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J. Tukler Henriksson, J. P. G. Bergmanson; Anatomical Explanation for Central to Peripheral Stromal Thickness Variation in the Mammalian Cornea. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4531.
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© ARVO (1962-2015); The Authors (2016-present)
The mouse like the human shows a thickness variation across the width of the cornea. The purpose of the present study was to provide an anatomical explanation for this thickness difference, which, in the mouse cornea, amounts to a 60 % decrease from center to periphery.
Six adult C57BL/6 mice were euthanized and 2 % glutaraldehyde fixative immediately applied to the cornea. The eyes were enucleated, immersed in fixative and corneas processed for histology using an established protocol. Keratocytes were counted light microscopically (LM) and ultrastructurally along an anterior/posterior line to establish if this count will give a quick indication of number of lamellae present. For lamellar and keratocyte counts and lamellar thickness assessments overlapping transmission electron microscopy (TEM) images were assembled into montages using PanaVue ImageAssembler and printed at a final magnification of 5000x. Histological criteria were used to obtain lamellar and keratocyte counts. The morphometry was conducted along a defined axial line at the corneal apex and at periphery. The central count was performed 77-113% from the peripheral landmark.
The C57BL/6 mouse central cornea had an average of 11.7 ± 0.3 keratocytes (LM), 9.8 ± 0.8 keratocytes (TEM) and 49.5 ± 8.0 lamellae compared to 7.8 ± 0.5 keratocytes (LM), 8.7 ± 0.8 keratocytes (TEM), and 35.5 ± 7.0 lamellae peripherally. The average lamellar thickness measured 2.1µm centrally but 1.8µm peripherally. Utilizing LM for keratocyte counts, TEM for lamellae counts and lamellar thickness measurements a statistically significant decrease (P< 0.005) in the number of keratocytes, lamellae and lamellar thickness was found in the peripheral compared to central cornea. However no statistical difference (P> 0.3) was found between the central and peripheral cornea when keratocytes were counted ultrastructurally.
Perhaps because LM requires a thicker section containing more keratocytes, it appears that this count provides a more reliable and easily obtained indication of number of lamellae present than TEM. The TEM count of lamellae is laborious but accurate showing that a 39% reduction in count peripherally. It is concluded that the thinning of the mouse corneal stroma towards the periphery is explained mainly by a decrease in lamellar count (50%) and to lesser degree by lamellar thickness (20%). This finding is suggestive of that not all corneal lamellae cross cornea limbus to limbus. The thickness differences in the human cornea may have a similar explanation.
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