Recommended approaches for the evaluation of testicular and epididymal toxicity

Animals. Use Sexually Mature Animals, Whenever Practical, for Evaluating Effects on Spermatogenesis. Unless the objective of the study dictates otherwise, screening studies in rodents should be conducted in animals that are sexually mature at termination of the study (>9 weeks in rat, >7 weeks in mouse). Ideally dogs should be 9-12 months of age at termination, to minimize confounding aspects of immaturity. If younger animals are used and equivocal results occur, it may be necessary to repeat the study in older animals. The difficulties in obtaining sexually mature primates and the associated handling hazards for laboratory personnel generally preclude their use in screening studies and therefore preclude assessment of treatment-related effects on spermatogenesis. For suspected testicular toxicants, specific studies in sexually mature animals should be considered (4-5 years of age for cynomolgous). Tissue Sampling Sample. Both Testes and Epididymides, Identify Left and Right Separately and Record Organ Weights. Distinction of unilateral from bilateral effects is important for evaluation of treatment related findings, therefore separating data sets for each side can provide important information. Histopathological findings in the testis and epididymis are often interelated; it is useful to be able to directly correlate such findings. Increases and decreases in testis and epididymal weight are important indicators of adverse effects. Tissue Fixation. Fix Testes from All Species from Studies of 13 Weeks Duration and Less in Modified Davidson's Fixative. Well-fixed tissue is critical for the detection of early and subtle changes in the testis. Regulatory guidelines discourage the use of formalin for testes and recommend Bouin's or an alternative. Modified Davidson's provides improved fixation properties over Bouin's and is significantly less hazardous and more convenient to use on a routine basis. The design of fertility studies relies on information gained from examination of the testes in general toxicity studies. Therefore, adequate fixation in all studies up to 13-weeks' duration is recommended. Tissue Trimming, Sectioning, and Staining. Embed Tissues in Paraffin Wax. Embed Transverse Sections of the Testes, to Include Part of the Rete. Embed Longitudinal Sections of the Epididymis, Incorporating the Caput, Corpus and Cauda. For Rodent Studies up to 28 Days, Examine PAS-H-Stained Testes. For Rodent Studies Over 28 Days and for All Dog and Primate Testes, Examine H&E Stained Testes. Wax sections provide adequate quality for evaluating screening studies. Resin embedding may be used for investigative studies. Transverse sections provide mostly round, cross-sectional profiles of tubules for evaluation. The rete is a potential site for compound-related findings. The various regions of the epididymis have different functions, morphology, and susceptibility for toxicity. The lumenal contents of the different regions of the epididymis also reflect different temporal events in the testes. PAS-H stained sections allows accurate identification of the tubular stage of spermatogenesis in rats and mice. This information is useful for identifying and evaluating early disturbances of spermatogenesis in studies of short duration (up to 28 days). PAS-H is less useful in the dog and primate due to its restricted staining of the acrosome. Microscopic Evaluation. Microscopic Evaluation of the Testis Should be Carried Out as a Qualitative Examination with an Awareness of the Spermatogenic Cycle. Recognition of the loss of specific populations of germ cells or the inappropriate presence of germ cells (eg, spermatid retention) relies on knowledge of the cellular associations of the different stages of the spermatogenic cycle. The pathologist must have an adequate understanding of the morphology of the different stages of the cycle in the species under investigation. An understanding of the kinetics of the cycle as well as the physiology and regulation of spermatogenesis is also necessary to allow interpretation of the patterns of changes seen and their toxicological significance. Quantitative measures of tubular stages and their frequency distribution are not recommended for screening studies. Nomenclature and Severity Grading for Disturbances in Spermatogenesis May Vary on a Case-by-Case Basis Depending on the Specificity of the Findings. Findings may be nonspecific and be adequately described by nonspecific terminology such as tubular atrophy. This is frequently the case following long-term exposure to a toxicant. Conversely they may be cell specific and stage specific and require detailed terminology to convey this specificity. This is more often the case in short-duration studies. Grading severity may be based on the proportion of tubules affected or on the proportion of germ cells affected or lost. The choice is dependent on the changes seen and the nomenclature used. Whatever system is used, it is important that the diagnostic and grading criteria are defined.