The skeletal system is composed of cartilage and bone and functions as a structural framework for the body, facilitating movement and protecting internal organs. Bone also serves as a reservoir for inorganic salts and cells of the immune system. During fetal development, long bones form through the process of endochondral ossification, and flat bones develop by intramembranous ossification. Both processes involve the aggregation of mesenchymal stem cells into mesenchymal condensations, which provide a microenvironment conducive for chondrogenesis. Following chondrocyte differentiation, the cells produce extracellular matrix components and the cartilage is then replaced by calcified bone during endochondral ossification. During intramembranous ossification, flat bones are formed from connective tissue that arises from mesenchymal condensations, rather than through a cartilage intermediate.
Throughout adulthood, many cell types and signaling pathways impact bone growth and remodeling. These include chondrocytes and osteoblasts derived from bone marrow mesenchymal stem cells, osteoclasts of hematopoietic origins, and hormones and other neuromodulators that help regulate the balance of osteoblastic bone formation and osteoclastic bone resorption. The dynamic processes of bone deposition and resorption continuously remodel bones, and this balance is crucial for maintenance of bone size, shape, and integrity. Disruptions of bone homeostasis accompany disorders that include osteoporosis, arthritis, and many heritable skeletal diseases.