Cystitis cystica, cystitis glandularis, and Brunn's nests in a feline urinary bladder.

females, but 20% of males, had a sciatic lesion graded 2.5 or 3; at 24 months, 80% of males and only 50% of females had a sciatic lesion graded 2.5 or 3. This suggests an earlier occurrence of the lesion in male than in female rats. This toxicity study used Charles River CD rats. Nerve lesions are known to occur in other strains also (Sprague Dawley, Wistar, and others), but a greater incidence has been reported in Charles River CD rats than in Sprague Dawley rats [3]. Although the cause of these degenerative neural changes is unknown [3], several predisposing factors may be suggested, including housing. The rats are kept in hanging wire-mesh cages for two years. This housing does not allow coprophagy and thus provision of bacteriallyderived nutrients such as thiamin and vitamin BE, which are important co-factors for nervous tissue. The rats are kept for several months on a surface where they must always grip and thus may develop plantar lesions (which are found frequently) that could lead to peripheral nerve lesions. The different pressures exerted by different body weights could thus explain the sex differences, although a genetic origin, as suggested by strain differences, cannot be ruled out. Nevertheless, whatever the cause of these nerve lesions, their development in aging normal rats must be taken into consideration in long-term toxicity experiments and may even be considered reason enough to eliminate the rat in species selection for long-term (over 18 month) neurotoxicity studies.