Histopathology atlas of animal model systems - overview of guiding principles.

Animal model systems represent an important adjunct and surrogate for studies of osteoarthritis (OA) in humans. They provide a means to study OA pathophysiology as well as aid in the development of therapeutic agents and biological markers for diagnosing and prognosing the disease. Thus, it is of great importance for the OA scientific community, both in academic as well as industrial research, to standardize scoring systems for evaluating the OA disease process and to make results between different studies comparable. The task of the histopathology initiative of OARSI was to achieve a consensus of scoring systems for the most important species used in OA animal model research (dog, guinea pig, horse, mouse, rabbit, rat, and sheep/goat), which are presented in the various chapters in this special volume of Osteoarthritis & Cartilage together with extra chapters on basic methodology (histochemistry, statistics, morphometry), the specific terminology and a general discussion of animal models in OA research. Standardized definitions are suggested for basic but essential terms such as "grading" and "staging" in order to promote their consistent use and thereby promote improved understanding and data interpretation across all model systems. Thus, this introductory chapter presents an overview of the guiding principles for assessment of important OA animal model systems. Use of such systems, independently or in conjunction with other systems in parallel, should facilitate comparability of results across animal model studies.