Reproductive and developmental toxicity studies are significant components of the regulatory approval process for chemicals and drugs. Sperm morphology, motility, and count are good indices of reproductive toxicity in males (Morrissey et al. 1989). These parameters are readily measured (repeatedly, if needed) and quantified. In contrast, few easily obtainable and quantifiable morphological markers are available for assessing reproductive toxicity in females. Instead, qualitative measures of reproductive toxicity are often used in females, including reduced fertility, abnormal maternal behavior, clinical signs (e.g., weight loss), or problems in lactation in dams or altered reproductive outcome, embryo toxicity, or terata in the conceptus. It has been proposed that ovarian follicle counts are a sensitive, quantitative measure of female reproductive toxicity and that their measurement should be added to reproductive toxicity studies (Bolon et al. 1997). Indeed, the ovary is a major target of xenobiotics affecting female fertility (Cooper et al. 1989, Mattison and Thomford 1989, Godowicz and Paulus 1985, Dobson and Felton 1983). Agents that destroy mature antral follicles may induce temporary infertility until damaged oocytes are replaced through recruitment from the pool of immature (small and growing) follicles. In contrast, compounds that deplete these two latter classes of follicles may lead to a permanent infertility and early reproductive senescence, e.g., accelerated menopause because the finite number of these progenitor oocytes cannot be replaced.
Studies in mice (Perez et al. 1997; Weitzman et al. 1992; Mattison et al. 1989, 1983; Takizawa et al. 1984) and rats (Flaws et al. 1994, Toaff et al. 1979) have actually shown that measuring differential follicle count is a sensitive means of estimating the extent of ovarian toxicity in females exposed to xenobiotics. These findings have led to the suggestion to use ovarian differential follicle count for both conventional safety assessment bioassays in animals and interspecies extrapolation between animals and humans. In fact, the U.S. Environmental Protection Agency (EPA) recently included ovarian follicle count in the TSCA/FIFRA study guidelines (U.S. EPA 1998).
This chapter reviews the methodology for the assessment of differential follicle counts in rats and mice, compares the various procedures as to sensitivity and time involvement, and discusses the potential uses of the counts. Data presented were previously published (Bolon et al. 1997, Bucci et al. 1998).