A comparative evaluation of the tissue responses associated with polymeric implants in the rat and mouse.

End product application is an important consideration when evaluating a material in an in vivo setting (Didisheim, Cardiovasc Pathol 1993;2:1S-2S). Small animal models allow high through-put evaluation of biocompatability. Previous preclinical evaluations have often used a rat subcutaneous model for the characterization of material-tissue interaction. Recent advances in genetic manipulation have provided mouse models with selective expression of a wide range of critical proteins. The rat model does not have many of the resources (i.e., knockouts, SCID, nude) that are present in mouse strains. The availability of these mice provides a resource to delineate the mechanisms regulating the healing associated with implants. However, before the mouse models can be used, they must be validated with respect to their ability to accurately assess tissue responses to materials. In this study the tissue responses after the implantation of expanded polytetrafluoroethylene (ePTFE) were compared between rat and mouse. Discs of ePTFE (30-microm internodal distance) were implanted in subcutaneous and epididymal fat tissue of rats (Sprague-Dawley) and mice (129-SVJ). After 5 weeks the samples were removed and evaluated for vascular density, inflammation, and fibrous encapsulation. No difference in the vessel density was observed within the peri-implant subcutaneous and adipose tissue or within the porous material. However, a significant difference was found in the number of activated macrophages and giant cells between these two species. Implants in the rat exhibited greater numbers of activated inflammatory cells in the peri-implant tissue. The data indicate that the mouse and rat provide a comparable model for evaluating angiogenesis and neovascularization associated with synthetic porous implants.