(Ultra) Structural Details of Cells and Matrix during Early Cartilage Regeneration after Chondrocyte Implantation in the Equine Knee Joint

Abstract

Objective: This pilot study of cartilage defect treatment was designed to establish the horse as a suitable animal model for MACT (Matrix-Associated Autologous Chrondrocyte Transplantation) transplants and to analyse the morphological aspects of repair tissue development and differentiation. Design: Hyaluronan-based and collagen-based biodegradable scaffolds were seeded with autologous chondrocytes and implanted into large (1.5×2 cm) defects in the trochlear ridge of the distal femur of three horses. A non-treated, empty defect was used as control. Three months after surgery, samples of the defect area were investigated using bright field and polarized light microscopy, immunohistochemistry and electron microscopy. Results: In MACT-treated lesions with good defect filling, the repair tissue integrated well into the defect and showed features of differentiation in transition to native cartilage; the matrix was partially masked by proteoglycans, strongly stained for collagen type II, and the fibres had the typical vertical arrangement of articular cartilage. In areas with less intense staining, collagen type II formed a network around the cells. Calcified cartilage was partially decalcified and osteoclasts as well as osteoblasts reorganised the subchondral bone. The control defect was almost empty. Conclusion: This study showed that the horse is a suitable animal model for MACT. Differentiation starts early after transplantation in the periphery of the defect. Collagen type II precedes proteoglycan deposition and undergoes a kind of vertical self-arrangement which is hypothesized to develop from a fibre network around the chondrocytes.