Ribs and sternum

The ribs are formed from the costal processes of the dense caudal parts of the sclerotomes. They are in contact with the neural arches and extend laterally between the myotomes. Chondrification begins at S18. Ossification commences in the fetal period, starting from a primary ossification centre near the costal angle. Above and below the ribs in the thoracic region, costal equivalents develop in the cervical (processus transversus), lumbar (processus costalis) and sacral segments (part of the pars lateralis).
At S17, the sternum is formed from bilateral condensations of the parietal mesoderm (sternal bands) near the median plane. By S21, the ribs lengthen and the sternal bands undergo chondrification to form sternal bars. At S23, they are shifted towards the median plane where they fuse together in a cranial to caudal direction.