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      Anatomy of cornea and ocular surface

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          Abstract

          Important functions of cornea in the eye include protecting the structures inside the eye, contributing to the refractive power of the eye, and focusing light rays on the retina with minimum scatter and optical degradation. Considerable advances have taken place in understanding the organization of collagen in the corneal stroma and its clinical significance. In this review, the structure and function of various components of cornea and ocular surface are presented.

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          Most cited references10

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          Central corneal endothelial cell changes over a ten-year period.

          To obtain longitudinal data to estimate long-term morphometric changes in normal human corneal endothelia. Ten years after an initial study, the authors rephotographed the central corneal endothelium of 52 normal subjects with the same contact specular microscope. The findings for the 10 subjects younger than 18 years of age at the initial examination were considered separately. For the remaining 42 adult subjects, the time between examinations averaged 10.6 +/- 0.2 years (range, 10.1 to 11 years). At the recent examination, these subjects' ages averaged 59.5 +/- 16.8 years (range, 30 to 84 years). Outlines of 100 cells for each cornea were digitized. For the 42 adult subjects, the mean endothelial cell density decreased during the 10.6-year interval from 2715 +/- 301 cells/mm2 to 2539 +/- 284 cells/mm2 (P < 0.001). The calculated exponential cell loss rate over this interval was 0.6% +/- 0.5% per year. There was no statistically significant correlation between cell loss rate and age. During the 10.6-year interval, the coefficient of variation of cell area increased from 0.26 +/- 0.05 to 0.29 +/- 0.06 (P < 0.001), and the percentage of hexagonal cells decreased from 67% +/- 8% to 64% +/- 6% (P = 0.003). For the 10 subjects 5 to 15 years of age at the initial examination, the exponential cell loss rate was 1.1% +/- 0.8% per year. Human central endothelial cell density decreases at an average rate of approximately 0.6% per year in normal corneas throughout adult life, with gradual increases in polymegethism and pleomorphism.
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            Morphology of corneal nerves using confocal microscopy.

            N Efron, P Soto (2001)
            The aim of the current study was to evaluate the distribution and morphology of corneal nerves as seen by means of white light confocal microscopy. This study analyzed images of corneal nerves that were obtained using the Tomey Confoscan slit scanning confocal microscope (40x/0.75 objective lens). The images were classified according to their location within the cornea. The objective and subjective evaluation of the images involved measuring, grading, or judging a number of parameters from both individual pictures and from each single nerve fiber within any image. The in vivo observations made in this work are in agreement with those of previous histologic studies. The general scheme of corneal innervation is described as originating from thick and straight stromal nerve trunks that extend lateral and anteriorly and give rise to plexiform arrangements of progressively thinner nerve fibers at several levels within the stroma. From there, nerve fibers perforate Bowman's layer and eventually form a dense neural plexus just beneath the basal epithelial cell layer, which is characterized by tortuous and thin beaded nerve fibers interconnected by numerous nerve elements; nerve fibers from this plexus are known to be responsible for the innervation of the epithelium. This study provides convincing evidence of the suitability of confocal microscopy to image corneal nerves, the only drawback being the limited resolution in terms of the differentiation of the ultrastructure of nerve bundles.
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              Collagen fibrils appear more closely packed in the prepupillary cornea: optical and biomechanical implications.

              The size and organization of stromal collagen fibrils influence the biomechanical and optical properties of the cornea and hence its function. How fibrillar structure varies with position across the cornea has not been fully characterized. The present study was designed to quantify the collagen fibril spacing and diameter across the normal human cornea and to relate this to the properties of the tissue. Small-angle x-ray diffraction was used to map in detail the variation in fibril spacing and fibril diameter along orthogonal medial-lateral and inferior-superior meridians of five normal human corneoscleral discs. Mean fibril diameters remained constant across all corneas up to the limbus, whereupon a sharp increase was observed. However, mean fibril spacing across the central 4 x 3 mm (prepupillary) cornea measured 5% to 7% lower than in the peripheral cornea. Collagen fibrils in the prepupillary cornea appear to be more closely packed than in the peripheral cornea. Anisotropy in fibril packing across the cornea has potential implications for the transparency and refractive index of the tissue. Biomechanically, it is possible that the higher packing density of stress-bearing collagen fibrils in the prepupillary cornea is necessary for maintaining corneal strength, and hence curvature, in a region of reduced tissue thickness. By inference, these results could have important implications for the development of corneal models for refractive surgery.
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                Author and article information

                Journal
                Indian J Ophthalmol
                Indian J Ophthalmol
                IJO
                Indian Journal of Ophthalmology
                Medknow Publications & Media Pvt Ltd (India )
                0301-4738
                1998-3689
                February 2018
                : 66
                : 2
                : 190-194
                Affiliations
                [1]Department of Ophthalmology, Krishna Institute of Medical Sciences, Hyderabad, Telangana, India
                Author notes
                Correspondence to: Dr. Mittanamalli S Sridhar, Department of Ophthalmology, Krishna Institute of Medical Sciences, Minister Road, Secunderabad, Hyderabad - 500 003, Telangana, India. E-mail: sri.vision@ 123456yahoo.co.in
                Article
                IJO-66-190
                10.4103/ijo.IJO_646_17
                5819093
                29380756
                56f75e96-d0a1-4e6a-9ddf-abbe7b2e75d2
                Copyright: © 2018 Indian Journal of Ophthalmology

                This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

                History
                : 26 July 2017
                : 20 September 2017
                Categories
                Review Article for Residents

                Ophthalmology & Optometry
                collagen,cornea,descemet's membrane,endothelium,epithelium,stroma
                Ophthalmology & Optometry
                collagen, cornea, descemet's membrane, endothelium, epithelium, stroma

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