ossifikationscentra

Ossifikationszentren, or ossification centers, are discrete sites in the developing skeleton where bone formation begins. They can arise via endochondral ossification within a cartilage model or via intramembranous ossification directly in mesenchyme. In long bones, ossification centers are categorized as primary and secondary; the primary centers form in the diaphysis before birth, while secondary centers develop later in the epiphyses and contribute to the bone’s growth and final shape.

During endochondral ossification, a primary ossification center appears in the diaphysis, followed by formation of a periosteal bone collar. Nutrient arteries invade and replace the cartilage with woven bone, creating the medullary cavity. After birth, secondary ossification centers appear in the epiphyses; these centers ossify but maintain the growth plate at the junction with the diaphysis, enabling longitudinal growth until puberty. At growth cessation, the epiphyseal plates close, leaving an epiphyseal line.

In intramembranous ossification, typical of flat bones such as the skull and clavicle, ossification centers form directly in condensed mesenchyme, with osteoblasts laying down bone matrix and trabeculae without a cartilage stage. Sutures and fontanelles persist until later.

Clinically, the pattern and timing of ossification centers inform skeletal maturity assessments. Variations include accessory ossification centers and physeal injuries, which can affect growth. Radiographs and imaging track the appearance, fusion, and closure of centers throughout development.