**3. Methods of assessing male reproductive capacity**

The determination of sperm concentration, morphology, and motility remains the primary clinical assay for male infertility (WHO, 1999). The WHO guidelines on semen quality provide reference semen parameters as follows: sperm concentration >20 million/ml, 50% motility, and at least 50% normal morphology (WHO, 1999). *Oligospermia* is the term for semen with <20 million/ml sperm in ejaculate, and *asthenospermia* for semen with sperm <50% motility or <25% rapid progressive motility, *teratospermia* for reduced percent sperm with normal morphology, and *azoospermia* for the absence of sperm in the ejaculate (Braude & Rowell, 2003). It is unknown whether endocrine disrupter exposure represents a subset of the environmental risk factors for male infertility, as there are few studies that examine exposure and human semen quality. Moreover, toxicants can affect the male reproductive system at one of several sites or at multiple sites. These sites include the testes, the accessory sex glands, and the central nervous system, including the neuroendocrine system.

#### **3.1 The contribution of experimental models**

The usefulness of experimental animal models is usually perceived as limited to hazard identification using the test protocols specified by regulatory agencies. However, animal models can also provide valuable support to reproductive risk assessment on many other fronts. If the focus is on human exposure, animal studies can be designed to confirm reproductive toxicity when initial observations in exposed humans are suggestive of an adverse effect. Furthermore, such observations can be extended across a wide range of exposures in animals, using any route of exposure and any specified dose versus time scenario. For example, when human exposure is likely to be acute or intermittent, animal models are ideal for defining critical exposure windows based on developmental stage or for revealing the pathogenesis of an effect at various times after exposure through recovery. This is particularly important with respect to male reproductive effects because alterations in semen quality or fertility may not become evident until sometime after the exposure, particularly if an early stage of spermatogenesis is targeted. A rodent model is most commonly used for the study of reproductive and developmental toxicity (Claudio et al., 1999). To use toxicology data derived from animal studies to advantage in risk assessment, it is critical to identify and understand species-specific differences in physiology and metabolism that may affect the response to the toxicant in question. Rodent models have, for example, been used to determine the relationship between sperm end points and function (fertility) (Chapin et al., 1997). Determining that a substance is toxic to the male reproductive system is only the first step: The next step is to examine its mechanisms of toxicity. Mechanistic information allows for predictions about the potential toxicity of individual compounds or complex mixtures in humans, for better understanding of the windows of vulnerability in the development of the male reproductive system, and for developments of possible preventive or curative measures. Acute short-term exposure models combined with serial exposure models give a complete picture of the range of effects (Claudio et al., 1999). Exposing animals over a long period of time allows for the detection of transgenerational effects from chemicals, such as male-mediated developmental effects. If developmental effects appear, researchers can go back and administer a dose during that critical period of development to refine knowledge about how such problems occur. Early developmental end points measurable in animal research include anogenital distance at birth, testis position, genital malformations, secondary sex characteristics, and serum hormone levels. Acute short-term exposures, on the other hand, can be useful for identifying critical windows of exposure. Acute exposures followed over time can help identify the pathogenesis of a lesion, isolate the cell type that is susceptible to damage (germ cells, spermatocytes, or spermatid), and determine genetic effects, including the repair capability of affected genes. Serial sacrifice studies are best used for identifying the earliest detectable pathologic changes in target organs, cells, or processes. Multigeneration studies, in particular continuous breeding studies, yield the most thorough assessment of the many complex processes that result in reproductive and developmental toxicity.
