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

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.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

A new method to determine rate-dependent material parameters of corneal extracellular matrix

The cornea protects internal ocular contents against external insults while refracting and transmitting the incoming light onto the lens. The biomechanical properties of the cornea are largely governed by the composition and structure of the stromal layer which is an extracellular matrix composed of collagen fibrils embedded in a hydrated soft matrix. The mechanical behavior of the corneal stroma has commonly been characterized using uniaxial tensile tests and inflation experiments. In the present study, unconfined compression experiments were used to investigate the influence of loading rates on compressive behavior of nineteen porcine corneal specimens. The experiments were performed at ramp displacement rates 0.15 μm/s (eight samples), 0.5 μm/s (six samples), and 1.0 μm/s (five samples). For all tests, a maximum compressive strain of 50% (five strain increments of 4% followed by three strain increments of 10%) was selected. The experimental data was analyzed by a transversely isotropic biphasic model and material parameters, i.e., the in-plane Young's modulus, the out-of-plane Young's modulus, and the permeability coefficient were calculated. It was observed that while the permeability coefficient decreased exponentially with increasing compressive strain, the in-plane and out-of-plane Young's moduli increased exponentially with increasing strain. Furthermore, it was found that the equilibrium stress was almost rate independent.

Assessment of collagen fibril spacing in relation to selected region of interest (ROI) on electron micrographs--application to the mammalian corneal stroma

AIMS

To evaluate measurements of collagen fibril spacing using different shaped regions of interest (ROI) on transmission electron micrograph (TEM) images of rabbit corneal stroma.

METHODS

Following glutaraldehyde fixation and phosphotungstic acid staining, TEM images of collagen fibrils in cross section were projected at a final magnification close to 250,000 × to obtain overlays. Interfibril distances (IFDs; center-to-center spacing) were measured within different ROIs of the same nominal area (0.25 μm(2) ) but different shape (with the length to width, L:W, ratio from 1:1 to 6:1). The IFD distribution was analyzed, and the 2D organization assessed using a radial distribution analysis.

RESULTS

The fibrils had an average diameter of 35.3 ± 3.8 (SD) nm, packing density of 393 ± 4 fibrils / μm(2) and a fibril volume fraction of 0.39 ± 0.02. IFDs ranged from 29 to 1400 nm depending on the shape of the ROI, with average values ranging from 263 to 443 nm. By artificially selecting IFD data only to a radial distance of 250 nm, the average IFDs were just 145-157 nm. The radial distributions, to 250 nm, all showed a nearest neighbors first peak which shifted slightly from predominantly at 45-54 nm with more rectangular ROIs. The radial distribution profiles could be shown to be statistically different if the ROI L:W ratio was 2:1 or greater.

CONCLUSION

Selection of an ROI for assessment of packing density and interfibril distances should be standardized for comparative assessments of TEMs of collagen fibrils.

Assessment of postnatal corneal development in the C57BL/6 mouse using spectral domain optical coherence tomography and microwave-assisted histology

The eyes of newborn mice are relatively underdeveloped and the lids remain closed for the first 2 weeks after birth. There after the eyes undergo a period of rapid growth for several weeks. Eventually the eyes reach an age at which many ocular structures stabilize for the remainder of the animal's life, or for others, growth is significantly slowed. The central corneal thickness (CCT) is a parameter commonly reported in corneal studies. However there is a large discrepancy in values reported for adult mice as well as a lack of comprehensive values covering the time from birth through adulthood. In this study we report, for the first time, the use of spectral domain optical coherence tomography (SD-OCT) for in situ and in vivo determination of CCT from P0 to P250 for C57BL/6 mice. SD-OCT provided a reliable measure of CCT and we fit the data to an exponential rise to maximum growth curve resulting in a value of 49 μm for P0 and a maximum adult value of 106 μm. By comparison, corneas processed for conventional histology produced CCT values approximately 30-35% thicker and with greater variability. Ex vivo real-time imaging during fixation revealed swelling and gross distortion of the cornea beginning after only 10-15 min in fixative. The fixation artifacts were not observed when the cornea was processed using an optimized microwave fixation protocol. CCT values measured in corneas fixed with the microwave process compared favorably with values obtained with SD-OCT. We conclude that for corneal research, mice younger than 8 weeks of age should not be considered as adults since they are still in a rapid phase of growth up until that time. In addition we report the first use of microwave processed histological specimens for visualizing the murine cornea. Tissue processed in this manner has minimal artifacts, a CCT equivalent to that measured in vivo by SD-OCT and ultrastructural detail comparable to conventional fixation methods.

