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A. J. Quantock, J. J. Doutch, B. P. Palka, R. D. Young, W. M. Petroll, K. K. H. Svoboda, K. M. Meek; Spectrophotometric Analysis of Avian Corneal Transparency Increasing From Embryonic Day 12 to Day 18. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4525.
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© ARVO (1962-2015); The Authors (2016-present)
Early photometric measurements of white light transmission through the developing chick cornea indicate an increase from approximately 40% at embryonic day 14 (E14) to around 80% by E18 (Coulombre & Coulombre. J Cell Comp Physiol 1958;51:1-11). The current study was conducted to investigate the spectral dependence and depth-profiled nature of light transmission through the developing chick cornea in this timeframe.
Spectrophotometric measurements through the centres of 80 freshly-isolated chick corneas (n=8-12, daily at E12 through E18) were made using a Pye Unicam SP8-100 double-beam spectrophotometer with a 1mm2 beam. Confocal Microscopy Through Focussing (CMTF) of corneas (n=5) at E14, E16 and E18 was carried out as described previously (Song et al IOVS 2003;44:548-57).
From E12 to E14 light transmission through the cornea does not change, but from E14 onwards increases across the whole 400nm to 700nm spectral range. Transmission is lowest at short wavelengths and highest and longer wavelengths for all developmental stages. At a mid-range wavelength of 550nm, transmission measures 28.7% (SD=5.7%) at E12, 25.4% (SD=2.3%) at E13, 27.6% (SD=4.7%) at E14, 37.0% (SD=5.2%) at E15, 44.7% (SD=3.2%) at E16, 57.8% (SD=3.8%) at E17, and 75.7% (SD=3.2%) at E18. CMTF indicates qualitatively more backscattered light in the anterior stroma at E14, E16 and E18. Theoretical analysis of light transmission does not rule out the possibility that a reduced corneal thickness and changing refractive index mismatch between the collagen fibrils and the extrafibrillar matrix contribute to the transparency increase as the cornea develops. However, the primary reason for enhanced light transmission appears to be a reduction in the size and number of spatial inhomogeneities in the stroma.
Corneal light transmission across the visible range increases with development in the chick, with the deep stroma becoming transparent in advance of more superficial regions. Potentially, changes in the morphology and light scattering characteristics of the keratocytes could influence corneal transparency. With regards to structural changes in the corneal extracellular matrix, however, a reduction in the size and number of collagen-free lakes in the stroma is judged to have most impact on the transmission of light.
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