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Front Back
Differentiated from the ventromedial part of somite layer, forms the vertebrae and ribs
Differentiated from the dosolateral part of somite layer, further splits into the myotome and dermatome regions.
Region of the dermomyotome that forms the myoblasts (muscle; primordial cells)
Region of the dermamyotome that forms the dermis (skin; fibroblasts)
Loosely organized embryonic connective tissue formed by mesodermal cells.
Intramembranous bone formation
One of two processes of bone formation in the axial skeleton. Mesenchyme forms membranous sheaths within which osteoblasts and osteoclasts (derived from mesenchyme) form bone.
Endochondral bone formation
One of two processes of bone formation in the axial skeleton. Chondrocytes (derived from mesenchyme) and osteoblasts within cartilage form bone.
How does the vertebrae develop?
Endochondral bone formation:
Mesenchymal cells from the somite corresponding to each vertebra migrates around the notochord and neural tube at W-4.
Resegmentation occurs
Notochord degenerates at vertebrae, but becomes the nucelus pulposus of the intervertebral discs. Annulus fibrosus (outer layer of the disc) formed by sclerotomal cells migrating to that site.
Vertebral arch formed by mesenchyme surrounding neural tube. Costal process in the thoracic region also formed by mesenchyme (rib attachment sites)
At week 6, chondrification centers appear in each vertebrae on each side, ossification begins. Initially there are dorsal and ventral centers in each vertebrae, but they eventually fuse.
The formation of the body of each vertebrae. Densely packed cells of one sclerotome fuses with loosely packed cells of the adjacent sclerotome to form the vertebrae.
Spina bifida occulta
Failure of the neural tube to close properly result in flaws in the formation of the vertebral arch in the lumbar vertebrae. Marked by a tuft of overlying hair. Very prevalent (10% of population). More serious forms can cause protrusions of the spinal cord, nerves, and minenges outside the spinal canal, associated with significan neurological deficits.
How do the ribs develop?
Derived from the mesenchyme on the costal processes of thoracic vertebrae.
Cartilaginous during embryonic period, but ossify during fetal development.
Grows into the lateral body wall
The upper 7 ribs contact the sternal bars (sternum). The lower 5 do not directly contact the sternum.

How does the sternum develop?
Forms from mesenchymal sternal bars in the ventrolateral body wall. Bars move medialy and fuse in the midline of the body. 
Contact the upper 7 ribs as they extend anteriomedially and form a cartilaginous connection with them.
What are the axial muscles?
Axial muscles consist of deep back muscles that extend the spine, muscles between ribs (intercostal), and muscles of the anterior abdominal wall.
What are the myotome layer divisions.
Divides into dorsal epaxial and ventral hypaxial divisions.
Epaxial division of myotome
Formed by differentiated myotome, forms the segmental muscles of the main body axis, extensor muscles of the neck and the vertebral column.
Segmental muscles derived from sacral and coccygeal myotomes degenerate, forming the dorsal sacrococygeal ligaments.
Hypaxial division of myotome
Formed by differentiated myotome, forms the scalene, prevertebral, geiohyoid, and infrahyoid muscles.
Hypaxial myotome at the thoracic division forms lateral and ventral flexor muscles of the vertebral column.
Hypaxial myotome at the lumbar division form the quadratus lumborum muscle. Also forms the intercostal muscles, abdominal muscles and abdominal muscles.
Hypaxial myotome at the sacrococcygeal division forms muscles of the pelvic diaphragm, striated muscles of the anus and muscles of the sex organs.
How does axial skin develop?
Middle portion of dermomyotome superficial to myotome forms the dermatome. Dermatome becomes skin of the back and trunk.
Axial Nerve Supply
The nerve supply to the dermomyotome, reflecting earlier resegmentation of the somites.
How does the axial nerve supply develop?
Corresponding spinal nerves grow out from the neural tube during somite (vertebrae) development, moving towards  the dermomyotome.
After resegmentation, spinal nerves pass between adjacent vertebrae and will initially innervate a myotome that span between those vertebrae. Innervation stays segmented even as myotomes fuse. Sensory nerve supply to the dermis will also reflect this segmentation into adulthood. Spinal nerves will branch into dorsal and ventral rami.
Dorsal Rami
A branching of the spinal nerves during axial nerve supply development.  Dorsal rami innervate the skin and muscles of the back.
Ventral Rami
A branching of the spinal nerves during axial nerve supply development. Ventral rami go to innervate the skin and muscles of the anterolateral trunk as well as the limbs.
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