When sclerotome surrounds the neural tube,
The position of sclerotomes is changed by
growth of the surrounding structures and not
by its migration.
Pax 1-gene play an important role in
development of vertebral column.
Forms from sclerotome portion of somites.
Typical vertebra consists of:
During 4th week, sclerotome cells from
around the notochord and spinal cord
migrate to opposing sclerotome on the other
side of the neural tube.
As development continues, sclerotome
portion of somites undergoes
RESEGMENTATION. This forms vertebra.
Patterning and shapes of vertebrae is
regulated by Hox genes.
Mesenchymal cells between cephalic and
caudal parts of original sclerotome fill the
space between the two vertebra bodies
forming the intervertebral disc.
Notochord regresses in the vertebra body.
Notochord enlarges and persists in the
intervertebral disc region.
This contributes to the NUCLEUS PULPOSUS
which is later surrounded by the ANNULUS
FIBROSUS. This gives rise to the intervertebral
Resegmentation causes myotomes to bridge
the IV disc. This gives the capacity to move
Intersegmented arteries pass midway over
Spinal nerves lie IV disc and leave through IV
As the vertebrae form, two primary curves of
the spine are established: THORACIC and
Later two secondary curves are established:
CERVICAL and LUMBAR curvatures.
-two vertebrae fuse asymmetrically or half a
-fewer than normal cervical vertebrae
-vertebrae fuse with abnormal shape
-associated with other defects.
- imperfect fusion of vertebral arch.
May involve only vertebral arches leaving the
spinal cord intact. In this case, bony defect is
covered by skin and no neurological deficit occur.
Spina Bifida Cystica
-neural tube fails to close
-vertebral arch fails to form
-neurological deficit depend on the extent of the
-occurrence is 1 in a 1000 births
-can be detected by ultrasound and prevented
folic acid to mother before conception.
If neural tissue is exposed, amniocentesis can
Bony part of the rib is derived from sclerotome
COSTAL CARTILAGE: formed by sclerotome
cells that migrate across the lateral somatic
frontier in the adjacent lateral plate mesoderm.
STERNUM: development in the parietal layer of
lateral plate mesoderm in the ventral body wall.
-two sternal bands form in the parietal layer of
the lateral plate mesoderm on the other side of
the midline and these fuse to form
cartilaginous models of the MANUBRIUM,
STERNEBRAE and XIPHOID PROCESS.
1. Extra rib/Accessory rib
-usually in the lumbar or cervical region
-attaches to 7th cervical vertebra
-iminges on the brachial plexus and subclavian
resulting in degrees of anaesthesia in the limb.
-may be unilateral or bilateral
-most common but cause no problem
2. Fused rib
-two ribs arise to form a single vertebrae
-associated with hemivertebrae (results from
failure of one of the centres to appear leading to
failure of half of the vertebrae to form)
1. Cleft sternum: arises when sternal band fail to
grow together in the midline.
2. Hypoplastic ossification centres, premature
fusion of sternal segment- particularly in infants
with heart disease; multiple manubrial
ossification(common with down’s syndrome)
-depressed sternum sunken posteriorly
-most common thoracic wall defect seen by
-due to overgrowth of costal cartilage that displace
the lower sternum posteriorly or abnormality of
ventral body wall closure.
-flattening of chest bilaterally with projecting
-projection of sternum resembles the keel of a boat.
-it has the same cause as P. excavatum.
Skeletal system develops from mesenchyme
Flat bones e.g. scapula, ribs, sternum)
undergo MEMBRANOUS OSSIFICATION.
Most bones undergo ENDOCHONDRIAL
OSSIFICATION and form hyaline cartilage.
Ossification centres appear and the
cartilage gradually ossifies.
NB: vertebrae are modified long bones.