Effect of corneal hydration on ultrasound velocity and backscatter

The cornea's acoustic properties (speed-of-sound, backscatter, attenuation) are related to its state of hydration. Our aim was to determine these properties as a function of corneal hydration using high-frequency ultrasound. Bovine corneas were suspended in a Dexsol-equivalent corneal preservation medium at 33 degrees C and then immersed successively in 75%, 50% and 25% medium and distilled water. Using a 38-MHz focused ultrasound transducer, we measured speed-of-sound and corneal thickness (n = 8) and stromal backscatter (n = 6) after 45-min immersion in each medium. Corneal speed-of-sound was modeled as a function of corneal thickness. We found the mean speed-of-sound to be 1605.4 +/- 2.9 m/s in normotensive medium. The maximum observed speed-of-sound was 1616 m/s. As we decreased medium tonicity, the cornea swelled and the speed-of-sound decreased, reaching 1563.0 +/- 2.2 m/s in water. Average corneal thickness increased from 969 +/- 93 microm in 100% medium to 1579 +/- 104 microm in water. Going from 100% medium to water, stromal backscatter (midband-fit) increased from -60.0 +/- 0.8 dBr to -52.5 +/- 3.5 dBr, spectral slope increased from -0.119 +/- 0.021 to -0.005 +/- 0.030 dB/MHz and attenuation coefficient decreased from 0.927 +/- 0.434 to 0.010 +/- 0.581 dB/cm-MHz. The observed correlation between acoustic backscatter and attenuation with the speed-of-sound offers a potential means for more accurate determination of speed-of-sound and, hence, thickness in edematous corneas.

Resolution and reproducibility of measures of the diameter of small collagen fibrils by transmission electron microscopy--application to the rabbit corneal stroma

AIM

To assess the impact of measurements of different numbers of small collagen fibrils at different final magnification values on the resultant mean values for fibril diameter (FD).

METHODS

Very high magnification (33,000x) electron micrographs were taken of the posterior-central zone of the corneal stroma from young adult rabbits (2 kg), printed at 46,000 or 50,000x, scanned at 400 d.p.i. and 35 mm slides prepared. These were projected to give final magnifications between 150,000x and 450,000x. An overlay of fibril outlines was prepared from the projected images and the fibril diameters (FD's) measured to within 0.5 mm.

RESULTS

On the overlays, at different projection magnifications, the average FD's ranged from 4 to 13.5 mm to allow measures to be made at a real resolution of between 3.5 and 1 nm. Using a fixed sized region of interest (ROI) on the overlays, the average values of FD's ranged between 24.9 and 31.7 nm, and could vary (for any particular micrograph) by up to 3.6 nm according to the number of FD's measured (n=34-384/ROI). Using a fixed number of FD measures (n=100) at different projection magnifications yielded average FD values from different corneas of between 25.1 and 35.2 nm, which could vary by up to 4.3 nm depending on the magnification used.

CONCLUSIONS

The results indicate that different average values for measures of fibril diameter of small collagen fibrils can easily be obtained according to the final magnification used and the number of fibrils measured, and that the overall averages can depend on whether the data sets are averaged or pooled. These aspects of the morphometry of small collagen fibrils therefore need to be carefully specified.

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.

A descriptive and quantitative study of the keratocytes of the corneal stroma of albino rabbits using transmission electron microscopy

The present morphometric study was designed to assess the dimensions and shape of keratocytes and their nuclei by transmission electron microscopy, and to assess these features in relation to the stromal lamellae. Corneas from 10 albino rabbits were fixed in 2% glutaraldehyde in cacodylate buffer (pH 7.4, 300 mOsm/kg) and embedded in Spurr's epoxy resin. Both transverse and coronal thin sections through the corneal stroma were prepared. The stromal lamellae had an average thickness of 2.45+/-1.15 microm. The average cell thickness of the keratocytes was 1.34+/-0.46 microm (range 0.49-4.76 microm), with the apparent cell thickness being related to the average anterior-posterior thickness of the adjacent lamellae (r = 0.424, P = 0.001)). The relative length and thickness of the cell nucleus, in transverse section, was measured to be 0.65+/-0.13 and 0.76+/-0.10 of the cell body section respectively. As assessed by planimetry, the area of the keratocyte cell body viewed in coronal section was 292+/-118 microm2, with a nucleus-to-cytoplasm ratio of 0.437+/-0.295. The electron micrographs confirmed the presence of gap junctions between keratocyte cell processes, and the occasional presence of centrioles in the cells. Some keratocyte processes were observed to extend from one face of the lamellae to the other, suggesting anterior-to-posterior cell communication. These studies indicate that the keratocyte cell thickness is influenced by the physical pressure exerted by adjacent stromal lamellae. The cell nucleus, while a dominant feature in transverse section, has a normal size in relation to the cell cytoplasm when viewed in coronal section.