Insulin-like growth factor-I gene expression patterns during spontaneous repair of acute articular cartilage injury

This study evaluated the constitutive insulin-like growth factor-I (IGF-I) gene expression pattern in spontaneously healing cartilage defects over the course of 16 weeks, and correlated the tissue morphology and matrix gene e xpression with IGF-I mRNA levels. Full-thickness 15 mm cartilage defects we re debrided in the femoral trochlea of both femoropatellar joints of 8 hors es and the healing defects examined 2, 4, 8, or 16 weeks after surgery. Sam ples were harvested for histologic assessment of tissue healing using H&E s taining, toluidine, blue histochemical reaction for proteoglycan deposition , and in situ hybridization and immunohistochemistry procedures to demonstr ate collagen type II mRNA and protein expression. Total RNA was isolated fo r Northern analysis to measure cartilage matrix molecule expression, and fo r semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR ) to determine IGF-I gene expression patterns in healing cartilage defects. Full-thickness cartilage defects in horses were slow to heal compared to s maller lesions in similar locations in other animals. However, a progressiv e decline in tissue cellularity and vascularity, and increased tissue organ ization were observed on H&E stained specimens over the 16-week experiment. Evidence of early chondrogenic repair was detected through collagen type I I in situ hybridization and immunohistochemistry. However, levels of collag en type II and aggrecan mRNA in lesions were not abundant on Northern analy sis indicating incomplete chondrogenesis. IGF-I message expression followed a cyclic pattern with low levels at 2 weeks, followed by an increase at 4 and 8 weeks, and a subsequent decline at 16 weeks. There was no direct corr elation between the stage of healing and cartilage matrix message expressio n, and the abundance of IGF-I mRNA in the healing lesions. In conclusion, t his study demonstrated that the spontaneous healing of articular defects wa s accompanied by a temporal fluctuation in IGF-I gene expression which was discoordinate to the steady rise in expression of cartilage matrix molecule s such as procollagen type II. (C) 2001 Orthopaedic Research Society. Publi shed by Elsevier Science Ltd. All rights reserved.