Ossification is the process of bone formation. It is the process of laying down new bone material by cells named osteoblasts. There are two types of bone ossification: intramembranous and endochondral. Intramembranous ossification is the direct laying down of bone into the primitive connective tissue (mesenchyme), while endochondral ossification involves cartilage as a precursor. Intramembranous ossification forms the flat bones of the skull, mandible, and hip bone, while endochondral ossification forms the long bones and other bones.
Ossification begins between the sixth and seventh weeks of embryonic development and continues until about age twenty-five, although this varies slightly based on the individual. Bone formation requires a template for development, which is mostly cartilage derived from embryonic mesoderm, but also includes undifferentiated mesenchyme (fibrous membranes) in the case of intramembranous ossification. By the time of birth, the majority of cartilage has undergone replacement by bone, but ossification will continue throughout growth and into the mid-twenties.
It is important to note that calcification is often confused with ossification, but they are not the same thing. Calcification is synonymous with the formation of calcium-based salts and crystals within cells and tissue, and it is a process that occurs during ossification, but not necessarily vice versa.
In summary, ossification is the natural process of bone formation, which involves laying down new bone material by cells named osteoblasts.
