Swelling rate of corneal stroma

Collagen fibril characteristics at the corneo‐scleral boundary and rabbit corneal stromal swelling

BACKGROUND

The aim of this investigation was to reassess the impact of the scleral rim on the swelling of the mammalian corneal stroma and to investigate the ultrastructural features of the scleral rim and corneal stromal tissues.

METHODS

The epithelium and endothelium were removed from corneas excised from three-month-old female rabbits. The resulting preparations consisted of the corneal stroma plus a surrounding scleral rim, excised corneal stroma or a nine-millimetre button of central corneal stroma. These preparations were immersed in a 35 mM bicarbonate-buffered mixed salt solution (equilibrated with five per cent CO2-air, pH 7.54 at 37 degrees Centigrade for nine hours. Some sclero-corneal preparations were fixed for light or transmission electron microscopy.

RESULTS

The initial rate of swelling of corneal stromal buttons was greatest at 127 +/- 8 per cent per hour, less for complete stromal preparations (118 +/- 9 per cent per hour) and least for sclero-stromal preparations (76 +/- 12 per cent per hour). The swelling continued over three to nine hours but sclera preparations swelled up to 40 per cent with no further swelling. Light microscopy demonstrated that the sclero-corneal rim tissue limited the swelling of the posterior corneal stroma. TEM sections of the episclera and sclera indicate that most fibril bundles show a radial orientation to the cornea. There are marked anterio-posterior differences in the collagen fibrils of the scleral surround that are distinctly different from previous reports. Average fibril diameters were 62.8 +/- 7.9 nm in the episclera, 122.4 +/- 18.9 nm, 133.5 +/- 51.9 nm and 56.5 +/- 11.2 nm in the anterior, mid- and posterior scleral stroma, compared to an average fibril diameter of 33.5 +/- 3.5 nm for the posterior corneal stroma.

CONCLUSIONS

When there is a scleral rim in place, the swelling of the corneal stroma is substantially less than for isolated corneal stroma. The effect can be attributed to the absence of a cut-edge effect for the sclero-corneal stromal preparation but the unique and largely radial arrangement of the collagen fibrils in the scleral rim plays a part in limiting the swelling of the adjacent corneal stroma. The heterogeneous nature of this sclero-corneal interface requires further investigation to define the mechanism of the effect.

Small Incision Femtosecond Laser-assisted X-ray-irradiated Corneal Intrastromal Xenotransplantation in Rhesus Monkeys: A Preliminary Study

BACKGROUND

Gamma-ray irradiation could significantly induce widespread apoptosis in corneas and reduced the allogenicity of donor cornea. And the X-rays may have similar biological effects. The feasibility and effects of X-ray-irradiated corneal lamellae have not been assessed yet.

METHODS

Different doses (10 gray unit (Gy), 20 Gy, 50 Gy, 100 Gy) of X-ray irradiated corneal lamellae were collected from SMILE surgery. These corneal lamellae were assessed by physical characterization, hematoxylin and eosin (H-E) staining, Masson's staining, TdT-mediated dUTP nick end labeling (TUNEL), cell viability assay and transmission electron microscopy (TEM). We selected the optimum dose (100Gy) to treat the corneal lamellae to be the grafts. The human grafts and fresh allogeneic monkey corneal lamellae were implanted into rhesus monkeys via the small incision femtosecond laser-assisted surgery, respectively. Clinical examinations and the immunostaining were performed after surgery.

RESULTS

There were no significant changes in the transparency of the corneal lamellae, but the absorbency of the corneal lamellae was increased. According to the H-E and Masson's staining results, irradiation had little impact on the corneal collagen. The TUNEL assay and cell viability assay results showed that 100Gy X-ray irradiation resulted in complete apoptosis in the corneal lamellae, which was also confirmed by TEM observations. In the following animal model study, no immune reactions or severe inflammatory responses occurred, and the host corneas maintained transparency for 24 weeks of observation. And the expression of CD4 and CD8 were negative in the all host corneas.

