Subcutaneous inflammation (panniculitis) in tibio-tarsal joint of rats inoculated with complete Freund's adjuvant.

Complete Freund's adjuvant (CFA)-induced arthritis in rats, which presents similar features to rheumatoid arthritis, is a model widely used in aetiopathogenetic and investigational drug studies. In this model, arthritis is induced by intradermal injection of Mycobacterium tuberculosis suspended in mineral oil in the hind footpad. Although the histopathology findings in the joint are well described, the marked subcutaneous features of panniculitis that concomitantly occur in this model have received no attention. The objective of this paper is to describe the subcutaneous histopathological features in 8 Wistar rats after intraplantar injection of CFA. We studied the subcutaneous histopathological features in 8 Wistar rats after intraplantar injection of CFA in the left hind paw. The levels of subcutaneous inflammation of the animals in this study were evaluated for the histological characteristics present in the tissue and scored with 4 parameters (acute inflammation, chronic inflammation with fibrosis, subcutaneous and profound soft tissue necrosis, and the presence of giant cells, neutrophils, macrophages and lymphocytes) on days 4, 7, 11 and 15 after induction. All animals developed intense subcutaneous inflammation characteristic of panniculitis, with predominance of acute changes in the initial period, with progression to a self-perpetuating chronic fibrotic process on day 15. These observations precede the joint changes. Besides being an interesting model for better studying diseases with panniculitis, our observations bring up issues concerning the possible relations between subcutaneous and joint inflammatory changes.