Fig. 1

Circadian rhythm influences bone/cartilage metabolism. Endochondral ossification involves the formation of cartilage template, differentiation and proliferation of chondrocytes, and gradual replacement by osteoblasts, resulting in bone formation. Intramembranous ossification entails the gradual differentiation of MSCs within soft tissues (or damaged tissues) into osteoblasts, followed by the formation of primary bone spicules and gradual deposition of mature bone tissue. Under physiological conditions, bone formation and resorption are dynamically balanced. Clock genes have been implicated in the regulation of the aforementioned bone/cartilage metabolism processes through various signaling pathways or networks