CONCLUSION

X-ray irradiated corneal lamellae could serve as a potential material for xenogeneic inlay, and the small incision femtosecond laser-assisted implantation has the potential to become a new corneal transplantation surgical approach.

[Post-laser in situ keratomileusis refractive changes induced by glaucoma in the absence of keratectasia]

CASE REPORT

We report the case of a 34 year-old man with progressive loss of visual acuity in his left eye, who was diagnosed with pigmentary glaucoma with an intraocular pressure of 32mmHg. Eight years previously, the patient had undergone refractive surgery (LASIK). Hypotensive treatment led to a significant refractive change accompanied by flattening of the corneal curvature.

DISCUSSION

We propose that a high intraocular pressure acting on a cornea weakened by refractive surgery can provoke corneal steepening, inducing refractive changes even in the absence of keratectasia.

Swelling pressure and hydration behavior of porcine corneal stroma

PURPOSE

The aim of this study was to characterize swelling pressure-thickness, swelling pressure-hydration and hydration-thickness relations of porcine cornea.

METHODS

Mechanical compression tests and free swelling experiments were performed on porcine cornea. A rheometer (DHR-2, TA Instruments) with a thermally controlled fluid chamber filled with 0.9% NaCl solution was used to measure the equilibrium swelling pressure of (n = 17) corneal stromal specimens. The samples were compressed incrementally and their swelling pressure-thickness relations were obtained. In parallel to this investigation, a transient digital imaging microscope (H800-CL, American Scope Inc.), a USB autofocus camera (UM05, ViTiny), and a precision weighing scale (AGZN100, Torbal) were simultaneously used to measure the weight-thickness relation of (n = 8) corneal specimens. This experimental study gave the thickness-hydration relationship required for expressing swelling pressure measurements as a function of hydration.

RESULTS

At the in vivo 666 ± 68 µm central corneal thickness, an average swelling pressure of 52 ± 13 mmHg and hydration of 3.36 ± 0.25 mg H2O/mg dry tissue were found. The swelling pressure was reported as functions of both tissue thickness and hydration. The average fixed charge density of ρF/F ~ 42.8 mM and dry density of 1.47±0.15 g/cm3 were found. The thickness-hydration relationship was only linear when the tissue thickness was within the range of physiological thickness.

CONCLUSION

Overall, the physiological hydration and swelling pressure of the porcine cornea were within the same range of those reported previously for other mammalian corneas such as steers, rabbits and humans. Nevertheless, the thickness-hydration behavior of the porcine cornea was only similar to that of the human cornea.

A physiological perspective on the swelling properties of the mammalian corneal stroma

PURPOSE

The present studies were designed to assess whether measurement of corneal stroma swelling in the laboratory, especially in non-physiological solutions, was associated with a measurable effect on the keratocytes.

METHODS

Complete corneal stroma preparations were made from quality- and age-selected recent post-mortem cattle eyes. These were either assessed immediately or incubated in three different solutions, namely a balanced salts solution with glucose (BSSG), isotonic phosphate buffered saline (PBS) or pure water. Incubations were carried out at 37 degrees C for 9h, and repeated measures of wet mass made so that the rates and extent of swelling could be determined. After incubation, an aqueous extract was made of the stroma for measurements of the levels the enzymes lactate dehydrogenase (LDH), aldehyde dehydrogenase (ALDH) and N-acetyl-beta-glucosaminidase.

RESULTS

The initial rates of swelling were lowest in BSSG, marginally faster in PBS and much faster in water. The secondary rates of swelling showed the same sequence being 10.0%/h in BSSG, 14.8%/h in PBS and 34.2%/h in water. Compared to non-incubated preparations, reductions in all three enzyme activities occurred. For LDH, these were 15% with BSSG, 40% in PBS and 80% with water. Similar results were seen with ALDH activity when comparing the three incubation solutions, while incubation in BSSG also resulted in a substantial (40%) reduction in N-acetyl-glucosaminidase activity.

