheral corneal epithelium.

CONCLUSIONS

The results strongly indicate that the squamous phenotype of the superficial corneal epithelial cells is characterised by a large size, large nucleus and low nucleo-cytoplasmic ratio. These morphological characteristics show a notable resemblance to data obtained from impression cytological studies on human conjunctival epithelial cells showing severe squamous metaplasia.

Averaged spacing and 2-d organization of collagen fibrils in the posterior cornea of the rabbit eye assessed by transmission electron microscopy

PURPOSE

To assess how reproducible collagen fibril spacing might be in the corneal stroma as viewed by transmission electron microscopy by calculating averaged values for the 2-D organization.

METHODS

One cornea from 8 albino rabbits (2 kg) was fixed in situ to preserve natural shape. Thin sections were stained with 2% phosphotungstic acid (PTA) and images taken of fibrils from the central-posterior stroma. After projection at 250,000 X magnification, an overlay was prepared of the fibrils. Using a 500 × 500 nm region of interest (ROI), the distances to all fibril centers were measured to 2 nm resolution.

RESULTS

The sets of fibrils had average diameters between 32.4 and 36.1 nm (group mean ± SD of 34.4 ± 4.2 nm). The mean fibril density was 396 ± 21 per square micrometer, with a fibril area fraction of 38.7 ± 3.9%. The mean distance to the literal nearest neighbor fibril center was 43.2 ± 4.5 nm. A radial distribution analysis showed a distinct nearest neighbors peak at 51 nm. This nearest neighbors peak had an average amplitude of 2.236 ± 0.315, with a broader secondary peak being evident in all data sets centered at 93 nm with an average amplitude of 1.166 ± 0.093 (or 53.3 ± 7.1% of the nearest neighbors peak).

CONCLUSIONS

Overall, these results show that a predictable 2-D organization can be demonstrated for collagen fibrils in rabbit corneas when consideration is given to sample selection and preparation and the image analysis strategy.

Assessment of goblet cell orifice distribution across the rabbit bulbar conjunctiva based on numerical density and nearest neighbors analysis

PURPOSE

To assess density and spatial distribution of the goblet cell orifices at the surface of the rabbit bulbar conjunctiva as an indicator of functional activity.

METHODS

Specimens of the superior or inferior bulbar conjunctiva from six healthy young adult (2 kg) pigmented rabbits were obtained using a special preparation technique by which the conjunctiva was carefully stretched out during fixation with buffered glutaraldehyde. The apical surface of the specimens was examined by scanning electron microscopy. From high magnification prints, the areas and dimensions of 32-49 orifices/image were measured. In addition, the centre-to-centre spacing and spatial distribution of the orifices were assessed using a nearest neighbors principle.

RESULTS

The bulbar conjunctival surface is composed of polygonal cells decorated with surface microplicae and in between which are individual goblet cell orifices. The goblet cell orifices are characterized by tending to be oval in shape (long:short dimensions ratio of 1.57 +/- 0.23) and usually having a distinct line of microvilli around the perimeter. The orifices had a wide range of areas (from 13 to 188 μm(2); group mean +/- SD of 54 +/- 36 μm(2)), and distribution of orifice areas was skewed or even bimodal. The overall orifice density was 387 +/- 68/mm(2), with the group-averaged nearest neighbors distance being 34 +/- 3 μm. Comparisons of the measured nearest neighbors distances to that for an optimum spacing based on numerical density reveals the goblet cell orifices to be slightly further apart and that they were not obviously in groups or clustered.

CONCLUSIONS

Goblet cell orifices at the bulbar conjunctival surface, a presumed indicator of functional secretory activity, appear to have reproducible density and a discrete and reasonably predictable spatial distribution.

Goblet cells of the normal human bulbar conjunctiva and their assessment by impression cytology sampling

Goblet cells of the conjunctiva are the main source of mucus for the ocular surface. The objectives of this review are to consider the goblet cells as assessed by various histological, cytological and electron microscopy methods, and to assess the consistency of published reports (over more than 25 years) of goblet cell density (GCD) from impression cytology specimens from nominally healthy human subjects. Reported GCD values have been notably variable, with a range from 24 to 2226 cells/mm² for average values. Data analysis suggests that a high density of goblet cells should be expected for the healthy human conjunctiva, with a tendency toward higher values in samples taken from normally covered locations (inferior and superior bulbar conjunctiva) of the open eye (at 973 +/- 789 cells/ mm²) than in samples taken from exposed (interpalpebral) locations (at 427 +/- 376 cells/mm²). No obvious change in GCD was found with respect to age, perhaps because the variability of the data did not allow detection of any age-related decline in GCD. Analyses of published data from 33 other sources indicated a trend for GCD to be lower than normal across a spectrum of ocular surface diseases.