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Skeletal System

Dr. NimirDr. Safaa

ObjectivesUnderstand the development of skull.Understand the development of limbs.Explain the mechanism of limb innervation.Discuss the anomalies of the limbs.Understand the development of vertebrae.Explain the anomalies of vertebrae.Understand the development of ribs &

sternum.

Skull:Is divided into two parts: 1. The neurocranium, which

protects the brain, consists of membranous and cartilaginous parts.

Membranous is derived from neural crest and paraxial mesoderm it forms flat bones e.g frontal,parietal, occipital which are seperated by sutures. Fontnelles are wide sutures.

Cartilaginous (chondrocranium) forms the base of the skull e.g sphenoid, temporal and base of occipital.

2. The viscerocranium, forms the skeleton of the face and is formed mainly from the first two pharyngeal arches.The first arch gives

rise to the maxilla, the zygomatic bone, part of the temporal bone and mandible.

The second pharyngeal arch gives rise to the incus, malleus and stapes.

Skull AnomaliesCranioschisis: The cranial vault fails to form and brain tissue is exposed to amniotic fluid brain tissue degenerates, resulting in AnencephalyCranial meningocele

& meningoencephalocele: Small defects in the skull through which meninges and/or brain tissue herniate .

Craniosynostosis :Caused by premature

closure of one or more sutures.

Scaphocephaly: Early closure of the

sagittal sutureAcrocephaly or tower

skull: Premature closure of the coronal suture.

Brachycephaly: The coronal and

lambdoid sutures close prematurely

Limb development:The limb buds appear at the end of the fourth week of

development. The upper limb appears first followed by lower 1 to 2 days later.

Initially buds consist of a mesenchymal core derived from the parietal (somatic) layer of lateral plate mesoderm that will form the bones and connective tissues , covered by a layer of ectoderm.

Ectoderm at the distal border of the limb thickens and forms the apical ectodermal ridge (AER).

AER induces adjacent mesenchyme to grow .

A. At 5 weeks. B. At 6 weeks. C. At 8 weeks.

At 6 weeks, there are hand and foot plates separated from the rest of the developing limb by a circular constriction marking the wrist and ankle; inside are the developing cartilagenous models.

Later, a second constriction appears; this marks the elbow and knee.

Fingers and toes are formed when cell death in the AER separates this ridge into five parts.

During the seventh week of gestation, the limbs rotate in opposite directions , upper rotates 90° laterally.Lower 90° medially.

Limb Anomalies Meromelia =partial absence of one or more of the extremities Amelia=complete absence (of one or more of the extremities Phocomelia =a form of meromelia the long bones are absent, and rudimentary

hands and feet are attached to the trunk by small, irregularly shaped bones. Micromelia =all segments of the extremities are present but abnormally short .

A. Child with unilateral amelia. B. Patient with a form of meromelia called phocomelia. The hands and feet are attached to the trunk by irregularly shaped bones.

A-Brachydactyly; the digits are shortened B-Syndactyly:If two or more fingers or toes are

fused,

C-Polydactyly= presence of extra fingers or toes. D-Cleft hand and foot (lobster claw deformity) =abnormal

cleft between the second and fourth metacarpal bones and soft tissues.

Clubfoot =The sole of the foot is turned inward, and the foot is adducted and plantar flexed.

Development of vertebrae:

Vertebrae form from the sclerotome portions of the somites.

Each vertebra is formed from the combination of the caudal half of one somite and the cranial half of its neighbor.

Vertebral AnomaliesScoliosis =lateral curving of the spine.Kyphosis =poterior curving of the spine.Lordosis =anterior curving of the spine.

Cleft vertebra (spina bifida)= imperfect fusion or nonunion of the vertebral arches.

Ribs and Sternum:

Ribs grow from the costal processes of thoracic vertebrae. Costal cartilages are formed by sclerotome cells.

Two sternal bands are formed in the parietal layer of the lateral plate mesoderm on either side of the body, and later fuse to form cartilaginous models of the manubrium, sternebrae and xyphoid process.