CONCLUSIONS

When immersed in an isotonic BSSG with added glucose at 37 degrees C, the swelling of a complete bovine corneal stroma is much less than smaller pieces of stroma, and also slightly less than if isotonic PBS was used. With the use of BSSG, little or no change in cytoplasmic enzyme activities occurred, but measurable decreases were noted with PBS and very substantial decreases when water was used, indicating a toxic effect on the keratocytes. The observation that substantial decreases in a lysosomal enzyme activity could occur even with the use of BSSG indicate substantial stress is imposed on the stroma during these types of experiments. Notwithstanding, the data collectively indicate that the keratocyte cells within the collagen matrix of the stroma can be substantially damaged and this needs to be taken into account in future experiments on the true physiology of the corneal stroma.

Swelling of the collagen-keratocyte matrix of the bovine corneal stroma ex vivo in various solutions and its relationship to tissue thickness

AIM

The mammalian corneal stroma, like some other connective tissues, can absorb fluid, swell and become oedematous. Since studies on the corneal stroma have been carried out with different types of preparations and solutions, inter-study comparisons are very difficult. A study was thus undertaken on a standardised preparation to assess the relative magnitude of this swelling and its relationship to thickness of the preparations.

METHODS

From selected recent post-mortem eyes of adult cattle, stroma preparations were cut from the central part of the cornea. These preparations were immersed in various solutions of known pH and osmolality, and the time-dependent changes in wet mass were assessed over 9 h at 37 degrees C. The relative rates and magnitude of the swelling of the tissue were then compared.

RESULTS

A reference value for stromal swelling was obtained by incubation in a 35 mM bicarbonate-buffered mixed salts solution equilibrated with 5% CO2-air (pH 7.60) where a 3.39-fold increase in wet mass and a 4.58-fold increase in thickness was realised in 9 h, at an initial rate of 76 +/- 3%/h. The swelling was essentially the same in an organic buffer-mixed salt solution (pH 7.5) but progressively greater in phosphate-buffered saline (pH 7.5), a range of phosphate buffers (10-67 mM, pH 7.5), NaCl solutions (0.025-1%) and with gross swelling observed in water (where a 15.9-fold increase in wet mass occurred along with a 25-fold increase in thickness, at an initial rate of 643 +/- 62%/h). Overall, the wet mass changes were strongly related to thickness (P < 0.001).

CONCLUSIONS

The results confirm that the selection of solution(s) for studies on corneal stromal swelling is critical. The swelling (oedema) is lower in a physiologically-relevant solution (similar to the aqueous humour of the eye). This indicates that the swelling tendency of the corneal stroma has been overestimated in the past, and that a similar discrepancy may also exist for studies on other connective tissues ex vivo when non-physiological experimental solutions are used.

In vitro hydration kinetics of recent post-mortem tissue versus pre-dried corneal stromal tissue

Both recent post-mortem and pre-dried corneal tissue has been used for laboratory studies of stromal swelling, but it has yet to be defined whether the same hydration (H value, mg H2O/mg dry mass) is obtained after extended re-hydration. Fresh or pre-dried pieces (8x8 mm squares) of ovine stromas were immersed in 1% NaCl for 24 hr at 37 degrees C, with the wet mass assessed regularly. Pre-drying was achieved in air for 7 days (with sulphuric acid, CaSO4 or silica gel as desiccants), or in an oven for 24 hr at 60, 70 or 80 degrees C. Fresh stroma preparations (</=2 hr post-mortem) had initial H values of 3.1, which increased to 27.8 after 24 hr in saline. After pre-drying over sulphuric acid, CaSO4 or silica gel, the H values after 24 hr in saline were 19.1, 13.1 and 7.3 respectively (all statistically different from fresh tissue (P<0.01). Following oven drying at 60, 70, and 80 degrees C, final H values of 8.5, 6.6, and 5.0 were obtained after 24 hr in saline. Recent post-mortem material showed sustained swelling over at least 24 hr. For pre-dried tissue, initial rates of hydration change over 1 hr were higher, but subsequent rates were substantially less after 2 hr. These studies indicate that fresh tissue should be used in studies concerning the swelling properties of the corneal stroma.

