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C.J. Connon, K.M. Meek, S. Kinoshita, A.J. Quantock; Anterior-posterior Development of Fibrillar Organisation in Chick Corneas . Invest. Ophthalmol. Vis. Sci. 2003;44(13):863.
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Purpose. In the latter stages of development the embryonic chick cornea undergoes significant changes in structure, composition and transparency. The arrangement of collagen fibrils in the corneal stroma is thought to play a crucial role in the development of corneal transparency. The current study was designed to investigate spatial alterations in the organisation of corneal collagen fibrils in the secondary chick cornea during development. Methods. Corneas taken from chick embryos at developmental days 14, 16 and 18 were processed for transmission electron microscopy. Ultrathin sections were stained with 2% uranyl acetate and 2% lead citrate. The relative positions and diameters of collagen fibrils visualised in cross section were quantified from 10 micrographs from anterior and posterior tissue regions at each time-point using a digital camera and image analysis software. For each micrograph a statistical measurement of the distance between each and every fibril (the radial distribution function, (RDF)) was calculated, as was the average fibril diameter. From the RDFs we calculated fibril number density, average interfibrillar spacing as well as level of local order in the fibrillar array. Results. At developmental days 14, 16 and 18, RDF traces indicated a greater level of short-range order in the collagen of the anterior stroma than the posterior stroma. Short-range order, however, increased with time in the posterior stroma so that the anterior/posterior distinction was least obvious at day 18. Between days 16 and 18 average interfibrillar spacings decreased in both anterior and posterior tissue regions. Fibril number densities also increased between days 16 and 18, especially in the posterior stroma. Collagen fibril diameters did not differ with depth or stage of development. Conclusions. Theories of corneal transparency require the stromal matrix to have some degree of regularity in the arrangement of its collagen fibrils. The secondary chick cornea becomes progressively transparent between days 14 and 18 of development as the stromal matrix compacts. The current work shows that over this time period the local-order of the collagen fibrillar array, as gauged by the sharpness of the primary RDF peak and the presence of subsidiary peaks corresponding to second and third nearest neighbour spacings, increases with time but is always greater in anterior tissue regions.
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