Composition of Cell Clusters in Torn Menisci and Their Extracellular Matrix Components

Cell clusters, or groups of cells sharing a single chondron-like structure, are frequently found in degenerated areas of the osteoarthritic (OA) meniscus. However, little is known about these meniscal clusters in humans. The aim of our study was to determine the composition of the extracellular matrix deposition around cell clusters in human OA menisci. Twenty-six menisci were obtained through total knee arthroplasty from patients with OA knee joints. The specimens were subjected to safranin O staining and immunostaining for Sry-type HMG box 9 (SOX9), type II collagen, and aggrecan. Their signal density after staining was assessed using ImageJ software. Five regions of interest were analyzed within each tissue sample. The SOX9, type II collagen, and aggrecan densities were considerably higher in cluster areas than in intact superficial layers of the meniscus. In addition, a substantial difference was detected between cluster areas and degenerative areas without cell clusters. We demonstrated that cell clusters localized near fissures and clefts showed remarkable uniformity in menisci exposed to a broad range of injuries. In addition, the chondrogenic proteins SOX9, type II collagen, and aggrecan were highly expressed in these tissues.