Chloride binding in the stroma of cultured human corneas

In the ox cornea, more than half of the non-diffusible, matrix negative charge is derived from the binding of free chloride ions. Because the magnitude of the net matrix charge is the dominant factor which determines the degree of stromal swelling, we investigated whether this phenomenon, stromal chloride binding, also occurs in human corneal stroma. Intrastromal ion concentrations were measured with radio-isotopes when human (outdated Eye Bank) corneas or (fresh) bovine corneas, physically clamped to maintain a constant hydration, were incubated in buffered 154 mM NaCl. The intrastromal chloride ion concentration was compared to the normalized concentrations of trace quantities of radio-labelled acetate and lactate ions. For human corneas, the intrastromal chloride ion concentration was found to be significantly higher (P < 0.001, t-test) than the normalized concentrations of both acetate and lactate ([Cl]i = 142.5 +/- 0.9 mM, (n = 9); [acetate]i = 131.2 +/- 1.2 mM, (n = 8); [lactate]i = 131.9 +/- 1.5 mM, (n = 5); all values are mean +/- S.E.M.). The sodium ion concentration was elevated ([Na]i = 176.0 +/- 1.8 mM, (n = 9)). These results demonstrate that chloride binding occurs to a significant extent in cultured human corneal stroma and suggest that chloride binding may be evident in the native human cornea.

Mussel adhesive protein: permeability characteristics when used as a basement membrane

Mussel Adhesive Protein (MAP) was applied to either denuded rabbit cornea, or dialysis membrane with a 50,000 dalton molecular weight cut-off. The permeability of corneal endothelium or bare stroma was compared with demambranized stroma coated on one surface with MAP. The permeability of dialysis membrane, both in the presence and absence of MAP, was also determined to inulin (5,000 daltons) and dextran (40,000 daltons). Corneal endothelial permeability was much less than that of bare stroma, and coating denuded stroma with MAP on one surface had no effect on stromal permeability. Dialysis membrane permeability was not reduced after coating with different thickness of MAP, indicating that MAP offers little resistance to the passage of even large non-electrolytes and can serve as a basement membrane for cellular or tissue attachment without impairment of nutritional supply to the overlying cells.

Effect of dextran-bound acetazolamide on rabbit corneal endothelial ion fluxes

The effects of a carbonic anhydrase inhibitor, acetazolamide, bound to a 72,000 dalton dextran (DBI) on bicarbonate and sodium fluxes across the isolated rabbit corneal endothelium have been examined. When DBI was present on the aqueous-facing endothelial surface, there was a marked inhibition of both the stromal to endothelial unidirectional and net flux of bicarbonate. This suggests that the exit of bicarbonate from the cell into the aqueous-facing solution is influenced by the enzyme carbonic anhydrase. No change was found in sodium fluxes under these same incubation conditions. When DBI was present on the stromal-facing surface of the endothelium, no changes were found in unidirectional or net bicarbonate fluxes; the sodium flux from stroma to endothelium was increased, however, with no change in net flux. This data implies that the link between sodium and bicarbonate movement across the endothelium is not a direct coupling between the transport of the two ions in the form of a symport (Na+:HCO3-) at the apical cell border.

Swelling studies of bovine corneal stroma without bounding membranes

1. The swelling characteristics of demembranated bovine corneal stroma were studied as a function of time and of the pH and ionic strength of the bathing solution. 2. Compared with other pH values, the stroma swelled least near pH 4. 3. In the pH range 6--10, increasing the pH resulted in an increase both in the rate of swelling and in the hydration reached in a given time. 4. At pH 2 and 4, a final constant value of hydration was attained. At higher pH values no such equilibrium was attained when the hydration of the tissue was followed for at least 100 hr. 5. The swelling at high pH values was consistent with the hypothesis that the Donnan-osmotic contribution is the major component of the swelling pressure. 6. The ionic strength dependence was complex. There was a general decrease of swelling with increase in the ionic strength (mu) until around mu = 0.1. The swelling at mu = 0.15 was greater than at mu = 0.1 and mu = 0.25. 7. The results were interpreted on the assumption that the Donnan-osmotic effect is the major component of the swelling pressure.