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P. N. Lewis, A. Quantock, K. Meek, C. Knupp; Three Dimensional Structural Organisation of Collagen Proteoglycan Interactions in the Cornea. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3924. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The accepted explanation for corneal transparancy is based on the high degree of positional order of the collagen fibrils within the cornea. This is achived through proteoglycans (PGs) that interconnect adjacent collagen fibrils. Detailed modes of interaction between collagen fibrils and PGs are not known.In this investigation we carry out a series of three dimentional (3D) reconstructions of PG collagen interactions, in order to elucidate their structural role and optomal 3D organisation in the cornea.
Sulphated PGs were visualised in bovine corneas by transmission electron microscopy after contrasting with cupromeronic blue (0.05% w/v). Tomographic tilt series of the anterior central region of the cornea were collected at 1° intervals over an angular range of +/- 60°. 3D reconstructions of tomagraphic data sets were carried out using EM3D software. Bovine corneas were briefly fixed and incubated in buffer with or without PG-degrading enzymes (chondroitinase ABC (1U/ml) or Keratanase I (5U/ml).
Transverse and longitudinal 3D reconstructions of cornea stroma (3D animations) showed long PGs interacting laterally with collagen fibrils bridging two or more collagen fibrils. Shorter PG chains between next-neighbour fibrils are also seen. Long PGs were keratanase resistant, but chondroitinase ABC sensitive and thus a form of CS/DS PG.
The 3D reconstructions along with the enzyme digestion patterns would appear to indicate two separate populations of PGs that have quite different functional roles. We propose a simple model that explains this distribution. We consider the two populations of PGs act synergistically to stabilise matrix architecture. The smaller PGs trying to push the fibrils apart while the longer PGs simultaneously preventing excessive spreading or expansion.
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