How does cartilage become bone during the development of vertebrae and ribs?

During the development of the vertebrae and ribs, the process of bone formation follows a sequence known as endochondral ossification.12

Initially, the axial skeleton develops from the sclerotome, a portion of the embryonic mesoderm. These cells migrate to form a mesenchymal blastema, which serves as the precursor for the vertebrae and ribs. This mesenchymal tissue then undergoes chondrification, where the cells are converted into cartilage cells, or chondrocytes, creating a cartilaginous model of the future bone.23

Once this cartilaginous framework is established, ossification centers appear. Within these centers, the cartilage gradually calcifies and the chondrocytes die out. Specialized cells called osteoblasts then invade the area, secreting a matrix called osteoid. Inorganic salts are subsequently deposited into this matrix to form mineralized bone. Through this process, the original cartilage model is progressively replaced by bone tissue.312