Outer nuclear layer (ONL) thickness of WT and Tg rats. ONL thickness is proportional to the number of PR nuclei, in Sprague-Dawley albino WT (WT) rats and the 3 different lines of Tg rats carrying the P23H (A) and 5 different S334ter (B) mutant rhodopsin transgenes. The values are the means of 54 measurements taken along the vertical meridian of one retina from 3–5 rats at each age. The variance from these means in most cases falls within the symbol, so error bars have been omitted for clarity. In most cases, the SEM was <1.0 mμ, and in all cases after P10, it was <2.0 mμ except for S334ter-3 at P15, where the SEM was 3.3 mμ. For conversion to rows of PR nuclei, in the 1-μm thick plastic sections, each rod nucleus is approximately 4.5 μm in diameter.