Group 1 – SEM IV – 2010

1By : Khalida , Yasmin & Abrar

The Development Of The Vertebral Column

A. Stage of formation of mesenchymal vertebral column.

Time development : during 4th week

1) Migration of sclerotomes :the sclerotomes migrate ventromedially to surround the spinal cord and

notochord

forming long mesenchymal column

this column retains its segmental origin as the sclerotomic segments are separated by less dense areas containing the intersegmental arteries

2) Differentiation of sclerotomic segment :

-Each sclerotome segment is differentiated into : a. less densed cephalic part b. more condensed caudal part

3) The development of intervertebral disc :

Mesenchymal cells between the cranial and caudal parts of each sclerotomic segment do not proliferate and give rise to the intervertebral discs

The peripheral part of which will form rings of fibrous tissue and

is called annulus fibrosus that surround the notochord

the enclosed notochord expands and undergoes nucoid degeneratiom

and form the central soft part of intervertebral disc called nucleus

pulposus

So in the intervertebrae disc there are two regions, the inner region is nucleus pulposus

and the outer region is annulus fibrosus.

Group 1 – SEM IV – 2010

2By : Khalida , Yasmin & Abrar

4) The development of the body (the centrum) of the vertebra :

The remaining caudal condensed part of each scelorotomic segment joins the cephalic less condensed part of scelorotomic segment caudal

Form mesenchymal centrum ( the primordium of the body of vertebra)

The notochord degenerates and disappears

5) The development of neural arch :

Scelorotomic tissue migrates backwards from both sides of the centrum of vertebra to surround the neural tube

The neural spine forms at the point of meeting of neural arch,posteriorly.

Sclerotomic tissue also extends laterally from both sides of the centrum to form 2 processes (costal process ventrally, transverse process dorsally)

B. Stage of formation of cartilaginous vertebral column

Time of appearance of chondrofication centres : during 6th week Process of chondrofication :

2 centres of chondrofication appear in centrum of vertebra

They fused together at the end of embryonic period (8th week)

Centres of chondrofication appear in neural arches

They fuse with each other and with the centrum

The spinous and transverse processes develop from extension of

chondrofication centres in the neural arch

Group 1 – SEM IV – 2010

3By : Khalida , Yasmin & Abrar

C. Stage of ossification of vertebral column

Primary ossification centers Secondary ossification centers

End of 8th week Time of development

After puberty

3 primary ossification centers i. 1 in the centrumii. 1 in each half of neural arch

Number 5 secondary ossifation cerntersi. 1 for the tip of the spinous

processii. 1 for the tip of each transverse

processiii. 1 for the superior rim of the

vertebral bodyiv. 1 for inferior rim of the

vertebral body

Process of ossification:

At birth each vertebra consists of 3 bony parts connected by cartilages.

The bony halves of the vertebral arch fuse together during the 1st

3 to 5 years. The arches articulate with the

centrum at cartilaginous neurocentral joints.

These joints disappear when the vertebral arch fuses with the centrum during the 3rd to 6th

year.

Fate of notochord:

1. The most cranial part of the notochord is incorporated in the basilar part of the occipital bone ant the posterior part of body of sphenoid bone.

2. The parts of the notochord in the bodies of the vertebrae degeneration and disappear.

3. The parts of the notochord in between the bodies of the vertebrae undergo mucoid degeneration to form the nucleus pulposus.

Fate of costal processes:

1. In cervical region they form the anterior and lateral boundary of the foramen transversum.

2. In thoracic region they form the ribs.

3. In lumbar region they fuse with the transverse processes.

4. In upper sacral region they unite tgether to form the anterior portion of the ala of sacrum.

Group 1 – SEM IV – 2010

4By : Khalida , Yasmin & Abrar

Curvature of the vertebral column

1. Primary curvature

In the fetus the entire vertebral column is flexed anteriorly like a "C"

anterior concavity

maintained at birth

seen in the thoracic and sacral regions. 2. Secondary curvatures

develop after birth

seen in the cervical and lumbar region

appears at the 3rd to 6th month when the child holds its head up and the lumbar curvature in the 6th month to 2 year range when the child sits up

Anomalies of vertebral column

1. Spina bifida

Incomplete fusion of the halves of the vertebral arches resulting in a midline defect usually in the lumbosacral region

Types : a) Spina bifida occulta( closed): 20% b) Spina bifida manifesta (open): 80%

-Meningocele: CSF only -Meningomyelocele( CSF+ Nerve fibers)

2. Hemivertebra

Failure of one of the chondrofication centres to appear and subsequent failure of half of the vertebra to form producing scoliosis.

3. Scralization of the fifth lumbar vertebra

5th lumbar vertebra is fused with the sacrum and as a result the number of lumbar vertebrae is 4 and the sacrum is formed of 6 vertebrae.

4. Lumbrization of the first piece of sacrum

Separation of the first piece of the sacrum to form a separate vertebra so the number of lumbar vertebrae is 6 and the sacrum is only formed of 4 sacral vertebrae.

5. Congenital Kyphosis

due to either failure of segmentation or defect in formation of vertebral body elements.

N/B : For pictures, please refer to group 1 student presentation.