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C. Knupp, P. N. Lewis, C. Pinali, R. D. Young, K. M. Meek, A. J. Quantock; Three-Dimensional Electron Tomography Throws New Light on the Structure of the Cornea. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4532.
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The transparency of the cornea is a consequence of the ordered hexagonal arrangement of the collagen fibrils in the corneal stroma. This arrangement is reached through the interaction of the corneal collagen fibrils with two families of proteoglycans, the chondroitin/dermatan sulphates and the keratan sulphates. Our purpose is to clarify the structural details of these interactions.
A series of three-dimensional reconstructions of bovine cornea were carried out using a JEOL 1010 transmission electron microscope at an original magnification of x25K. Our samples were treated for conventional microscopy and stained with cupromeronic blue to improve proteoglycan visibility. Tilt series at 1 degree intervals were taken from -60 to +60 degrees, and real space reconstructions, 400 nm by 400 nm by 100 nm were calculated using EM3D.
It is apparent from our reconstructions that there are no systematic six-fold interactions between collagen and proteoglycans in the hexagonal array. Instead, proteoglycans form long electron dense strands that hold two or more collagen fibrils together. Since these strands are oriented in different directions, proteoglycans bridges are formed between all adjacent collagen fibrils to gives rise to the observed three-dimensional corneal fibrillar packing.
Careful analysis of the reconstructions obtained led to the proposal of a new model of how interfibrillar distances and hexagonal arrangement in the corneal stroma are controlled. In this model, adjacent collagen fibrils experience two opposite forces, a repulsive force arising from osmotic pressure and an attractive force arising from the thermal motion of the proteoglycans.
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