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ABSTRACT
Background: Considerable concern exists in the scientific community regarding potential effects of endocrine disruptive or modulating environmental agents on male and female reproductive development and capacity. Existing data show that in utero and postnatal exposure of rodents to endocrine modulating chemicals can influence the timing and progression of sexual differentiation and/or maturation (e.g., balano-preputial separation and vaginal opening). Methods: Sexual maturation data from various types of littering studies using International Gold Standard (IGS) Crl Sprague-Dawley rats were evaluated for consistency with both historical observations and published values from other laboratories. In addition, litters from two developmental neurotoxicology studies were statistically analyzed to identify whether increasing the numbers of pups per litter evaluated affected the interpretation of sexual maturation data sets. Results: Control values for preputial separation and vaginal opening ages ranged from PD 45.0 to 48.0 and from PD 32.0 to 34.0, respectively, regardless of the number of pups evaluated per litter. However, statistically significant delays in sexual maturation were present when three rats/sex/litter were evaluated that were not present when only one randomly selected rat/sex/litter was evaluated. Conclusions: Standardized procedures and criteria are required to provide consistent intra-laboratory values and reduce inter-laboratory differences in sexual maturation observations. When such criteria are used, these endpoints provide sensitive measures for detecting alterations in sexual maturation. However, our analyses demonstrate that the ability to detect statistically significant and biologically important differences in these endpoints is sometimes impaired by the currently common practice of evaluating only one randomly selected rat/sex/litter. Evaluation of three rats/sex/litter improved the sensitivity of the statistical analysis in detection of treatment-related effects and reduced the probability of identifying a false negative result.