A Practical Approach, Second Edition=Ronald D. Ho.pdf

886 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONhuman) for the compound in question. Several important parameters that must be considered indesigning a study to test for effects on the male reproductive system are the duration of exposure,appropriate routes of exposure, level of exposure associated with adverse risks in humans, choiceof the test species, and the endpoints selected for study. Other parameters that should be consideredinclude appropriate fixatives for the preservation of reproductive tissues and the most appropriatemethods to detect changes in the selected endpoints.The endpoints commonly observed in male reproductive toxicity testing include animal bodyweight, fertility, reproductive organ weights, sperm quality (morphology and motility), spermquantity (cauda epididymal sperm numbers and testicular homogenization-resistant spermatid numbers),histopathological observations on reproductive organs (and in particular the testis), anogenitaldistance, and when necessary, reproductive endocrine profiles. Many of the endpoints obtainedfrom these studies are interrelated. The strength of collecting this type of data lies in the fact thatdata obtained on one endpoint can help corroborate effects observed on other endpoints. It istherefore suggested that a weight-of-evidence approach be taken when interpreting male reproductivetoxicity data obtained in regulatory testing protocols.The endpoints observed in the animal test system represent various manifestations of outcomesof altered sperm production, sperm quality, and sperm quantity. A multitude of reproductive effectscan be observed from exposure to a toxicant, but the effects are unequal. Effects observed shouldnot only be classified as to their severity but also as to whether they are reversible. For example,compounds that destroy stem cells will have a long-term irreversible effect on the testis, whereasanother compound may have only a short-term effect on a different cell population.The following presents a brief discussion of some endpoints of male reproductive toxicitycommonly collected and their use in the risk assessment process. Among the endpoints of malereproductive toxicity commonly assessed, two endpoints have received the most attention in thelast 10 years: evaluation of testicular histopathology and assessment of sperm motility. Theseendpoints are discussed first and are followed by a discussion of other selected endpoints.1) Testicular Histopathology — Careful histopathological examination of the testis is recognizedas a sensitive method to identify effects on spermatogenesis. The sensitivity of this methodcan be increased or decreased depending on the quality of the tissue fixation and the embedmentof the testicular tissues. In the past, histopathological evaluation of the testis identified only majorchanges between control and treated groups. Subtle lesions (retained spermatids, missing germ celllayers) may not have been observed because the tissue preservation methodology was substandardand pathologists were not familiar with testicular morphology and the kinetics of spermatogenesisof the test species examined. Testicular morphological changes can be used to identify injury thatis not severe enough to cause a reduction in fertility or sperm production in the rat. Morphologicalchanges that occur in the absence of functional changes (i.e., libido, sperm motility, fertility) shouldbe interpreted as an indication of the potential for the compound to affect spermatogenesis adverselyin the human. Although histopathological changes may fail to reveal treatment related effects,consideration should be given to the possible presence of other testicular or epididymal effects notdetected histologically that may affect reproductive function.Excellent articles and books have been written describing methods that can be used to preservetesticular tissue as well as to evaluate testicular histopathology. 20,21 These resources provide variousapproaches for the qualitative and/or quantitative assessment of testicular lesions. Included in thebooks are detailed descriptions of the various stages of the cycle of the seminiferous epitheliumand extensive information on tissue preparation and examination, and interpretation of observationsfrom normal microscopic and ultrastructural histology of many experimental animals.The histological picture seen after toxic insult may provide insight into the site and/or mechanismof action of the toxicant on a particular reproductive organ. If similar targets or mechanismsexist in humans and other test species, the data can be utilized for extrapolation between speciesand may allow a prediction of the extent of damage and the chances for recovery. 22© 2006 by Taylor & Francis Group, LLC