ossification

Ossification is the biological process by which bone tissue forms. It occurs during fetal development, supports growth in childhood, and continues in adulthood through bone remodeling and repair. Ossification differs from calcification, which is mineral deposition in soft tissues without the formation of bone.

There are two principal pathways that build most skeletal bone. Intramembranous ossification forms bone directly within mesenchymal tissue. Mesenchymal cells differentiate into osteoblasts, which secrete osteoid that mineralizes into woven bone. This pathway forms most of the flat bones of the skull, parts of the clavicle, and facial bones. Endochondral ossification forms bones through a cartilaginous template. A cartilage scaffold is produced by chondrocytes; blood vessels invade, and osteoprogenitor cells differentiate into osteoblasts that lay down osteoid on the scaffold, which mineralizes into bone as the cartilage is progressively replaced. Growth plates at the ends of long bones enable lengthwise growth until they close after puberty.

Cellular players include osteoblasts, osteocytes (mature bone cells embedded in matrix), and osteoclasts (bone-resorbing cells that participate in remodeling). Bone is continually remodeled through cycles of formation and resorption, modulated by hormones, mechanical load, vitamin D, calcium and phosphorus levels, and growth factors.

Ossification also has clinical relevance in healing fractures and in growth disorders. Pathological ossification, or heterotopic ossification, can occur in soft tissues after injury or surgery, producing bone in locations where it is not normally present.