Which part of a long bone is primarily responsible for its growth?

The epiphysis is primarily responsible for the growth of a long bone due to its role in the process of endochondral ossification. This is a critical mechanism that enables long bones to lengthen during childhood and adolescence.

Inside the epiphysis, the growth plate, also known as the epiphyseal plate, is where new bone is formed. This plate contains chondrocytes, which are cartilage-forming cells that proliferate and expand, pushing the existing bone further away from the joint and allowing the bone to lengthen. As individuals mature, the epiphyseal plates gradually close and turn into solid bone, which marks the end of growth in length.

While other parts of the long bone, such as the diaphysis (the shaft of the bone), the periosteum (the outer covering of the bone), and the endosteum (the lining of the inner bone cavity), play vital roles in bone health, remodeling, and providing structural support, they do not directly contribute to the lengthening of the bone in the way that the epiphysis does. Therefore, the epiphysis is crucial for bone growth during the developmental years.