Embryology of the Axial Skeleton

Development of the Axial Skeleton

• The axial skeleton consists of the skull (cranium), vertebral column, sternum, and ribs.
• Development of the vertebral column begins with the development of the notochord during gastrulation.

Fate of the Notochord

• In the region of the bodies of vertebrae, the notochord degenerates.
• Between the bodies of vertebrae, the notochord forms the central part, nucleus pulposus, of the intervertebral discs.
• The annulus fibrosus part of the intervertebral discs is formed by the mesoderm surrounding the notochord.

Development of the Vertebral Column

• The vertebral column develops from somites (42 pairs) derived from paraxial mesoderm.
• The notochord induces the formation of the sclerotomes from the paraxial mesoderm.
• The sclerotomes grow dorsally around the neural tube and form the vertebral (neural) arch.
• Ventrolaterally, costal processes develop that give rise to ribs in the thoracic region.

Ossification of the Vertebral Column

• Ossification of the vertebral column occurs in two stages: chondrification and ossification.
• Chondrification centers appear at 6 weeks (cartilage).
• By the end of 8 weeks, the 3 primary ossification centers appear (bone).
• The 5 secondary ossification centers appear at puberty.
• Fusion of bony halves of vertebral arch occurs at 3-5 years.
• Fusion of centrum with vertebral arch occurs at 4-6 years.
• All centers unite around 25 years.

Spina Bifida

• Spina bifida is a neural tube defect that occurs when two embryonic neural arches fail to fuse with each other.
• It is one of the most serious vertebral defects.
• It occurs in 1 per 1000 births and may be prevented in many cases by giving folic acid to mothers prior to conception.

Types of Spina Bifida

• Spina bifida occulta: the closed type, only one vertebra is affected, and skin overlying it is intact.
• Spina bifida cystica: the open type, which is the most severe and complex form of spina bifida.

Development of the Ribs

• Ribs are formed from the ventral extensions of the sclerotomic mesenchyme that forms the vertebral arches.
• Accessory ribs may develop, such as accessory lumbar rib and accessory cervical rib.

Development of the Sternum

• The sternum develops from two mesenchymal vertical plates on either side of the midline.
• These plates are called mesenchymal sternal bars.
• The two sternal bars fuse with each other in a craniocaudal sequence to form the cartilaginous model of the sternum.
• The cartilaginous model of the sternum gets ossified to form the sternum.

Ossification of the Sternum

• Ossification of the sternum occurs in a craniocaudal sequence during 5th, 6th, 7th, 8th, and 9th months.
• Manubrium and body ossify by five double centers from above downward.
• The fusion is complete by 25 years of age.
• Xiphoid process appears during the 3rd month and fuses completely with the sternum at about 40 years.