**1. Introduction**

The extracellular matrix (ECM) is the intricate scaffolding, which surrounds and supports cells, and helps to organize them into tissues and organs. The composition of the extracellular matrix varies based on tissue type and organ function, and this matrix affects many cellular behav‐ iors including proliferation, differentiation, and wound healing. Matricellular proteins, present in the immediate environment of the cell, help to regulate and modulate these functions. We

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present a discussion of the important matrix elements of cartilage and bone, and the support‐ ing connective tissues of the body, with regard to their function in health and disease. The major aim of this book chapter is to summarize the role of extracellular matrix proteins and their roles in chondrogenesis and osteogenesis. Additionally, we will discuss the importance of the CCN proteins—a family of secreted extracellular matrix‐associated proteins—in functional path‐ ways of chondrogenesis and osteogenesis, and include a discussion of the role of CCNs in pathophysiology of these tissues. The reader will gain an increased awareness of the impor‐ tance of CCN proteins in a wide range of important functional pathways, providing in‐depth focus on their role in bone and cartilage development and repair. These proteins have also been implicated in many human diseases and thus are important targets for drug discovery and development.
