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Shannon Getz, Micah A Chrenek, Natecia Williams, Jeffrey H Boatright, J M Nickerson; Early Morphological Changes in IRBP Knockout Mice. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3468.
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Because IRBP knockout (KO) mice develop profound myopia and show a slow retinal degeneration after P23, we posit that IRBP plays an essential role in eye development and setting the number of retinal cells. Excessive ocular enlargement for IRBP KO begins some time between P7 and P10 (Wisard et al. IOVS. 2011; 52:5804-11). This coincides with “inner rod” development and death (Young, RW. J Comp Neurol. 1984; 229:362-73). To more accurately define when myopic elongation begins in IRBP KO mice, we measured globe dimensions, ophthalmic, and histological features in IRBP KO and C57BL/6J (WT) mice each day from postnatal (P) day 5 to 10.
IRBP KO and WT mice were sacrificed from P5 to P10. Axial, nasal-temporal (N-T) and superior-inferior (S-I) measurements were conducted with a laser micrometer. The eyes were stained and the number of nuclei and retinal layer thickness were measured. For IRBP mRNA localization, fluorescence in situ hybridization was performed with IRBP KO and WT at P60. TUNEL staining was used to evaluate apoptosis. SD-OCT and fundus images were taken at P15 and P30.
IRBP mRNA was found exclusively in the inner segments and the outer nuclear layer in WT, but not in the IRBP KO. At each age, the mouse body weight of WT and KO strains was the same. From P5 to P7, the eye weight, axial, N-T, and S-I measurements were the same in both mice but increases (in IRBP KO over WT) were first detected at P8. Each retinal layer showed a slight decrease in thickness in the KO. The full thickness of the retinal layers showed slight thinning in the KO, beginning at P8. SD-OCT confirmed histological thickness measures at P15 & P30. A trend towards fewer “Inner rods” was found in the IRBP KO. No difference in fundus appearance was detected.
Abnormalities in the IRBP KO began at P8 with a distinct increase of eye size in all three dimensions and eye weight. The WT and IRBP KO have the same body weight, demonstrating that the changes in eye size do not correspond to a change in body weight. IRBP KO nuclear counts and retinal layer thicknesses were less than WT, but not significantly. These changes suggest that IRBP plays an important developmental role in eye shape and size, affecting phenotype gradually beginning at a very discrete age, P8, with minimal impact early on retinal thickness, which may imply an excess total number of retinal cells, that are lost later (P23 and later).
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