There are two types of ossification centers – primary and secondary. A primary ossification center is the first area of a bone to start ossifying. It usually appears during prenatal development in the central part of each developing bone. In long bones the primary centers occur in the
diaphysis/shaft and in irregular bones the primary centers occur usually in the body of the bone. Most bones have only one primary center (e.g. all long bones except clavicle) but some irregular bones such as the os coxae (hip) and vertebrae have multiple primary centers. A secondary ossification center is the area of ossification that appears after the primary ossification center has already appeared – most of which appear during the postnatal and adolescent years. Most bones have more than one secondary ossification center. In long bones, the secondary centers appear in the
epiphyses. At the end of the formation of the secondary ossification center, the only two areas where the cartilage remains is at the
articular cartilage covering the epiphysis and at the
epiphyseal plate between the epiphysis and diaphysis. endochondral ossification: Primary endochondral ossification begins with the formation of a chondrocyte template. Afterwards, chondrocytes undergo hypertrophy beginning from the mid-
diaphysis, eventually extending to the epiphyseal poles, vasculature invades the forming bone transporting mesenchymal stromal cells and hypertrophic cells undergo apoptosis. Mesenchymal stromal cells differentiate into osteoblasts and then osteocytes. Secondary ossification occurs at the
epiphysis post-natally and bone formation initiates at the center and extends peripherally. ==References==