Embro

DEVELOPMENT OF VERTEBRAE AND VERTEBRA COLUMN, RIBS AND

STERNUM.

GROUP 3

INTRODUCTION When sclerotome surrounds the neural

tube, notochord develops. 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.

Development of Axial Skeleton

Forms from sclerotome portion of somites. Typical vertebra consists of:

1. -vertebra arch2. -foramen3. -body4. -transverse process5. -spinal process

Vertebrae

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.

Formation of vertebral column

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 disc.

Resegmentation causes myotomes to bridge the IV disc. This gives the capacity to move the spine.

Intersegmented arteries pass midway over vertebral bodies.

Spinal nerves lie IV disc and leave through IV foramina.

As the vertebrae form, two primary curves of the spine are established: THORACIC and SACRAL curvatures.

Later two secondary curves are established: CERVICAL and LUMBAR curvatures.

SCOLIOSIS-two vertebrae fuse asymmetrically or half a

vertebra missing. KLIPPEL-FEIL SEQUENCE-fewer than normal cervical vertebrae-vertebrae fuse with abnormal shape-associated with other defects.

CLINICAL CORRELATES

CLEFT VERTEBRA- imperfect fusion of vertebral arch.

1. May involve only vertebral arches leaving the spinal cord intact. In this case, bony defect is covered by skin and no neurological deficit occur.

2. Spina Bifida Cystica-neural tube fails to close-vertebral arch fails to form-neurological deficit depend on the extent of the lesion-occurrence is 1 in a 1000 births-can be detected by ultrasound and prevented

administering

folic acid to mother before conception. If neural tissue is exposed, amniocentesis

can be used

Bony part of the rib is derived from sclerotome cells.

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.

DEVELOPMENT OF RIBS AND STERNUM

RIB DEFECTS1. Extra rib/Accessory rib-usually in the lumbar or cervical region Cervical ribs-attaches to 7th cervical vertebra-iminges on the brachial plexus and subclavian

artery resulting in degrees of anaesthesia in the limb.-may be unilateral or bilateral Lumbar ribs-most common but cause no problem

CLINICAL CORRELATES

2. Fused rib -occurs posteriorly -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)

STERNUM1. 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)

PECTUS EXCAVATUM

-depressed sternum sunken posteriorly-most common thoracic wall defect seen by paediatricians.-due to overgrowth of costal cartilage that displace the lower sternum posteriorly or abnormality of ventral body wall closure. PECTUS CARINATUM

-flattening of chest bilaterally with projecting sternum.

-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.

SUMMARY