# Development of Vertebrae and Ribs > Development of vertebrae and ribs for MBBS Anatomy: sclerotome resegmentation, notochord fate, ossification timeline, cervical rib and congenital anomalies. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/development-of-vertebrae-ribs - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Development of Vertebrae and Ribs", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/development-of-vertebrae-ribs ## Direct answer Every vertebra in your spine is built from two half-somites that were never neighbours: during week four, sclerotome cells from each somite migrate around the notochord and neural tube, and then resegment so that the caudal half of one sclerotome fuses with the cranial half of the next to form each vertebral centrum — which is why segmental arteries cross the waist of vertebral bodies. The notochord persists only as the nucleus pulposus of each disc, the annulus fibrosus coming from surrounding sclerotome. Chondrification centres appear in week six, ossification begins in the embryonic period with a centrum plus two neural-arch centres, arch halves fuse posteriorly in the first year and neurocentral synchondroses close by three to six years, and ring epiphyses arrive at puberty, fusing in the early twenties. Ribs grow from the costal processes of thoracic vertebrae; their persistence at C7 makes the cervical rib of thoracic outlet syndrome. ## What you must remember - **Somites and their timetable:** about 42-44 pairs of somites form from paraxial mesoderm; the sclerotome compartment migrates in week four during neurulation's closing phases. - **Resegmentation:** caudal half of sclerotome N plus cranial half of N+1 make one vertebra — the intersegmental vertebra; arteries and nerves stay intersegmental, so they cross the disc and lie on the body's waist. - **Notochord fate:** becomes the nucleus pulposus of each intervertebral disc; its remnant in adults shrinks with age; the annulus fibrosus is sclerotome-derived — the classic single-line viva question. - **Chondrification (week six):** two centres in the centrum fuse, two in each neural arch; failure of one centrum centre gives a hemivertebra — a congenital scoliosis cause. - **Ossification centres:** three primary per vertebra (one centrum, two arch halves), appearing in the late embryonic period; arch halves fuse posteriorly around the first year, arch-to-centrum (neurocentral) synchondroses fuse at three to six years. - **Secondary centres:** at puberty — one for the spinous process tip, two for transverse processes, and two annular ring epiphyses rimming each body's upper and lower faces — fusing by about 21-25 years. - **Ribs and their variants:** costal processes enlarge into ribs only at thoracic levels; in the neck they become the anterior bar of the transverse process and the costotransverse bar; a hypertrophied C7 costal element — the cervical rib — compresses the lower trunk and subclavian artery. - **Clinical anomalies:** spina bifida occulta (posterior arch fusion failure, most common at L5-S1), Klippel-Feil sequence (cervical segmentation failure — short webbed neck) and hemivertebra, a congenital scoliosis cause. ## Drawing resegmentation so it finally makes sense Picture two adjacent somites, each with dense caudal and loose cranial halves. The caudal (denser) half of the upper somite and the cranial half of the lower one fuse across the old boundary to make a vertebra — so the intervertebral disc forms where the boundary used to be, and the segmentally arranged myotome and its nerve root now lie at the level of the disc, not the bone. That single shift explains why spinal nerves exit at disc level and why segmental arteries cross the waist of each body. Now the costal elements: at thoracic levels they separate as ribs at synovial costovertebral joints; in cervical and lumbar regions they fuse into the transverse processes; at C7 an independent costal element can enlarge into a cervical rib dropping the lower trunk over it — a young woman with medial arm pain, thenar wasting and a positive Adson test is the textbook picture. ## Where students slip Students state that one somite makes one vertebra — resegmentation means each vertebra is intersegmental, from two adjacent somites; examiners isolate exactly this point. They also assign the whole disc to the notochord — only the nucleus pulposus is notochordal, the annulus is sclerotomal. On dates, the commonest slips are fusing the arch halves "at birth" (about one year) and forgetting the ring epiphyses of puberty. The five-marker format is "development of a typical vertebra with anomalies" — resegmentation, chondrification, three primary centres, then spina bifida occulta and hemivertebra. ## Frequently asked questions ### What does the notochord become in the adult? The nucleus pulposus of each intervertebral disc, surrounded by the sclerotome-derived annulus fibrosus. No other notochordal derivative persists in the spine. ### What is resegmentation of sclerotomes? The caudal half of one sclerotome fuses with the cranial half of the next to form each vertebra, making vertebrae intersegmental. This is why nerves and vessels cross the disc and body waist. ### How many primary ossification centres does a typical vertebra have? Three — one in the centrum and one in each half of the neural arch. The arch halves fuse posteriorly at about one year, and neurocentral synchondroses close by three to six years. ### What is a cervical rib embryologically? A hypertrophied costal process of the seventh cervical vertebra, complete or fibrous. It compresses the C8-T1 lower trunk and the subclavian artery — thoracic outlet syndrome. ### Name two developmental anomalies of vertebral formation. Spina bifida occulta from failure of posterior arch fusion (commonest at L5-S1), and hemivertebra from failure of one chondrification centre, causing congenital scoliosis. Klippel-Feil sequence is segmentation failure in the neck.