Chondrification and ossification of the vertebral column

At S17, chondrification of the blastemal or mesenchymatous vertebral column commences. It starts from three chondrification centres, one in the vertebral centrum and one in each of the neural processes. During month 3, at the beginning of the fetal period, the neural processes merge together and form the neural arch. Ossification of the cartilaginous vertebral column begins in the vertebral centra, the costal processes (both endochondral) and in the neural arches (perichondral). Up until birth, the ossification centres extend to form zones of ossification. Cartilaginous zones of fusion remain in the vertebrae between the vertebral centrum and the neural arch, and dorsally in the region of the forming spinal process. The costal heads in the synovial joint between the neural arch and costal process are also still cartilaginous. These zones of fusion become ossified by the time the child reaches age three. Secondary ossification centres appear at about the time of puberty: apophyses (at the processes) and epiphyses (annular osseous centres or ridges on the superior and inferior surfaces of the vertebral body). The fusion of the epiphyses through synostosis with the vertebral bodies continues until age 18.