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G.A. Engbretson, R.B. Sacunas, S.C. Chamberlain, K.G. Townsend; Rod Outer Segment Growth and Photoreceptor Distribution in the Larval Xenopus Retina . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2812.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: We studied the pattern of normal retinal growth to provide a baseline against which effects of transgenic manipulation can be compared. Methods: In vitro fertilized eggs of Xenopus laevis were raised under normal laboratory conditions. Tadpoles were staged (Niewkoop & Faber, 1956), fixed and sectioned for microscopy. The retina was divided into 40 µm segments in 1 µm plastic sections that were within 100 µm caudad and cephalad of the optic nerve head. Rods and cones were counted in each segment. The length and diameter of optimally aligned rod outer segments (ROS) were also measured. Eye diameter was measured at the position of the optic nerve. Results: Mean eye diameter increases slowly from about 400 µm at stage 42 to about 500 µm at stage 47. Between stages 47 and 51 (mean diameter ≈ 900 µm) eye growth with stage is much more rapid. Two regression lines with a 10-fold difference in slope fit these two growth phases well. As eye diameter increases between 400 and 900 µm, ROS length increases and that relationship can be expressed by a linear regression of the form, ROS length = -2.9 µm + 0.045 x eye diameter (R=0.96). ROS diameter also increases linearly with eye diameter over this same range and that relationship is expressed by the regression, ROS diameter = 4.3 µm + 0.004 x eye diameter (R=0.80). In both cases a large and statistically significant portion of the variation in ROS dimension is explained by regression on eye diameter. At all stages between 47 and 51, cones were more prevalent in the dorsal retina. In stage 51 animals cones constituted 54% of the photoreceptors in the dorsal third of the retina. This percentage decreased to about 39% in the center third and 19% in the ventral third. The dorsal-ventral difference in cone density was only slightly smaller in younger animals. Animals raised in complete darkness from gastrulation exhibited similar dorsal-ventral cone density difference. Conclusions: The rate of eye growth in normal X. laevis larvae is not constant during development. However, ROS length and diameter are highly predictable from eye diameter. Unlike adult animals, the larvae exhibit a strong dorsal-ventral difference in cone density. Absence of illumination during development does not appear to affect this asymmetric distribution.
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