Changes in hydration, protein and proteoglycan composition of the collagen-keratocyte matrix of the bovine corneal stroma ex vivo in a bicarbonate-mixed salts solution, compared to other solutions

The use of phospholipase A(2) to prepare acellular porcine corneal stroma as a tissue engineering scaffold

This study was to develop a method using phospholipase A(2) (PLA(2)) to prepare acellular porcine corneal stroma (APCS) for tissue engineering. The APCS was prepared from native porcine cornea (NPC) that was treated with 200 U/ml PLA(2) and 0.5% sodium deoxycholate (SD). The removal of DNA content, representing decellularization efficiency, reached to 91%, while all hydroxyproline and 80% of glycosaminoglycan were retained in the APCS when compared with NPC. The residual PLA(2) and SD were 0.35+/-0.04 U/mg dry weight and 4.3+/-0.8 ng/mg dry weight respectively. The extracts of APCS had no inhibitory effects on proliferation of corneal epithelial and endothelial cells as well as keratocytes. There was no sign of infiltration of neutrophilic leukocytes or leukomonocytes at 2 weeks after subcutaneous implantation of APCS. The prepared APCS displayed similar light transmittance to NPC. There were no significant differences in the areal modulus and curvature variation between APCS and NPC. Rabbit lamellar keratoplasty showed that the grafts of APCS were epithelialized completely in 8+/-2 days, and their transparency was restored in 84+/-11 days when the light transmittance of APCS-transplanted corneas displayed no significant difference compared with native corneas. Corneal neovascularization, corneal deformation, and graft degradation were not observed within 12 months.

Collagen fibril assembly during postnatal development and dysfunctional regulation in the lumican-deficient murine cornea

The transparent cornea is the outer barrier of the eye and is its major refractive surface. Development of a functional cornea requires a postnatal maturation phase involving development, growth and organization of the stromal extracellular matrix. Lumican, a leucine-rich proteoglycan, is implicated in regulating assembly of collagen fibrils and the highly organized extracellular matrix essential for corneal transparency. We investigated the regulatory role(s) of lumican in fibril assembly during postnatal corneal development using wild type (Lum+/+) and lumican-null (Lum-/-) mice. In Lum+/+ mice, a regular architecture of small-diameter fibrils is achieved in the anterior stroma by postnatal day 10 (P10), while the posterior stroma takes longer to reach this developmental maturity. Thus, the anterior and the posterior stroma follow distinct developmental timelines and may be under different regulatory mechanisms. In Lum-/- mice, it is the posterior stroma where abnormal lateral associations of fibrils and thicker fibrils with irregular contours are evident as early as P10. In contrast, the anterior stroma is minimally perturbed by the absence of lumican. In Lum+/+ mice, lumican is expressed throughout the developing stroma at P10, with strong expression limited to the posterior stroma in the adult. Therefore, the posterior stroma, which is most vulnerable to lumican-deficiency, demonstrates an early developmental defect in fibril structure and architecture in the Lum-/- mouse. These defects underlie the reported increased light scattering and opacity detectable in the adult. Our findings emphasize the early regulation of collagen structure by lumican during postnatal development of the 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.

Transparency of the bovine corneal stroma at physiological hydration and its dependence on concentration of the ambient anion

De-epithelialised and de-endothelialised bovine corneal stromas with a hydration of 3.2 equilibrated at 154 mM NaCl and buffered at pH 7.4 had their optical density (400-750 nm) measured. Stromas equilibrated against 10, 20, 30, 50 or 100 mM NaCl made isotonic to 154 mM NaCl by supplementing with sorbitol were progressively more transparent as NaCl increased. Hypertonic equilibration against 300, 600 or 1000 mM NaCl resulted in a progressive loss of transparency compared with 154 mM NaCl. Light scattering as a function of wavelength fitted a lambda(-3) function well for 10, 30, 50, 100 and 154 mM NaCl preparations between 450 and 650 nm, but not at higher wavelengths. However, hypertonic 300, 600 and 1000 mM NaCl preparations showed a lambda(-2) dependence in the 450-750 nm range. Experiments with 154 mM NaCl and either 0 or 300 mM sorbitol suggested that the changes in light scattering in hypertonic preparations are unlikely to be caused by osmotic alterations to the stromal keratocytes. Psychophysical studies of the optical transmission function of preparations indicated that corneal stromas dialysed against 154 mM NaCl had usable optical properties, but preparations dialysed against 10 mM NaCl were effectively unable to transmit an image. The results are related to the known increase of fixed negative charge in the corneal matrix when chloride ions are adsorbed onto the matrix. It is suggested that the ordering force between corneal collagen fibrils, generated in part by anion binding, may be crucial to the physiological functioning of the visual system.

Follicular fluid responds endothermically to aqueous dilution

BACKGROUND

Studies suggest that ovarian follicles are cooler than their surrounding tissues. The mechanism of this remarkable phenomenon is unclear. We postulate that endothermic reactions accompany the growth-associated hydration of follicular fluid.

METHODS

We performed two types of experiment, using human and animal follicular fluids. In the first, saline (50 microl) was injected into follicular fluid (500 microl) held in an equilibrated incubator, with monitoring of sample temperature. In the second, an adiabatic microcalorimeter recorded thermal shifts after injection of buffer (10 microl) into previously dialysed samples (1.4 ml). The relevance of changes observed was assessed by mathematical modelling.

RESULTS

In the incubator study, 9/17 bovine and 6/12 human fluids showed a temperature fall (0.05-0.2 degrees C). Cooling was delayed by up to 2 min but sustained for 7-25 min. Remaining fluids showed no change. In the microcalorimeter, 4/9 human, 4/6 bovine, 5/5 porcine and 1/4 equine samples showed an endothermic response. Remaining samples showed either no response (bovine) or exothermy (human, equine). Pre-concentration of human follicular fluid amplified the endothermy or reversed the exothermy. Modelling indicated that the incubator-type response was of appropriate magnitude to explain follicular hypothermy.

CONCLUSION

Follicular fluid responds endothermically to aqueous dilution and may contribute to follicular cooling during growth.