Graeme Wilson

Non-uniform swelling properties of the corneal stroma

The swelling properties of the anterior and posterior stroma of bovine and rabbit corneas were examined by three different methods. The results show that the posterior stroma can swell more than the anterior stroma. Also, at a given swelling pressure, the posterior stroma is more hydrated than the anterior stroma. In a third experiment it is demonstrated that the posterior stroma is also more hydrated in the living eye. It is concluded that the stroma is not a homogeneous structure. The superficial stroma has different physiological properties from the deep stroma. These differences must be taken into account in theoretical models of corneal deturgescence.

Spectral sensitivity in jumping spiders (Araneae, Salticidae)

Summary1.We report here a psychophysical technique for studying the spectral sensitivity of jumping spiders (family Salticidae), based on a newly discovered oculomotor reflex.2.Our results, obtained fromMaevia inclemens (Salticidae), are compatible with electrophysiological findings of retinal cells maximally sensitive in the green and ultraviolet regions of the spectrum.3.Sensitivity to longer wavelengths (>650 nm) has been controversial. In our study jumping spiders are shown to have a broad spectral sensitivity function extending from the ultraviolet (330 nm) to the deep red (700 nm).

Investigation of sloughed corneal epithelial cells collected by non-invasive irrigation of the corneal surface

This paper reports the development of a non-contact corneal irrigation chamber (NC-CIC) which enables non-invasive collection of epithelial cells from the corneal surface of human subjects. Cells were viewed by fluorescence microscopy following vital staining with acridine orange (AO). Staining characteristics revealed two corneal epithelial cell types: cells with (i) green and (ii) orange-red cytoplasmic staining. The green cytoplasmic stain appeared to indicate a more viable cell. Multi-cell aggregates were regularly collected from the corneal epithelial surface. Groups of up to seven epithelial cells were obtained. Quantitative studies of corneal epithelial cell sloughing, using isotonic NaCl (305 mOsm/kg) and isotonic basic tear solution (BTS, 305 mOsm/kg) as irrigating solutions, involving hourly irrigations between 8 a.m. and 4 p.m. were conducted. Consistently higher cell counts were obtained with NaCl. Using BTS, data scatter was reduced sufficiently to reveal significant differences in sloughing rate as a function of time of day. Instillation of one drop of 0.5% proparacaine caused a significant, but gradual, increase in epithelial cell sloughing rate over a period of hours, as indicated by subsequent BTS irrigations of the cornea. Since the NC-CIC technique is able to discriminate these effects, it may be an appropriate system for in vivo studies of the relationship between corneal epithelial cell mitosis and sloughing.

THE EFFECT OF ULTRAVIOLET-B IRRADIATION ON THE CELL SHEDDING RATE OF THE CORNEAL EPITHELIUM

Abstract. In this study cell shedding rates of the corneal epithelium were determined in vitro following a single suprathreshold dose of ultraviolet‐B irradiation. Rabbit corneas were excised and superfused in pairs. The epithelial and endothelial surfaces were bathed with solutions containing essential ions and adjusted to appropriate pH and osmolality. One cornea in each pair was irradiated, while the other cornea served as a control. Shed cells were periodically collected from the corneal surface and the shedding rate determined by counting the number of cells in a 50 min time period. Following a latent period of about 3 h, the epithelial shedding rate accelerated, and remained elevated for at least 8 h after irradiation. This result suggests that suprathreshold doses of ultraviolet‐B irradiation disrupt the normal orderly cell shedding process and homeostatic equilibrium of the corneal epithelium. The elevated cell shedding rate exposes subsurface nerve endings and causes the characteristic pain of photokeratitis.

Cell content of tears following overnight wear of a contact lens

The cell content of the precorneal tears of eight subjects was sampled following overnight wear of a contact lens in one eye. This was compared with morning samples taken from the other eye, and afternoon samples taken several days later from both eyes. In the morning the median number of desquamated epithelial cells in the precorneal tears was 62 cells in the lens-wearing eye and 119 cells in the control eye. The difference was not statistically significant. In the afternoon the medians were 18.5 cells in one eye and 7.5 cells in the other. There were relatively few leucocytes in the afternoon, the median was 2 leucocytes in one eye and 9 in the other. In the morning the median was 8,154 leucocytes in the lens-wearing eye and 6,500 in the control eye. The data show that large numbers of leucocytes are present on the ocular surface during sleep. There was no evidence that the number of leucocytes or the number of epithelial cells was altered by the presence of the contact lens.

