Comparison of Morphology of Bulbar Conjunctival Cells Assessed by Impression Cytology versus Scrape and Smear Methods

Impression cytology of the healthy equine ocular surface: Inter-observer agreement, filter preservation over time and comparison with the cytobrush technique

BACKGROUND

The cytobrush technique is commonly used to sample the equine ocular surface. Impression cytology (IC) is an innovative noninvasive method, which allows for the collection of superficial layers of ocular epithelium.

OBJECTIVES

The aims of this study were to compare the cytobrush and IC techniques on healthy equine ocular surfaces, to assess the agreement between observers with different levels of expertise, and to test the preservability of filters over time.

METHODS

Twenty-four horses were sampled within 10 minutes of slaughter using IC on the left eye and the cytobrush technique on the right eye. May-Grünwald-Giemsa stained specimens were evaluated by two observers with different levels of expertise. Morphologic features were evaluated using a 4-grade system. The IC samples were re-evaluated after 6 months to examine filter preservation.

RESULTS

In IC samples, corneal and conjunctival cells were clearly separated. Goblet cells were found in five and 17 filters by observer 1 and 2, respectively. Using the cytobrush technique, corneal and conjunctival cells were present but mixed. Goblet cell cellularity, preservation, and enumeration were higher with the IC technique compared with the cytobrush technique (P = 0.013; P = 0.004; P = 0.031, respectively). The inter-observer agreement for the IC technique was moderate to fair. In 7/24 IC samples re-evaluated after 6 months, cellular morphology was impaired, and the overall score was significantly lower.

CONCLUSIONS

IC is an innovative noninvasive method, which allows for sample collection with higher cellularity and preservation. Moreover, the identification of goblet cells is easier. For these reasons, IC could be interesting and useful as a complementary diagnostic cytologic method in clinical practice.

Corneal Surface and Superficial Cells as Viewed by Scanning Electron Microscopy and Impression Cytology Sampling

PURPOSE

To objectively compare sizes of corneal epithelial surface cells visualized by scanning electron microscopy (SEM) or collected by impression cytology (IC).

METHODS

Corneas of recent postmortem sheep eyes were either glutaraldehyde fixed for SEM or IC samples taken, glutaraldehyde fixed, and then stained with Giemsa. The corneal epithelial surface was examined at ×200, with the borders of all cells marked and dimensions and areas measured. For SEM, the cells were classified as having light, medium, or dark electron reflex and by the number of cell sides.

RESULTS

SEM revealed a mosaic of polygonal features with different electron reflexes according to the density of cell surface microplicae. The cell area and longest dimension were predictably dependent on the number of cell sides. Compared with the overall average cell area of 1598 μm(2) (range 200-3900 μm(2)), the dark reflex cells were predominantly the largest, with an average area of 2435 μm(2) and having the average longest dimension of 59.9 μm(2). Cells collected by IC had a range of areas from 1150 to 4800 μm(2) but were predominantly large and had an average area of 2802 μm(2) with the average cell longest dimension averaging 64.4 μm(2). IC-sampled cells had nucleocytoplasmic ratio values consistent with their having a squamous phenotype.

CONCLUSIONS

Cells sampled by IC from the corneal epithelial surface are predominantly those that are enlarged (and flattened out) in preparation for desquamation, a feature also attributed to the dark reflex cells seen in SEM.

Assessment of Consistency in Assignment of Severe (Grade 3) Squamous Metaplasia to Human Bulbar Conjunctiva Impression Cytology Cell Samples

Nucleus-to-cytoplasm (N:C) characteristics of published images of human bulbar conjunctival cells designated as showing severe (grade 3) squamous metaplasia were retrospectively assessed. From publications over a 40-year period, measurements were made from images of cell and nucleus size (based on areas and dimensions) and four different calculations were made for nucleo-cytoplasmic (N:C) ratios. From 54 published images, the mean nucleus-to-cytoplasm area ratio (NU/CYT AREA ratio) was 0.145 +/- 0.077 (range 0.052 to 0.346), compared to two different reference set values of 0.069 +/- 0.017 and 0.080 +/- 0.021. Similarly, a nucleus-to-cytoplasm length ratio (as LNLONG) was 0.308 +/- 0.080 (range 0.191 to 0.475) compared to reference values of 0.226 +/- 0.032 and 0.236 +/- 0.034. Similar differences in reference values were obtained using two other N:C ratio calculations. A wide range of values was found for morphometric N:C indices from published images, especially those without a scale bar, indicating a high incidence of inconsistent grading assignments. Overall, only about 30% of the published images showed morphological features consistent with severe squamous metaplasia (i.e., with the rest neither enlarged nor showing substantially changed N:C ratios) with no substantial pictorial evidence indicating that cells from the human bulbar conjunctiva have pyknotic nuclei. Current evidence indicates that grade 3 squamous metaplasia cells should be substantially enlarged.

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.