When does cartilage start to be replaced by bone?

During the process of endochondral ossification, cartilage is gradually replaced by bone. This process begins in utero and continues after birth until skeletal maturity is reached.

The first step in endochondral ossification is the formation of a cartilage model of the bone. This model is then invaded by blood vessels, which bring in osteoblasts, cells that build new bone. The osteoblasts lay down new bone on the surface of the cartilage, while the cartilage cells in the center of the bone model die. As more and more bone is laid down, the cartilage model is gradually replaced by bone.

The process of endochondral ossification is essential for the development of the skeleton. It allows bones to grow in length and width, and it provides them with the strength they need to support the body.