The cell shedding rate of the corneal epithelium — a comparison of collection methods

PURPOSEnTo determine the spontaneous shedding rate of cells from the rabbit corneal epithelium using different harvesting methods.nnnMETHODSnCells were collected from the rabbit corneal epithelium using two in vitro methods and three in vivo methods. Cells were counted and the shedding rate calculated after adjustment for the collection time.nnnRESULTSnThe shedding rates obtained from the in vitro methods were (cells/min/cornea, mean +/- SE): 78.0 +/- 9.4 (corneal superfusion), and 5.4 +/- 1.3 (whole-eye perfusion). For the in vivo corneas, the shedding rates were: 10.0 +/- 2.3 (corneal superfusion), 8.1 +/- 0.4 (corneal immersion), and 14.5 +/- 1.5 (corneal irrigation). In vitro corneal superfusion was significantly different from the other four methods (P < 0.01).nnnCONCLUSIONSnThe results suggest that the spontaneous cell shedding rate of the in vivo rabbit corneal epithelium is 5 to 15 cells/min/cornea. This is much lower than estimates of about 100 cells/min/cornea based upon in vitro corneal superfusion. One explanation of this slow shedding rate is that factors which were absent during our collection methods (such as blinking) would normally increase shedding. Another possibility is that cells in the corneal epithelium may have a much longer life span than previously reported; rather than a few days, the epithelium could take several months to completely replace all cells. Whatever the explanation, the measured spontaneous shedding rate does not complement the reported production rate of new cells. It is necessary to revise our understanding of the kinetics of epithelial homeostasis.

The effect of hypoxia on the shedding rate of the corneal epithelium

The reason a cell leaves the corneal epithelial surface at a particular time is not understood. It is likely that a cell must depend on metabolic energy to accomplish this task successfully and with minimal disruption to the epithelial surface and barrier. The hypothesis under test is that the epithelium is directly dependent on atmospheric oxygen to maintain a normal cell shedding rate. Rabbit corneas were excised in pairs and the surfaces bathed with appropriate media for 400 minutes at 304 mOsm/kg and pH 7.4. The epithelial surface of one cornea was bathed with a normoxic solution as a control, while the other cornea was anoxic. Solutions were collected from the epithelial surface at 50 minute intervals. Cell counts of shed corneal epithelial cells were made by staining with acridine orange and viewing with fluorescence microscopy. Stromal thickness was measured at the beginning and at 400 minutes to confirm hypoxia. The results show that hypoxia does reduce the rate at which cells are shed, at least for collections made during the first 50 minutes (P < 0.01). However, by 150 minutes there was no difference between the hypoxic cornea and the control. This result suggests that cell shedding is dependent on oxygen only during the early part of a period of hypoxia. When the hypoxia is prolonged, other mechanisms intervene which preserve the normal shedding rate.

CHARACTERIZATION OF CELLS SHED FROM THE OCULAR SURFACE IN NORMAL EYES

The corneal and conjunctival epithelia constitute the ocular surface, which is a dynamic layer under constant renewal. As corneal epithelial cells migrate anteriorly and centripetally,1,2 the cells terminally differentiate and flatten. Cells reach the surface, tight junctions form between adjacent epithelial cells, and cells apically differentiate. Older surface cells, presumably the large, dark squamous cells viewed by scanning electron microscopy (SEM),3–5 slough into the tear film as new epithelial cells move anteriorly to replace them. The conjunctiva replaces itself similarly,6 although the cells may be more cuboidal7 or polyhedral8 in shape. The renewal process of the normal ocular surface can be studied by examination of cells shed from the surface.

The Size of Corneal Epithelial Cells Collected by Contact Lens Cytology from Dry Eyes

The success of impression cytology as a technique for collecting conjunctival cells1 suggested to us that a contact lens could be used to collect cells from the corneal surface2. This paper describes the size of corneal epithelial cells collected using contact lens cytology (CLC) from patients with dry eye.