Figure 16.25.

Example of a generally accepted grading scale (normal→minimal→slight→moderate→severe) to evaluate retinal atrophy in rat toxicity studies. (A) Normal retina: the different retinal layers are distinct, photoreceptor inner and outer segments are elongated, and outer nuclear layer is intact. (B) Slight retinal atrophy: different layers remain distinct, but the outer nuclear layer is reduced in thickness, and macrophages are present in the photoreceptor layer. (C) Moderate retinal atrophy: the RPE, photoreceptors, and most of the outer nuclear layer are lost; the outer plexiform layer is not present. The inner nuclear layer remains relatively intact albeit irregular. (D) Severe retinal atrophy: there is marked disruption of the normal retinal architecture with loss of retinal layer organization. Remnants of inner nuclear and ganglion cells remain.

de Vera Mudry et al. (2013); reprinted by permission of Sage Publications.