The Sheep Cornea: Structural and Clinical Characteristics.

PURPOSE: The aim of this study was to perform qualitative and quantitative analyses to characterize the corneas of young, healthy sheep. MATERIALS AND METHODS: Eight healthy male sheep, 10 months to 1 year of age, were included as experimental subjects. Central corneal thickness was measured using a handheld pachymeter, and an Easygraph corneal topographer provided topographic maps. Microstructural imaging of corneal layers was achieved by using the Heidelberg Retina Tomograph III Rostock Corneal Module in vivo corneal microscope (IVCM). An Ocular Response Analyzer (ORA) provided quantitative measurements of intraocular pressure (IOP), corneal hysteresis (CH), and corneal resistance factor. Tissue histology and immunohistochemistry were carried out to obtain detail on the corneal layers. RESULTS: Light microscopy and immunohistochemical labeling revealed a stratified epithelium, a limbus with numerous limbal crypts, a thick basement membrane, a thin Bowman's layer, a thick corneal stroma with a dense population of keratocytes, and a thick, hyper-reflective Descemet's membrane. Using IVCM, the cell density of the basal layer was noted to be significantly higher than that of other epithelial cell types. The density of keratocytes was significantly higher (P value = 0.0223) in the anterior compared to the posterior stroma. The endothelial cells were organized in a characteristic honeycomb pattern. The mean and standard deviation values for central corneal pachymetry were 623.14 ± 19.5 µm and 616.37 ± 34.87 µm for the left and right eyes, respectively. ORA-derived mean values for IOPcc and CH for the left and right eyes were 14.93 ± 1.73 mm Hg and 15.16 ± 2.02 mm Hg and 3.56 ± 0.72 mm Hg and 3.73 ± 0.49 mm Hg, respectively. CONCLUSIONS: The anatomical and clinical characteristics of the sheep cornea, as outlined in this study, make the sheep a suitable and relevant model for corneal research. This study provides researchers with important data on the suitability of sheep as a model for ophthalmic experiments.

Aberrant Patterns of Key Epithelial Basement Membrane Components in Keratoconus.

PURPOSE: In the cornea, the epithelial basement membrane (BM) plays an important role in maintaining corneal integrity and homeostasis. Aberrations in this vital structure are associated with several corneal pathologies including keratoconus. The aim of this study was to investigate the expression of key structural components of the epithelial BM in keratoconic corneas and to identify and describe any aberrant patterns. METHODS: Immunohistochemical labeling of key BM components including fibronectin, laminin, and type IV and VII collagen was performed in healthy and keratoconic corneas. RESULTS: Clear changes in the BM components in the keratoconic corneas were seen with the key structural components either being absent or forming a discontinuous pattern. Another aberrant pattern, the expression of BM proteins, particularly fibronectin, laminin, and type IV collagen, in the anterior stroma of keratoconic corneas was also observed. CONCLUSIONS: These results indicate the activation of keratocytes into the fibroblast and myofibroblast wound phenotypes and the potential source of corneal scarring commonly observed in keratoconic corneas. Our data also support the hypothesis of dysregulated collagen synthesis and breakdown in the keratoconic cornea, in particular, the BM, and suggest a role for the BM in initiation and progression of keratoconus.

Keratocytes are induced to produce collagen type II: A new strategy for in vivo corneal matrix regeneration.

The stroma, the middle layer of the cornea, is a connective tissue making up most of the corneal thickness. The stromal extracellular matrix (ECM) consists of highly organised lamellae which are made up of tightly packed fibrils primarily composed of collagens type I and V. This layer is interspersed with keratocytes, mesenchymal cells of neural crest origin. We have previously shown that adult corneal keratocytes exhibit phenotypic plasticity and can be induced into a neuronal phenotype. In the current study we evaluated the potential of keratocytes to produce collagen type II via phenotypic reprogramming with exogenous chondrogenic factors. The cornea presents a challenge to tissue engineers owing to its high level of organisation and the phenotypic instability of keratocytes. Traditional approaches based on a scar model do not support the engineering of functional stromal tissue. Type II collagen is not found in the adult cornea but is reported to be expressed during corneal development, raising the possibility of using such an approach to regenerate the corneal ECM. Keratocytes in culture and within intact normal and diseased tissue were induced to produce collagen type II upon treatment with transforming growth factor Beta3 (TGFß3) and dexamethasone. In vivo treatment of rat corneas also resulted in collagen type II deposition and a threefold increase in corneal hardness and elasticity. Furthermore, the treatment of corneas and subsequent deposition of collagen type II did not cause opacity, fibrosis or scarring. The induction of keratocytes with specific exogenous factors and resulting deposition of type II collagen in the stroma can potentially be controlled by withdrawal of the factors. This might be a promising new approach for in vivo corneal regeneration strategies aimed at increasing corneal integrity in diseases associated with weakened ectatic corneal tissue such as keratoconus.

Transdifferentiation of chondrocytes into neuron-like cells induced by neuronal lineage specifying growth factors.

We previously reported that neural-crest-derived stromal cells from adult human and rat corneas can differentiate into neuron-like cells when treated with neuronal lineage specifying growth factors. However, it remains unclear whether this level of cell plasticity is unique to the corneal stromal cell population present in the eye. In this study, non-neural-crest-derived chondrocytes from the xiphosternum of adult rats were subjected to the same differentiation protocol. Cells of the adult rat xiphosternum can also differentiate into neuron-like cells when treated with neurogenic differentiation specifying growth factors. After 1 week in neurogenic differentiation culture conditions, the chondrocytes changed from a round to a stellate morphology and started to express neuron-specific protein neurofilament-200 (NF-200), microtubule associated protein-2 (Map-2), and ß-III tubulin. Lineage-specifying growth factors can affect changes in morphology and protein expression of adult cells in culture, findings that challenge the notion of a restricted differentiation potential of adult cell populations and questions the stability of the differentiated state of cells.

Cells from the adult corneal stroma can be reprogrammed to a neuron-like cell using exogenous growth factors.

Cells thought to be stem cells isolated from the cornea of the eye have been shown to exhibit neurogenic potential. We set out to uncover the identity and location of these cells within the cornea and to elucidate their neuronal protein and gene expression profile during the process of switching to a neuron-like cell. Here we report that every cell of the adult human and rat corneal stroma is capable of differentiating into a neuron-like cell when treated with neurogenic differentiation specifying growth factors. Furthermore, the expression of genes regulating neurogenesis and mature neuronal structure and function was increased. The switch from a corneal stromal cell to a neuron-like cell was also shown to occur in vivo in intact corneas of living rats. Our results clearly indicate that lineage specifying growth factors can affect changes in the protein and gene expression profiles of adult cells, suggesting that possibly many adult cell populations can be made to switch into another type of mature cell by simply modifying the growth factor environment.